Dietary protein is arguably the most important macronutrient for human health. Even its name acknowledges its importance as the word protein comes from the Greek proteios, meaning primary or first.1 Proteins are long chains of amino acids connected by peptide bonds. Amino acids from dietary protein provide the body with nitrogen, hydrocarbons, sulfur, and peptides and are used for metabolic, physiological, and structural purposes including muscle growth and maintenance, pH and fluid balance, transport nutrients, and the production of enzymes, hormones, neurotransmitters, antioxidants, immune system mediators, connective tissue, non-essential and conditionally essential amino acids, and more.1 In a starvation state, proteins from skeletal muscles can also be catabolized to produce ATP, the body’s primary energy currency.

There are 20 amino acids that are important for human health, 9 of which are considered essential and can only be obtained through diet and 6 of which are conditionally essential, meaning that the human body can synthesize them from other amino acids, but often not in enough quantities to meet the body’s requirements. Inadequate intake of essential amino acids results in negative nitrogen balance, loss of appetite, extreme fatigue, and nervous irritability.1 Under-consumption of dietary protein results in micronutrient deficiencies, cardiovascular dysfunction, edema, impaired immune function, impaired metabolism, muscle weakness, and, in children, stunted growth and cognitive impairment.1 Inadequate dietary protein has been connected to insulin resistance, dyslipidemia, hypothyroidism, increased oxidative stress, infertility, and more.

In the US, the current Recommended Dietary Allowance (RDA) of protein for a healthy adult is 0.8 grams of protein per kilogram of body weight (kg/bw) daily to meet nitrogen balance requirements. This doesn’t take into account protein requirements to maintain optimal skeletal-muscle or health issues that increase protein requirements. To meet these protein requirements, dietary protein intake should be between 1.1-1.6 grams of protein per kg/bw.1,2,3 Aging, acute and chronic illnesses, and certain medications increase the body’s protein requirements.2 As we age the stomach produces less hydrochloric acid to break down proteins into amino acids so more protein intake is required to maintain muscle mass and normal cellular functions; this same principle applies to those taking acid blocking medications and diseases that affect digestion.2,3 Aging and chronic diseases can also cause anabolic resistance to dietary protein that results in impaired muscle growth and maintenance of dietary protein.2 Anabolic resistance can be caused by reduced digestive capacity of dietary proteins, increased excretion of dietary proteins, decreased availability of dietary amino acids, lower perfusion of muscle, decreased muscle uptake of amino acids, and reduced protein synthesis.2 Inflammatory conditions such as cancer, cardiovascular disease, COPD, and diabetes increase metabolism and inactivity from illness or lifestyle also result in increased protein requirements, especially in older adults.2

There is a lot of concern in some circles about the negative effects of high protein diets, especially when it comes to kidney function, bone health, cancer, and metabolic diseases, and those arguments are often used against low carbohydrate and Paleo style diets. These concerns are primarily a myth. Clinical trials show that healthy adults can tolerate dietary protein as high as 3.0 grams per kg/bw daily for the short term, and 1.5-2.0 grams per kg/bw for long term without damage to the kidneys.1,4,5 Only amounts of dietary protein greater than 2.0 grams per kg/bw increases nitrogen load on the gastrointestinal tract, liver, and kidneys.1 A tiny subsection of men, but not women, with primary glomerular disease see slower kidney function decline with moderate protein diets (0.6-0.8 grams per kg/bw) and another clinical trial noted that protein consumption within that same range failed to slow kidney function decline; despite this, low protein diets are recommended for everyone with chronic kidney disease.6,7

An alkaline diet is promoted as a way to reduce risk of osteoporosis based on the acid-ash hypothesis. This hypothesis is based primarily on the false premise that “acidic” foods like protein, especially red meat, cause mineral losses from bone and tissues. This premise appears to be based on the bone demineralization experienced by those with chronic kidney disease extrapolated to apply to everyone regardless of kidney function.8.9 Research has shown that while lower urinary pH results in progressively greater calcium excretion, there’s no evidence that a lower, more acidic, diet contributes to bone loss.10 Nor is there evidence that the increased urinary calcium originated from bone and tissues; it’s more likely that calcium absorption changes with the foods ingested.10 Calcium absorption is greatly dependent upon dietary constituents with vitamin D, lactose, other sugars, sugar alcohols, and protein increasing absorption of calcium and fiber, phytic acid, oxalic acid, unabsorbed fatty acids, and excessive elemental cations decreasing absorption of calcium.11 Calcium levels are also tightly rate-limited both inside and outside cells, with parathyroid hormone, calcitriol, and calcitonin all playing roles in calcium homeostasis, which is why optimal levels of vitamin D are essential to bone health.11 Another reason given for avoiding “acidic” foods according to the acid-ash hypothesis is that the higher phosphate levels in meat contributes to bone demineralization and osteoporosis.12 However, increased phosphate consumption from “acidic” whole foods like meat, dairy, and grains decreases urinary calcium excretion and increases calcium balance within the body, reducing risk for bone loss and osteoporosis.12 Based on the research, higher protein diets improve bone health rather than increases bone loss as suggested by proponents of alkaline and other low protein diets.

Another concern about dietary protein is the association of high levels of advanced glycation end products (AGEs) within the body and diseases such as cancer, cardiovascular disease, and diabetes. AGEs are formed from a reaction between sugars and proteins, either in foods or within the body.13 During high heat cooking the Maillard reaction results in browned or caramelized foods like seared steaks, crispy fried potatoes, toasted bread, and roasted coffee beans that gives foods additional flavor, aroma, and color. There’s a growing number of doctors and dietitians who think these dietary AGEs (dAGEs) are a major source of high blood and tissue levels of AGEs and people should reduce their dietary intake by using shorter cook times, lower temperatures, and adding acidic ingredients such as lemon juice or vinegar when cooking proteins.13 However, the research on dietary AGEs is not convincing. Discussion and concern about AGEs should focus on endogenous production of them via the reaction of high glucose levels in blood to amino acids and not exogenous sources from food.14 High glucose concentrations is a significant factor in endogenous production and accumulation of AGEs.14 This in vivo production and accumulation of AGEs due to high blood glucose may be one reason why insulin resistance is strongly associated with cardiovascular disease, Alzheimer’s disease, and other chronic degenerative diseases the same as high blood levels of AGEs. In which case, a diet that significantly reduces blood glucose levels and normalizes insulin sensitivity is more important for the prevention and treatment of chronic lifestyle diseases than reducing dAGEs by changing cooking methods.

When choosing proteins for your diet, the quality and quantity of the protein determines its value.1 There are four primary ways that proteins are assessed: protein efficiency ratio (PER), biological value, net protein utilization, and protein digestibility corrected amino acid score (PDCAAS).15 The PER measures the effectiveness of protein through animal growth and is a poor measurement for human usage of protein.15 Biological value calculates the amount of nitrogen utilized from food with eggs used as the standard at 100.15 Net protein utilization measures retained nitrogen from protein and is similar to biological value.15 PDCAAS is determined by the Food & Agriculture Organization and World Health Organization (FAO/WHO) based on essential amino acid requirements for preschool aged children adjusted for fecal digestibility.15 A comparison of these four assessments on a limited number of common proteins are in the chart below.15 In general animal products such as meat, fish, poultry, eggs, and dairy contain all 9 essential amino acids and have higher digestibility than plant proteins. Many health agencies hesitate to recommend animal proteins based on the percentage of saturated fats in them, though as discussed last week, this concern is overblown. Clinical trials show that plant protein sources often have inadequate amounts of the amino acids methionine, lysine, and/or leucine and have lower digestibility which results in lower muscle protein synthesis and lower anabolic properties.16 This makes plant-based diets a poor choice for nutritional interventions to support skeletal-muscle mass gain, growth, or maintenance in vulnerable populations such as children, older adults, and those with chronic diseases.

Protein Type

Protein Efficiency Ratio

Biological Value

Net Protein Utilization

PDCAAS

Beef

2.9

80

73

0.92

Casein

2.5

77

76

1.00

Egg

3.9

100

94

1.00

Milk

2.5

91

82

1.00

Soy protein

2.2

74

61

1.00

Wheat gluten

0.8

64

67

0.25

Whey protein

3.2

104

92

1.00

1. Wu, G. Dietary protein intake and human health. Food Funct. 2016; 7: 1251-1265. doi: 10.1039/C5FO01530H.

2. Deutz NE, Bauer JM, Barazzoni R, Biolo G, Boirie Y, Bosy-Westphal A, et al. Protein intake and exercise for optimal muscle function with aging: recommendations from the ESPEN Expert Group. Clin Nutr. 2014; 33(6): 929-936. doi: 10.1016/j.clnu.2014.04.007.

3. Courtney-Martin G, Ball RO, Pencharz PB, Elango R. Protein Requirements during Aging. Nutrients. 2016; 8(8): 492. doi: 10.3390/nu8080492.

4. Devries MC, Sithamparapillai A, Brimble KS, Banfield L, Morton RW, Phillips SM. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis. The Journal of Nutrition. 2018; 148(11): 1760-1775. doi: 10.1093/jn/nxy197.

5. Martin WF, Armstrong LE, Rodriguez NR. Dietary protein intake and renal function. Nutr Metab (Lond). 2005; 2: 25. doi:10.1186/1743-7075-2-25.

6. Rosman JB, Langer K, Brandl M, Piers-Becht TP, van der Hem GK, ter Wee PM, et al. Protein-restricted diets in chronic renal failure: a four year follow-up shows limited indications. Kidney Int Suppl. 1989; 27: S96-102.

7. Williams PS, Stevens ME, Fass G, Irons L, Bone JM. Failure of dietary protein and phosphate restriction to retard the rate of progression of chronic renal failure: a prospective, randomized, controlled trial. Q J Med. 1991; 81(294): 837-855.

8. Bonjour JP. Nutritional disturbance in acid–base balance and osteoporosis: a hypothesis that disregards the essential homeostatic role of the kidney. Br J Nutr. 2013; 110(7): 1168-1177. doi: 10.1017/S0007114513000962.

9. Zheng CM, Zheng JQ, Wu CC, Lu CL, et al. Bone loss in chronic kidney disease: Quantity or quality? Bone. 2016; 87: 57-70. doi: 10.1016/j.bone.2016.03.017.

10. Fenton TR, Eliasziw M, Lyon AW, Tough SC, Hanley DA. Meta-analysis of the quantity of calcium excretion associated with the net acid excretion of the modern diet under the acid-ash diet hypothesis. Am J Clin Nutr. 2008; 88(4): 1159-66.

11. Gropper, SS., Smith, JL. Advanced Nutrition and Human Metabolism. Sixth Edition. Belmont, CA: Wadsworth, Cengage Learning; 2013: 427-431.

12. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Phosphate decreases urine calcium and increases calcium balance: A meta-analysis of the osteoporosis acid-ash diet hypothesis. Nutrition Journal. 2009; 8(1): 41. doi: 10.1186/1475-2891-8-41.

13. Uribarri J, Woodruff S, Goodman S, Cai W, Chen X, Pyzik R, et al. Advanced Glycation End Products in Foods and a Practical Guide to Their Reduction in the Diet. J Am Diet Assoc. 2010; 110(6): 911-16.e12. doi:10.1016/j.jada.2010.03.018.

14. Luevano-Contreras C, Chapman-Novakofski K. Dietary Advanced Glycation End Products and Aging. Nutrients. 2010; 2(12): 1247-1265. doi: 10.3390/nu2121247.

15. Hoffman JR, Falvo MJ. Protein – Which is Best? Macronutrient Utilization During Exercise: Implications For Performance And Supplementation. J Sports Sci Med. 2004; 3(3): 118-130.

16. van Vliet S, Burd NA, van Loon LJ. The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. J Nutr. 2015;145(9): 1981-1991. doi: 10.3945/jn.114.204305.

 

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