ALL ABOUT THE PROTEIN FOODS GROUP

TABLE CONTENT  

• What foods are in the Protein Foods GroupProtein is incredibly important.Protein - What Is It and Why Do We Care?Protein Can Help You Lose Weight (and Prevent You From Gaining it in The First Place).How Protein Can Help You Lose Weight Naturally.Protein Makes You Lose Weight, Even Without Conscious Calorie Restriction .Protein Consumption and the Elderly: What Is the Optimal Level of Intake?What foods are in the Protein Foods Group?

All foods made from meat, poultry, seafood, beans and peas, eggs, processed soy products, nuts, and seeds are considered part of the Protein Foods Group. Beans and peas are also part of the Vegetable Group. For more information on beans and peas, see Beans and Peas Are Unique Foods.



Select a variety of protein foods to improve nutrient intake and health benefits, including at least 8 ounces of cooked seafood per week. Young children need less, depending on their age and calorie needs. The advice to consume seafood does not apply to vegetarians. Vegetarian options in the Protein Foods Group include beans and peas, processed soy products, and nuts and seeds. Meat and poultry choices should be lean or low-fat.

Delicious High-Protein Foods to Eat


People argue about carbs, fats, and everything in between.
However, almost everyone agrees that protein is important.

Eating plenty of protein has numerous benefits.
It can help you lose weight (especially belly fat), and increase your muscle mass and strength, to name a few.

The recommended daily intake (RDI) is 46 grams for women, and 56 grams for men .

However, many health and fitness experts believe that we need much more than that.

Here is a list of delicious foods that are high in protein.

1. Eggs

Whole eggs are among the healthiest and most nutritious foods on the planet.

They are loaded with vitamins, minerals, healthy fats, eye-protecting antioxidants and brain nutrients that most people don't get enough of.

Whole eggs are high in protein, but egg whites are almost pure protein.
Protein content: 35% of calories in a whole egg. 1 large egg contains 6 grams of protein, with 78 calories.

2. Almonds

Almonds are a popular type of tree nut.

They are loaded with important nutrients, including fiber, vitamin E, manganese and magnesium.

Protein content: 13% of calories. 6 grams per 1 ounce (28 g) serving, with 161 calories.

Other High-Protein Nuts

Pistachios (13% of calories) and cashews (11% of calories).

3. Chicken Breast

Chicken breast is one of the most popular protein-rich foods.
If you eat it without the skin, the majority of the calories in it come from protein.

Chicken breast is also very easy to cook, and tastes delicious if you do it right.

Protein content: 80% of calories. 1 roasted chicken breast without skin contains 53 grams, with only 284 calories.

4. Oats

Oats are among the healthiest grains on the planet.

They are loaded with healthy fibers, magnesium, manganese, thiamin (vitamin B1) and several other nutrients.

Protein content: 15% of calories. Half a cup of raw oats contains 13 grams, with 303 calories.

5. Pumpkin Seeds

Pumpkins contain edible seeds called pumpkin seeds.
They are incredibly high in many nutrients, including iron, magnesium and zinc.
Protein content: 14% of calories. 1 ounce (28 g) contains 5 grams of protein, with 125 calories.
Other High-Protein Seeds
Flax seeds (12% of calories), sunflower seeds (12%) and chia seeds (11%).


6. Fish (All Types)

Fish is incredibly healthy, for various reasons.
It is loaded with various important nutrients, and tends to be very high in heart-healthy omega-3 fatty acids.

Protein content: Highly variable. Salmon is 46% protein, with 19 grams per 3 ounce (85 g) serving, with 175 calories.


7. Peanuts

Peanuts are incredibly delicious.

They are high in protein, fiber, magnesium and many studies show that they can help you lose weight.
Peanut butter is also high in protein, just make sure not to eat too much as it is quite "more-ish."

Protein content: 16% of calories. One ounce (28 g) contains 7 grams, with 159 calories.

8. Lentils

Lentils are a type of legume.

They are high in fiber, magnesium, potassium, iron, folate, copper, manganese and various other nutrients.
Lentils are among the world's best sources of plant-based protein, and are an excellent food for vegetarians.

Protein content: 27% of calories. 1 cup (198 g) of boiled lentils contains 18 grams, with 230 calories.


9. Milk

Milk is highly nutritious, but the problem is that a huge percentage of the world's adults are intolerant to it.
However, if you tolerate milk and enjoy drinking it, then milk can be an excellent source of high-quality protein.
Milk contains a little bit of almost every single nutrient needed by the human body.
It is particularly high in calcium,phosphorus and riboflavin (vitamin B2).

Protein content: 21% of calories. 1 cup of whole milk contains 8 grams of protein, with 149 calories.

10.Broccoli

Broccoli is an incredibly healthy vegetable, loaded with vitamin C, vitamin K, fiber and potassium.
Broccoli is also loaded with various bioactive nutrients believed to help protect against cancer.
Calorie for calorie, it is high in protein compared to most vegetables.

Protein content: 20% of calories. 1 cup of chopped broccoli (96 grams) contains 3 grams of protein, with only 31 calories.

11.Lean Beef

Lean beef is very high in protein, and also tastes delicious.
It is loaded with highly bioavailable iron, vitamin B12 and numerous other nutrients.

Protein content: 53% of calories. One 3 ounce (85 g) serving of cooked beef with 10% fat contains 22 grams of protein, with 184 calories.

If you're on a low-carb diet, feel free to eat fatty cuts of beef instead of lean beef.

Protein is incredibly important.

If we don't get enough from the diet, our health and body composition suffers.

However, there are vastly different opinions on how much protein we actually need.
Most official nutrition organizations recommend a fairly modest protein intake.
The DRI (Dietary Reference Intake) is 0.8 grams of protein per kilogram of body weight, or 0.36 grams per pound.

This amounts to:

56 grams per day for the average sedentary man.46 grams per day for the average sedentary woman.

Although this meager amount may be enough to prevent downright deficiency, studies show that it is far from sufficient to ensure optimal health and body composition.

It turns out that the "right" amount of protein for any one individual depends on many factors... including activity levels, age, muscle mass, physique goals and current state of health.

Protein - What Is It and Why Do We Care?

Proteins are the main building blocks of the- body.
They're used to make muscles, tendons, organs and skin.

Proteins are also used to make enzymes, hormones, neurotransmitters and various tiny molecules that serve important functions.

Without protein, life as we know it would not be possible.

Proteins are made out of smaller molecules called amino acids, which are linked together like beads on a string. The linked amino acids form long protein chains, which are then folded into complex shapes.
Some of these amino acids can be produced by the body, while we must get others from the diet. The ones we can not produce and must get from our foods are called the "essential" amino acids.

Protein is not just about quantity. It's also about quality.

Generally speaking, animal proteinprovides all the essential amino acids in the right ratio for us to make full use of them (only makes sense, since animal tissues are similar to our own tissues).

If you're eating animal products (like meat, fish, eggs, or dairy) every day, then you're probably already doing pretty well, protein-wise.
If you don't eat animal foods, then it is a bit more challenging to get all the protein and essential amino acids that your body needs (good article on this here).
Most people don't really need protein supplements, but they can be useful for athletes and body builders.

Protein is a structural molecule assembled out of amino acids, many of which the body can’t produce on its own. Animal foods are usually high in protein, with all the essential amino acids that we need.


Protein Can Help You Lose Weight (and Prevent You From Gaining it in The First Place)

Protein is incredibly important when it comes to losing weight.
As we know... in order to lose weight, we need to take in fewer calories than we burn.

Eating protein can help with that, byboosting your metabolic rate (calories out) and reducing your appetite (calories in). This is well supported by science (1).
Protein at around 25-30% of calories has been shown to boost metabolism by up to 80 to 100 calories per day, compared to lower protein diets.

But probably the most important contribution of protein to weight loss, is its ability to reduce appetite and cause aspontaneous reduction in calorie intake. Protein is much more satiating than both fat and carbs .

In a study in obese men, protein at 25% of calories increased feelings of fullness, reduced the desire for late-night snacking by half and reduced obsessive thoughts about food by 60% .

In another study, women who increased protein intake to 30% of calories ended up eating 441 fewer calories per day. They also lost 11 pounds in 12 weeks, just byadding more protein to their diet.

But protein doesn't just help you lose... it can also help prevent you from gaining weight in the first place.

In one study, just a modest increase in protein from 15% of calories to 18% of calories reduced the amount of fat people regained after weight loss by 50%.

A high protein intake also helps to build and preserve muscle mass (see below), which burns a small amount of calories around the clock.

By eating more protein, you will make itmuch easier to stick to whichever weight loss diet (be it high-carb, low-carb or something in between) you choose to follow.

According to these studies, a protein intake around 30% of calories may be optimal for weight loss. This amounts to 150 grams per day for someone on a 2000 calorie diet.


How Protein Can Help You Lose Weight Naturally


Protein is the single most important nutrient for weight loss and a better looking body.

A high protein intake boosts metabolism, reduces appetite and changes several weight-regulating hormones.
Protein can help you lose weight and belly fat, and it works via several different mechanisms.

This is a detailed review of the effects of protein on weight loss.

Protein Changes The Levels of Several Weight Regulating Hormones

Your weight is actively regulated by your brain, particularly an area called the hypothalamus.

In order for your brain to determine when and how much to eat, it processes multiple different types of information.

Some of the most important signals to the brain are hormones that change in response to feeding.

A higher protein intake actually increases levels of the satiety (appetite-reducing) hormones GLP-1, peptide YY and cholecystokinin, while reducing your levels of the hunger hormone ghrelin.
By replacing carbs and fat with protein, you reduce the hunger hormone and boost several satiety hormones.

This leads to a major reduction in hunger and is the main reason protein helps you lose weight. It can make you eat fewer calories automatically.


Digesting and Metabolizing Protein Burns Calories

After you eat, some calories are used for the purpose of digesting and metabolizing the food.
This is often termed the thermic effect of food (TEF).

Although not all sources agree on the exact figures, it is clear that protein has amuch higher thermic effect (20-30%) compared to carbs (5-10%) and fat (0-3%) (11).
If we go with a thermic effect of 30% for protein, this means that 100 calories of protein only end up as 70 usable calories

Protein Makes You Burn More Calories (Increases "Calories Out")

Due to the high thermic effect and several other factors, a high protein intake tends to boost metabolism.

It makes you burn more calories around the clock, including during sleep.

A high protein intake has been shown to boost metabolism and increase the amount of calories burned by about 80 to 100 per day.

This effect is particularly pronounced during overfeeding, or while eating at a caloric surplus. In one study, overfeeding with a high protein diet increased calories burned by 260 per day.

By making you burn more calories, high protein diets have a "metabolic advantage" over diets that are lower in protein


Protein Reduces Appetite and Makes You Eat Fewer Calories

Protein can reduce hunger and appetite via several different mechanisms.
This can lead to an automatic reduction incalorie intake.

In other words, you end up eating fewer calories without having to count calories or consciously control portions.

Numerous studies have shown that when people increase their protein intake, they start eating fewer calories.

This works on a meal-to-meal basis, as well as a sustained day-to-day reduction in calorie intake as long as protein intake is kept high.

In one study, protein at 30% of calories caused people to automatically drop their calorie intake by 441 calories per day, which is a huge amount.

So, high protein diets not only have a metabolic advantage - they also have an "appetite advantage," making it mucheasier to cut calories compared to lower protein diets.


Protein Cuts Cravings and Reduces Desire for Late-Night Snacking

Cravings are the dieter's worst enemy.

They are one of the biggest reasons why people tend to fail on their diets.

Another major problem is late-night snacking. Many people who have a tendency to gain weight get cravings at night, so they snack in the evening. These calories are added on top of all the calories they ate during the day.

Interestingly, protein can have a powerful effect on both cravings and the desire to snack at night.

This graph is from a study comparing a high-protein diet and a normal-protein diet in overweight men:

The high-protein group is the blue bar, while the normal-protein group is the red bar.

In this study, protein at 25% of calories reduced cravings by 60% and cut the desire for late-night snacking by half!

Breakfast may be the most important meal to load up on the protein. In one study in teenage girls, a high-protein breakfast significantly reduced cravings


Protein Makes You Lose Weight, Even Without Conscious Calorie Restriction

Protein works on both sides of the "calories in vs calories out" equation. It reduces calories in and boosts calories out.

For this reason, it is not surprising to see that high-protein diets lead to weight loss, even without intentionally restricting calories, portions, fat or carbs.

In one study of 19 overweight individuals, increasing protein intake to 30% of calories caused a massive drop in calorie intake:

In this study, the participants lost an average of 11 pounds over a period of 12 weeks. Keep in mind that they only addedprotein to their diet, they did not intentionally restrict anything.

Although the results aren't always this dramatic, the majority of studies do show that high-protein diets lead to significant weight loss .

A higher protein intake is also associated with less belly fat, the harmful fat that builds up around the organs and causes disease.

All that being said, losing weight is not the most important factor. It is keeping it off in the long-term that really counts.

Many people can go on "a diet" and lose weight, but most end up gaining the weight back.

Interestingly, a higher protein intake can also help prevent weight regain. In one study, a modest increase in protein intake (from 15 to 18% of calories) reduced weight regain after weight loss by 50%.

So not only can protein help you lose weight, it can also help you keep it off in the long-term


Protein Helps Prevent Muscle Loss and Metabolic Slowdown

Weight loss doesn't always equal fat loss.

When you lose weight, muscle mass tends to be reduced as well.

However, what you really want to lose is body fat, both subcutaneous fat (under the skin) and visceral fat (around organs).

Losing muscle is a side effect of weight loss that most people don't want.

Another side effect of losing weight is that the metabolic rate tends to decrease.

In other words, you end up burning fewer calories than you did before you lost the weight.

This is often referred to as "starvation mode," and can amount to several hundred fewer calories burned each day.

Eating plenty of protein can reduce muscle loss, which should help keep your metabolic rate higher as you lose body fat.

Strength training is another major factor that can reduce muscle loss and metabolic slowdown when losing weight.

For this reason, a high protein intake and heavy strength training are two incredibly important components of an effective fat loss plan.

Not only do they help keep your metabolism high, they also make sure that what is underneath the fat actually looks good. Without protein and strength training, you may end up looking "skinny-fat" instead of fit and lean


 How Much Protein is Optimal?

The DRI (Dietary Reference Intake) for protein is only 46 and 56 grams for the average woman and man, respectively.

This amount may be enough to prevent deficiency, but it is far from optimal if you are trying to lose weight (or gain muscle).

Most of the studies on protein and weight loss expressed protein intake as a percentage of calories.

According to these studies, aiming for protein at 30% of calories seems to be very effective for weight loss.

You can find the number of grams by multiplying your calorie intake by 0.075. For example, on a 2000 calorie diet you would eat 2000 * 0.075 = 150 grams of protein.

You can also aim for a certain number based on your weight. For example, aiming for 0.7-1 gram of protein per pound of lean mass is a common recommendation (1.5 - 2.2 grams per kilogram).

It is best to spread your protein intake throughout the day by eating protein with every meal.

Keep in mind that these numbers don't need to be exact, anything in the range of 25-35% of calories should be effective


How to Get More Protein in Your Diet

Increasing your protein intake is simple. Just eat more of protein-rich foods.

These include:

Meats: Chicken, turkey, lean beef,pork, etc.Fish: Salmon, sardines, haddock, trout, etc.Eggs: All types.Dairy: Milk, cheese, yogurt, etc.Legumes: Kidney beans, chickpeas, lentils, etc.You can find a long list of healthy high-protein foods in this article.

If you're eating low-carb, then you can choose fattier cuts of meat. If you're not on a low-carb diet then try to emphasize lean meats as much as possible. This makes it easier to keep protein high without getting too many calories.

Taking a protein supplement can also be a good idea if you struggle to reach your protein goals. Whey protein powder has been shown to have numerous benefits, including increased weight loss.

Even though eating more protein is simple when you think about it, actually integrating this into your life and nutrition plan can be difficult.

I recommend that you use a calorie/nutrition tracker in the beginning. Weigh and measure everything you eat in order to make sure that you are hitting your protein targets.

You don't need to do this forever, but it is very important in the beginning until you get a good idea of what a high-protein diet looks like.

Does Protein Have any Negative Health Effects?

Protein has been unfairly blamed for a number of health problems.

It has been said that a high protein diet can cause kidney damage and osteoporosis.
However, none of this is supported by science.

Although protein restriction is helpful for people with pre-existing kidney problems, protein has never been shown to cause kidney damage in healthy people (20, 21).
In fact, a higher protein intake has been shown to lower blood pressure and help fight diabetes, which are two of the main risk factors for kidney disease.

If protein really does have some detrimental effect on kidney function (which has never been proven), it is outweighed by the positive effects on these risk factors.
Protein has also been blamed for osteoporosis, which is strange because the studies actually show that protein can help prevent osteoporosis.

Overall, there is no evidence that a reasonably high protein intake has any adverse effects in healthy people trying to stay healthy



Protein Consumption and the Elderly: What Is the Optimal Level of Intake?


Abstract

Maintaining independence, quality of life, and health is crucial for elderly adults. One of the major threats to living independently is the loss of muscle mass, strength, and function that progressively occurs with aging, known as sarcopenia. Several studies have identified protein (especially the essential amino acids) as a key nutrient for muscle health in elderly adults. Elderly adults are less responsive to the anabolic stimulus of low doses of amino acid intake compared to younger individuals. However, this lack of responsiveness in elderly adults can be overcome with higher levels of protein (or essential amino acid) consumption. The requirement for a larger dose of protein to generate responses in elderly adults similar to the responses in younger adults provides the support for a beneficial effect of increased protein in older populations. The purpose of this review is to present the current evidence related to dietary protein intake and muscle health in elderly adults.

Keywords: protein, aging, muscle, requirements, anabolic response, protein synthesis, elderly

1. Introduction

The United States is experiencing considerable growth in its elderly adult population. By 2015, the population aged 65 and over is projected to reach nearly 84 million [1]. Maintaining independence, quality of life, and health is crucial for elderly adults [2]. One of the major threats to living independently is the loss of muscle mass, strength, and function that progressively occurs with aging, known as sarcopenia [2,3]. A loss or reduction in skeletal muscle function often leads to increased morbidity and mortality either directly, or indirectly, via the development of secondary diseases such as cardiovascular disease, diabetes, and obesity [3,4]. The prevalence of obesity among elderly adults has also increased over the last several decades. For example, the prevalence of obesity among men aged 65–74 increased from 31.6% in 1999–2002 to 41.5% in 2007–2010. Between 2007 and 2010, approximately 35% of adults aged 65 and over were obese [5]. One reason for the increase in obesity could be due to body composition shifts that occur as we age, resulting in a higher percentage of body fat and decreases in muscle mass with age [6]. Both sarcopenia and obesity act synergistically, which increases the risk of negative health outcomes and earlier onset of disability [2].

Nutrition plays an essential role in the health and function of elderly adults [7]. Inadequate nutrition can contribute to the development of both sarcopenia and obesity [3,8]. As life expectancy continues to rise, it is important to consider optimal nutritional recommendations that will improve health outcomes, quality of life, and physical independence in elderly adults [5]. Several studies have identified protein as a key nutrient for elderly adults (reviewed in [3,8]). Protein intake greater than the recommended amounts may improve muscle health, prevent sarcopenia [9], and help maintain energy balance, weight management [10], and cardiovascular function [11,12,13]. Benefits of increased protein intake include improved muscle function and the prevention onset of chronic diseases, which can increase quality of life in healthy elderly adults [3]. Therefore, the purpose of this review is to present the current evidence related to dietary protein intake and muscle health in elderly adults.

2. Optimal Protein Intake for Elderly Adults

2.1. Dietary Protein Recommendations

Traditionally, protein recommendations have been based on studies that estimate the minimum protein intake necessary to maintain nitrogen balance [3,8]. However, the problem with relying on these results is that they do not measure any physiological endpoints relevant to healthy aging, such as muscle function. The current dietary recommendations for protein intake include the dietary reference intakes (DRI) for macronutrients, which include an estimated average requirement (EAR), a recommended dietary allowance (RDA) and an acceptable macronutrient distribution range (AMDR) [14]. In the case of daily protein intake, the EAR for dietary protein is 0.66 g/kg/day and the Food and Nutrition Board recommends an RDA of 0.8 g/kg/day for all adults over 18 years of age, including elderly adults over the age of 65. The RDA for protein was based on all available studies that estimate the minimum protein intake necessary to avoid a progressive loss of lean body mass as determined by nitrogen balance [3,8]. The Food and Nutrition Board recognizes a distinction between the RDA and the level of protein intake needed for optimal health. Therefore, the recommendation for the ADMR includes a range of optimal protein intakes in the context of a complete diet (10%–35% of daily energy intake come from protein [14]), which makes the ADMR more relevant to normal dietary intake than the RDA [3].

2.2. Protein Requirements for Elderly Adults

Experts in the field of protein and aging recommend a protein intake between 1.2 and 2.0 g/kg/day or higher for elderly adults [3,8,15]. The RDA of 0.8 g/kg/day is well below these recommendations and reflects a value at the lowest end of the AMDR. It is estimated that 38% of adult men and 41% of adult women have dietary protein intakes below the RDA [16,17].

Most published results, based on data from either epidemiological or short-term studies, indicate a potential beneficial effect of increasing protein intake in elderly adults. These data demonstrate that elderly adults, compared with younger adults, are less responsive to low doses of amino acid intake [18]. However, this lack of responsiveness in healthy older adults can usually be overcome with higher levels of essential amino acid (EAA) consumption [18]. This is also reflected in studies comparing varying levels of protein consumption [19], suggesting that the lack of muscle responsiveness to lower doses of protein intake in elderly adults can be overcome with a higher level of protein intake. The requirement for a larger dose of protein to generate responses in elderly adults similar to the responses in younger adults provides the support for a beneficial effect of increased protein in older populations [8].

The mechanism by which dietary protein affects muscle is through the stimulation of muscle protein synthesis and/or suppression of protein breakdown by the absorbed amino acids consumed in the diet [20,21]. There appears to be an EAA threshold when it comes to stimulating muscle protein synthesis. Ingestion of relatively small amounts of EAA (2.5, 5 or 10 g) appears to increase myofibrillar protein synthesis in a dose-dependent manner [22]. However, a larger dose of EAA (20–40 g) fail to elicit an additional effect on protein synthesis in young and older subjects. Similar results were observed after the ingestion of either 113 or 340 g of lean beef containing 10 or 30 g EAA, respectively [23]. Despite a threefold increase in EAA content, there was no further increase in protein synthesis in either young or older subjects following consumption of 340 g versus 113 g of protein. There are fewer data regarding the response of protein breakdown to different levels of protein or amino acid intake. The balance between protein synthesis and breakdown is discussed in more detail below.

2.3. Essential Amino Acid Requirements for Aging Adults

Essential amino acids, especially the branched-chain amino acid leucine, are potent stimulators of muscle protein synthesis. Studies have focused on the stimulation of muscle protein synthesis via the protein kinase mTORC1 (mechanistic target of rapamycin complex 1) [24,25,26], but the in vivo significance of this mechanism as a regulator of the rate of protein synthesis in human subjects is not yet proven. Several studies demonstrate that maximal stimulation of muscle protein synthesis is possible with 15 g of EAA (reviewed in [20]). This translates to ~35 g of high quality protein per meal delivering ~15 g of EAA. A larger amount of lower quality protein, which contains a lower content of EAA, would be required to achieve the same functional benefits. The addition of nonessential amino acids to a supplement containing EAA does not result in additional stimulation of muscle protein synthesis [27], indicating that the quality of the protein, or its amino acid profile, is a key determinant of the functional potential of protein in muscle health. This is supported by several studies demonstrating that the ingestion of milk proteins, compared with the ingestion of soy protein stimulates muscle protein synthesis to a greater extent after resistance exercise, owing to the higher content of EAA in milk protein [28,29,30,31]. The data from the Health, Aging and Body Composition study support these findings [31], showing that intake of animal protein (with greater content of EAA), but not plant protein, was significantly associated with the preservation of lean body mass over three years in older adults [31]. In that study, individuals in the highest quintile of protein intake had 40% less loss in lean body mass than those in the lowest quintile of protein intake [31].

2.4. The Importance of Protein Quality

When considering protein intake, it is also important to consider total energy intake. Age is associated with a progressive decline in basal metabolic rate (BMR) at a rate of 1%–2% per decade after 20 years of age [32,33,34]. This reduction in BMR is closely associated with the loss in fat-free mass, including muscle, and the gain of less metabolically active fat [35] that occurs as we age [33]. In fact, studies suggest that BMR adjusted for the change in fat-free mass is 5% lower in elderly adults compared to younger adults [35]. This implies that aging adults require a lower daily energy intake. However, the extent to which BMR may increase or decrease with age depends on the balance between weight gain with age, tending to increase BMR, and aging, which decreases BMR [35].

Although older adults typically eat less than younger adults, including less protein [15,16], it is important for aging adults to consider total caloric intake when choosing a protein source to incorporate in the diet. The discrepancies in quality between animal and plant protein sources go beyond the amino acid profiles. When the energy content of the protein source is accounted for, the caloric intake needed to meet the EAA requirements from plant sources of protein is considerably higher than the caloric intake from animal sources of protein [36]. This is important to consider since obesity, especially with aging, is a major public health concern. Obesity is the most predominant factor limiting mobility in the elderly [37].

2.5. Dietary Protein and Muscle Anabolic Response in Elderly Adults

There is abundant evidence that muscle plays a central role in the prevention of many chronic diseases, including diabetes and obesity [38]. In addition, evidence that optimal health for elderly adults is dependent on maintaining muscle mass is emerging [3,8]. EAAs are the primary nutrients responsible for the maintenance of muscle mass and function, but elderly individuals have reduced anabolic sensitivity to amino acids (termed anabolic resistance). An increasing amount of evidence suggests that a minimum threshold of EAA needs to be reached to elicit an anabolic muscle response, and older individuals require a higher concentration of amino acids compared to younger individuals.

Optimal protein intake per meal can be defined as the minimal dose of protein intake that results in the maximal anabolic response and thus can help maintain or improve muscle mass (reflected as lean body mass) and function over time. It has been reported that the optimal dose of dietary protein consumption in a meal that results in a near maximal anabolic response is ~35 g/meal [23] or 0.40 g/kg/meal of high-quality protein in elderly adults [19], translatable to 1.2 g/kg/day or 96 g/day for an 80 kg elderly adults. The optimum amount for elderly adults (0.24 g/kg/meal) is approximately 70% greater than that for young adults (0.8 g/kg/day) [19], indicating an age-associated anabolic resistance to dietary protein. It is likely that elderly individuals need more protein intake to achieve a maximal anabolic response per meal considering the varying degrees of quality of protein eaten in the real world. In a typical American diet, the consumption of the majority of total daily protein intake skews toward dinner (~50% of total amount; ~40–60 g protein) [16,17,39] that clearly exceeds the “optimal” protein dose (i.e., ~35 g protein/meal) without extra stimulation of anabolic response. This led to an interesting hypothesis that spreading daily protein intake evenly throughout the day can result in a greater cumulative anabolic response than the skewed pattern of protein intake [40]. If this is the case, elderly adults can gain benefits regarding improvement in muscle mass and strength, and related functions, simply by adopting even distribution pattern of equal amounts of protein intake [40]. However, the rationale behind this hypothesis is largely incorrect, as the hypothesis was solely based on data on muscle protein synthesis (MPS), which is only one half of the equation determining net anabolic response (i.e., net anabolic response = protein synthesis minus protein breakdown).

The significance of simultaneous measurement of both protein synthesis and breakdown is dependent on a number of catabolic conditions (i.e., loss of muscle mass over time) such as type I diabetes, cancer cachexia, and burn injury, in which the rate of protein synthesis is typically not blunted but actually normal or often increased [41], due largely to the increased availability of amino acids secondary to an accelerated rate of protein breakdown. This issue is important when quantifying the net anabolic response to dietary protein intake. Furthermore, although net anabolic response at the muscle level is the most relevant physiological response, the whole body is potentially involved in the anabolic response to protein ingestion, as approximately half of the total body protein turnover occurs at non-muscle tissues, particularly gut tissue [42]. Thus, determination at the muscle level could underestimate total anabolic response. For example, a large portion of the amino acids absorbed from a meal is retained in gut proteins that turn over rapidly [42,43], particularly following a mixed meal, due largely to a systemic insulin response [42]. Those amino acids can, in turn, be released into the blood over time as a result of a protein breakdown and be used for incorporation into new proteins in muscle. This is of particular importance in situations where older adults consume a protein intake greater than the amount that stimulates a maximal MPS.

Consistent with this notion, our recent findings showed that similar MPS responses were achieved by two doses of protein intake (40 g vs.70 g), while a greater net protein synthesis at whole-body level was achieved with a meal containing 70 g of protein due to the suppression of breakdown amplifying the anabolic effect of the stimulation of synthesis [44]. Furthermore, we have directly tested the “distribution” hypothesis at two protein levels (0.8 g or 1.5 g protein/kg/day) in mixed meals and found no beneficial effects of an even distribution pattern of protein intake on net anabolic response at whole-body level and MPS [45]. Instead, we found the higher protein intake (i.e., 1.5 g/kg/day) resulted in a greater anabolic response at whole-body level and MPS. Strikingly, the positive anabolic response achieved with both levels of protein intake was largely due to reductions in protein breakdown, indicating the importance of simultaneous determination of both protein synthesis and breakdown, as protein synthesis actually declined with 0.8 g protein/kg/day, regardless of the distribution patterns. Furthermore, the same study [45] showed that whole body anabolic response increased linearly with increasing amount of protein intake (dose range: ~6.4–91.7 g), without evidence of plateau in older adults [45]. These results extended previous findings shown by the Deutz group [46,47], indicating that the amount of total protein, but not the pattern of protein intake, is of importance with respect to maximizing anabolic response. Importantly, the linear relationship between the amount of protein intake and anabolic response has been recognized for more than half a century, as determined by a nitrogen balance technique, although the anabolic response beyond RDA for protein (i.e., 0.8 g protein/kg/day) has been ignored [48]. Therefore, data indicate that there is no practical limit to the anabolic response in increasing amount of dietary protein intake.

Taken together, the data do not support the notion that a maximal anabolic response is stimulated with ~35 g of high quality protein per meal [23] or 0.4 g/kg/meal (1.2 g/kg/day) for older adults [19]. The “even distribution hypothesis” was based on this limit of anabolic response [40], but that hypothesis ignored many important factors in determining true net anabolic response. These factors include the quality of protein consumed, the contribution of protein breakdown to the net anabolic response, and the potential involvement of whole body response, all of which result in the considerable underestimation of the maximal anabolic response. It is therefore unreasonable to base recommendations for the optimal level of protein intake in elderly adults on the idea that the maximal effective dose of protein is ~35 g per meal. If the goal of the optimal level of protein intake is considered to be the amount needed to maximally stimulate protein anabolism (i.e., synthesis minus breakdown), then consumption of dietary protein in accord with the higher end of the AMDR (35% of total calories) is reasonable. Unfortunately, long-term studies assessing the effect of this level of dietary protein consumption on functional outcomes in elderly adults have not been performed.

2.6. Dietary Protein and Anabolic Signaling in Muscle of Elderly Adults

Signaling through mTORC1 is involved in the regulation of several anabolic processes in the body including protein synthesis [26,49,50]. In skeletal muscle, amino acids signal through mTORC1 to initiate the process of protein synthesis [25,51,52,53]. The translation initiation factors 4E-BP1 (eukaryotic initiation factor 4E binding protein 1) and p70S6K (ribosomal protein S6 kinase) are downstream targets of mTORC1 [51,52,53]. Signals provided by EAA, especially leucine, are required for full activation of this pathway [25,51,54]. Muscle becomes resistant to the normal stimulatory effects of postprandial leucine concentrations with increasing age [18], which may result in the reduced stimulation of the mTORC1 pathway and reduced activation of translation initiation and subsequent MPS. This could be due to a reduced sensitivity to leucine with age, to less efficient absorption of leucine from the gut, or to the fact that the dietary protein intake tends to decrease with age [8,55,56].

Age-related muscle loss may involve a decreased response to EAA due to decreased phosphorylation of mTORC1 and p70S6K [22]. In response to 10 g of EAA, mTORC1 phosphorylation, or activation, while significantly increased in skeletal muscle of elderly adults, is still significantly lower in younger adults [22]. Guilletet al. [57] found that p70S6K phosphorylation is not stimulated in older adults after infusion with leucine. These findings are supported by Fry et al. [58] who found that elderly adults, compared with young adults, have significantly reduced phosphorylation of mTORC1 and translation initiation factors after a bout of resistance exercise. Gene expression of proteins associated with muscle protein synthesis and satellite cell function also differ between young and elderly adults in response to exercise and supplementation with EAA [59]. While no difference was found between young and elderly in the fasted state, there was a significant decrease in protein (REDD1, TSC1, TSC2, and IGF1 receptor) expression six hours post-exercise and EAA intervention in elderly adultsversus young adults [59]. In addition, after only seven days of bed rest, elderly adults had a reduced response to EAA ingestion resulting in no increase in MPS, activation of translation initiation factors (4E-BP1 and p70S6K), and no increase in amino acid transporters [60]. Elderly adults also had decreased LAT1 (L-type amino acid transporter) and SNAT2 (sodium-coupled neutral amino acid transporter 2) following seven days of bed rest [60]. These findings are further supported in a study by Tanner et al. [61], who found that, after five days of bed rest, elderly adults (but not younger adults) had reduced amino acid-induced anabolic sensitivity, resulting in decreased muscle protein synthesis. In this study, elderly adults had increased MURF1 gene expression at baseline and increased AMPKα phosphorylation after bed rest, which is suggestive of increased muscle protein breakdown [61]. These data are important because they demonstrate how quickly an injury or hospital stay could decrease skeletal muscle function. While all of these data suggest a potential role of changes in sensitivity of mTORC1 and related factors in the anabolic response as well as anabolic resistance in elderly adults, it must also be acknowledged that the nature of the data is correlational and thus does not definitively prove a cause–effect relationship. To this end, it has recently been shown that consumption of a very small dose of EAA (3 g) can stimulate muscle protein anabolism equivalently to 20 g of whey protein in the absence of any time-coincident changes in initiation factor activity [62].

3. Conclusions

Elderly adults are less responsive to the anabolic stimulus of low doses of amino acid intake compared to younger adults . However, this lack of responsiveness in elderly adults can be overcome with higher levels of protein consumption . This is also reflected in studies comparing varying levels of protein intake . This suggests that the lack of muscle responsiveness to lower doses of protein in older adults can be overcome with a higher level of protein intake. The requirement for a larger dose of protein to generate responses in elderly adults similar to the responses in younger adults provides the support for a beneficial effect of increased protein in elderly populations.The consumption of dietary protein consistent with the upper end of the AMDRs (as much as 30%–35% of total caloric intake) may prove to be beneficial, although practical limitations may make this level of dietary protein intake difficult. The consumption of high-quality proteins that are easily digestible and contain a high proportion of EAAs lessens the urgency of consuming diets with an extremely high protein content.