Protein

Are you worried about protein deficiency?

Protein is an important nutrient and a popular topic of controversy. The amount of protein needed for human health is an area of continuous debate. In critical hospital settings, insufficient calories and protein in the diet can lead to a deadly state of muscle wasting and starvation known as protein-energy malnutrition (PEM). This can manifest as kwashiorkor, which involves a decrease in muscle mass, an impaired immune system, and swelling in the abdominal area. This condition is seen in the quintessential infomercial about poverty-stricken African children. PEM can also result in marasmus, which is characterized by a severely emaciated appearance. A combination of these two conditions is often seen in starving and malnourished patients.1

Conversely, in a low-calorie diet, high protein intake can exceed the liver’s capacity to convert nitrogen to urea, which may lead to nausea, diarrhea, acidosis, and even death (especially in the absence of sufficient fat or carbohydrate). This phenomenon is also known as the rabbit starvation syndrome. In addition, multiple case studies demonstrate a correlation between increased protein intake and renal dysfunction.2 Healthy people may have renal clearance at high protein intake. On the other hand, once calorie needs are met, which is usually the case in the American diet, excess protein ends up as fat stored in the body.

Protein is made of a sequence of amino acids joined together by peptide bonds. Hormones, enzymes, transporters, organ muscle fibers and skeletal muscle tissue are all made of different proteins. Human tissue is made of varying combinations of 20 amino acids, which are created, broken down, and recycled in the body on a regular basis.3 For example, mammalian muscle tissue contains high proportions of glutamine, glutamic acid, alanine, glycine and lysine among other amino acids.4 The concentration of recycled protein in the body is several fold greater than the amount consumed in the daily diet. About one third of the amino acids come from muscle proteins, whereas a large proportion are derived from internal organs and other tissue. An estimated 340g of amino acids are used as a result from the turnover process in the body, while only 90g may come from the diet each day.5

What is protein made of?

Adults require eight amino acids, which cannot be synthesized in the body so they must be consumed in the diet. These eight essential amino acids include isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The other 12 are nonessential because they are made from other compounds in the body. Nonessential amino acids include histidine, cystine, glycine, glutamic acid, alanine, serine, asparginine, tyrosine, aspartic acid, proline and arginine. Histidine is essential for infants, while children have a limited ability to synthesize arginine.6 Researchers still use histidine in studies that evaluate essential amino acids. The proportion of essential amino acids found in mammalian muscle tissue is highest in leucine, lysine, and valine but lowest in tryptophan, methionine, and tyrosine concentrations.7

Common protein myths

Ironically, much of the current dietary recommendations for protein and specific amino acids are based on vastly outdated research. In 1938, Sumner etal examined the biological value of milk and egg protein in human subjects.8 Egg protein was determined to have high biological value in a 1947 animal experiment.9 This data is part of the basis for modern claims about animal protein being superior to plant protein.

A recent article in Shape magazine states “Animal proteins are always complete proteins, but plant proteins like those found in beans and in many grains are not.” The author, Mike Roussell MD, also mentions, “Unlike complete proteins, an incomplete protein does not contain all nine essential amino acids.” In this case, the article explains several benefits of including plant-based proteins in the diet while simultaneously emphasizing the need for some animal-based proteins in the diet. Roussell first describes plant proteins as incomplete and later refers to tofu as a complete protein.10 His explanation is an improvement from the old myths of protein quality only being associated with animal products. However, the assumption that humans need both types of protein, animal and plant, is erroneous.

Another Internet article offers dietary advice for maximizing protein synthesis after exercise sessions, “Take in about 20 grams of protein for an optimal response. You can easily accomplish this with a scoop of whey powder added to your post-exercise sports drink or smoothie.”11 The data is mixed concerning protein intake before or after exercise. One study found that six grams of a mixed essential amino acid solution given before a workout translates into increased muscle protein synthesis more than the same solution given immediately after or one hour after the workout.12

Bolster etal discovered an increase in muscle protein fractional synthetic rate (FSR) for low and moderate protein diets versus a high protein diet post endurance exercise. FSR involves the creation of muscle proteins while fractional breakdown rate (FBR) involves the degradation of muscle proteins. Resistance exercise routines leave the body in a negative protein balance because FBR is greater than FSR. Researchers, in this case, expected that a high protein diet would raise the amount of muscle made and stored in the body. The low protein diet was the current RDA of 0.8 grams of protein per kilogram of body weight, the moderate protein diet was 1.8 grams per kilograms, and the high protein diet was 3.6 grams per kilogram. 13

Borsheim etal used a six gram essential amino acid solution on subjects one hour after a resistance workout and saw an increase in net muscle protein balance. In fact, muscle protein synthesis increases 3.5 fold when the six grams of essential amino acid supplement is given regardless of carbohydrate content. 14

What is a complete protein?

Quality or biologic value of protein is determined by evaluating the essential amino acid content of a food item. A complete protein is one with adequate amounts of all eight essential amino acids while an incomplete protein is low in one or more essential amino acid. If the definition stated that an incomplete protein must totally lack one or more essential amino acid, then most plant proteins could be considered complete proteins. The majority of plant foods contain all eight essential amino acids (For examples, see table 1.0). A given plant protein may be low in one or more amino acids when compared to animal foods but this does not mean it is nutritionally inadequate. Nutrition experts realize that a variety of foods make up a healthy diet and no one food should be depended upon as the sole protein source in an individual diet. In the grand scheme of human nutrition, it is inconsequential for one specific food to contain a perfect ratio of all eight amino acids. Perhaps, for this reason, a specific clinical amino acid deficiency, ie ‘lysine deficiency’ or ‘methionine deficiency’, is rare at best.

Historically, high protein quality was associated with animal products and plant protein was considered inferior in quality. Eggs are used as the gold standard of protein food considering their even spread of essential amino acids. Modern texts, however, are beginning to acknowledge that essential amino acids are equal in quality regardless of food source and many plant foods contain all of the essential amino acids in nutritionally adequate quantities. This allows plant protein foods to be equal and in some cases superior nutritional substitutes for animal protein foods.

Do plant foods have complete proteins?

Soybeans and other legumes are particularly concentrated in protein compared with other plant foods. Nevertheless, most plant foods, such as nuts, seeds, grains, and vegetables, contain all of the essential amino acids. Animal products, such as eggs, beef, chicken, and milk, are more concentrated sources of protein. They are also typically high in cholesterol, saturated fat, and calories when compared with plant foods. Table 1.0 compares 20 grams of protein for each food and shows that animal and some plant proteins are similar in essential amino acid composition. For this group of foods, chia seeds are relatively high in methionine, while eggs are slightly high in threonine.

Table 1.0

Essential Amino Acids (g) in 20g of Protein

 

Food

Serv

Cal

Phen

Val

Tryp

Meth

His

Iso

Leu

Lys

Thr

Egg

2/3c

220

1.1

1.4

.27

.62

.5

1

1.7

1.4

.9

Chicken

1/2c

133

.8

1

.23

.56

.62

1

1.5

1.7

.85

Milk 2%

2.5c

305

1

1.3

.25

.53

.61

1

1.9

1.7

.88

Chia

4.4oz

413

1.2

1.2

.54

.73

.66

.99

1.7

1.2

.88

Soy

1c

220

.9

.6

.23

.26

.49

.56

1.3

1.4

.62

Pea

2.4c

323

.76

.9

.14

.31

.4

.74

1.2

1.2

.77

Lentil

1c

253

1

1

.18

.17

.56

.86

1.4

1.4

.71

Cornmeal

2c

883

1

1

.14

.41

.6

.71

2.4

.56

.74

Tofu

280g

154

1.2

1.1

.34

.26

.51

1

1.8

1.4

.87

Quinoa

2.5c

555

.85

.85

.24

.44

.59

.73

1.2

1.1

.6

 

Table created by Jennifer Swallow, RD, LD. Data source is USDA National Agricultural Research Service, National Agricultural Library: Nutrient Data Laboratory. [Accessed Jan 30 2013] http://ndb.nal.usda.gov/ndb/search/list

References:

  1. Bern C, Zucker JR, Perkins BA, Otieno J, Otoo AJ, Yip R. Assessment of potential indicators for protein-energy malnutrition in the algorithm for integrated management of childhood illness. Bull World Health Organ 1997; 75s 1: 87-96.
  2. Bilsborough S, Mann N. A review of issues of dietary protein intake in humans. Int J Sport Nutr Exerc Metab 2006; 16: 129-152.
  3. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ. Modern nutrition in health and disease. 10th ed. Baltimore: Lippincott Williams & Wilkins; 2006; 23-35.
  4. Bergstom J, Furst P, Noree LO, etal. J Appl Physiol 1974; 36: 693-7.
  5. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ. Modern Nutrition. Lippincott Williams & Wilkins 2006; 23-35.
  6. Block RJ, Weiss KW. Amino Acid Handbook: Methods and Results of Analysis. Springfield, IL:Charles C Thomas, 1956.
  7. Sumner EE, Muelin JR. The biological value of milk and egg proteins in human subjects. J Nutr, 1938;16: 141.
  8. Bricker ML, Mitchell HH. The protein requirements of the adult rat in terms of the protein contained in egg, milk and soy flour. J Nutr 1947; 34: 491.
  9. McArdle WD, Katch FI, Katch VL. Sports and exercise nutrition. 3rd ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins; 2009.
  10. Roussell M. Shape Online [Accessed Jan 30 2013] http://www.shape.com/healthy-eating/diet-tips/ask-diet-doctor-whats-best-protein-source
  11. Schulmann D. Get Enough Protein Post-Workout. [Accessed Jan 30 2013] http://www.active.com/nutrition/Articles/Get_enough_protein_after_your_next_workout
  12. Wolfe RR. Skeletal muscle protein metabolism and resistance exercise. J Nutr 2006; 525s-528.
  13. Bolster DR, Pikosky MA, Gaine C, Martin W, Wolfe RR, Tipton KD, Maclean D, Maresh CM, Rodriguez NR. Dietary protein intake impacts human skeletal muscle protein fractional rates after endurance exercise. J Physiol Endocrinol Metab 2005; 289: e678-683.
  14. Borsheim E, Tipton K, Wolf SE, Wolfe RR. Essential amino acids and muscle protein recovery from resistance exercise. Am J Physiol Endocrinol Metab 2002;283: e648-657.
  1. Bilsborough S, Mann N. A review of issues of dietary protein intake in humans. Int J Sport Nutr Exerc Metab 2006; 16: 129-152.
  2. Van Loon LJC, Kies AK, Saris WHM. Protein and protein hydrolysates in sports nutrition. Int J Sport Nut Exerc Metab 2007; 17: s1-4
  3. Katsanos CS, Kobayashi H, Sheffield-Moore M, Aarsland A, Wolfe RR. A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly. Am J Physiol Endocrinol Metab 2006; 291: e381-387.
  4. Drummond MJ, Dreyer HC, Pennings B, Fry CS, Dhanani S, Dillon EL, Sheffield-Moore M, Volpi E, Rasmussen BB. Skeletal muscle protein anabolic response to resistance exercise and essential amino acids is delayed with aging. J Appl Physiol 2008; 104: 1452-1461.