Nutritional Benefits of Meat: A White Paper

This project compared the nutrient density, protein quality and glycemic response of various sources of protein, and evaluated the benefits of protein on bone health, immune function, inflammation, muscle wasting, and cognition.

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Objectives

The objectives of this project were (1) to compare the nutrient density, protein quality and glycemic response of various sources of protein, and (2) to evaluate the benefits of protein on bone health, immune function, inflammation, muscle wasting, and cognition.  A summary from the white paper follows.

Conclusions

A “nutrient dense” food contains a generous amount of nutrients relative to its caloric content.  Meat, poultry, fish, dry beans and peas, eggs, nuts, and seeds supply many nutrients including protein and essential amino acids, B vitamins (niacin, thiamin, riboflavin, B6 and B12), vitamin E, iron, zinc, and magnesium. Beef, lamb, pork, and poultry all provide 20-30 g of protein / 100 calories; however the iron content of poultry is less than half that in beef.  The soybean products, boiled green soybeans and tofu, contain about half the amount of protein as red meat and poultry, less iron and no vitamin B12. While corn, rice and kidney beans contain significantly fewer calories than the animal protein sources, they also contain less protein and iron, and no vitamin B12.  Soybeans, black beans, pinto beans and kidney beans contain substantially more iron than lamb or beef, however, the iron in animal food products is present in the heme form, while that in vegetable protein sources is in the non-heme form.  Absorption of heme iron is efficient while that of non-heme iron is not.  Zinc and niacin are required for a number of enzymes in the energy production pathways.  The majority of the zinc and niacin in the American diet are derived from meat, poultry and fish.  Vitamin B12 is required for the formation of red blood cells and for fatty acid metabolism in the formation of the myelin sheath surrounding nerves.  It is found in essentially all animal food sources, particularly in fish, milk, eggs, meat, poultry and liver, but not in vegetable foods.

The amino acid balance in an “ideal” protein (5.5% lysine, 3.5% sulfur-containing amino acids, 4% threonine, 1% tryptophan and 7% leucine) is based on egg protein, the standard to which all others are compared. Lamb, pork, chicken breast, turkey breast, tuna and salmon meet all the requirements; beef meets the proportional requirements with the exception of tryptophan. Soy beans (boiled, mature), black beans and kidney beans meet all the proportional requirements except the sulfur-containing amino acids.  Pinto beans and split peas meet all except for the sulfur-containing amino acids and threonine.  The most commonly deficient proportional essential amino acid among the vegetable-based protein sources are the sulfur-containing amino acids.

The glycemic response is the increase in blood glucose after consuming a food.  The concept of the Glycemic Index has been controversial because it reflects how quickly a food raises blood sugar levels, but it does not take into account the total carbohydrate content in a serving of the food which is an important component of the glycemic response.  However, animal-derived muscle foods contain no carbohydrate, so they do not increase blood sugar. In addition, dietary protein significantly reduces the glycemic response of carbohydrate.  Vegetable protein sources contain varying amounts of carbohydrate which must be considered in the total glycemic load of the food.

Animal protein intake has been suggested as a cause for demineralization of bone due to its effects on urinary calcium excretion.  There is no consistent evidence for superiority of one type of protein over another on calcium metabolism, bone loss prevention or fracture risk reduction, however total protein does appear to be positively related to bone loss prevention.  Plant-based diets are often high in phytate and fiber content, which decrease iron, zinc, and calcium bioavailability.  They lack vitamin B12, are of low energy density, and contain poorer quality protein than diets that contain some animal-based products.  This over-all lack of nutrient availability and protein quality may have a greater impact on bone health than any one factor alone.

Nutritional deficiency of protein is often associated with impaired immune responses (cell-mediated immunity, phagocyte function, cytokine production, secretory antibody response, antibody affinity, and the complement system).  Glutamine, the most abundant amino acid in the body, is a “conditionally” essential amino acid because, when the body is under extreme stress (during infection or inflammation), the need for glutamine increases, however the production does not. Red meat and poultry contain greater than 4 mg glutamic acid/100 g while plant foods contain significantly less, particularly beans (< 2 mg/100 g). Low vitamin B6 intake is also associated with impaired immune function. Meat, poultry and fish are generally good sources of vitamin B6 (>0.3 mg/100 g) while most plant-derived foods contain considerably.

 It has been suggested that increasing iron stores from iron derived from red meat initiates oxidative damage and inflammation via free radicals. Increasing red meat intake can reduce leukocyte counts.  Some studies suggest decreased (rather than increased) oxidative stress and inflammation when lean red meat intake is increased at the expense of dietary carbohydrate-rich foods. Overall differences in dietary patterns (“Western” vs “prudent” diets) have been associated with various measures of inflammation, lipoprotein-lipid profile, serum glucose and insulin. However, weight (BMI) appears to be a bigger factor in development of chronic diseases of an inflammatory nature than does protein source.  Taurine, glutathione, and choline found in animal food products appear to down-regulate inflammatory responses.

Loss of muscle mass (wasting) is especially apparent during the aging process. There is a growing body of research indicating that dietary protein intakes above the recommended daily allowance help maintain muscle function and mobility.For muscle protein maintenance to exceed degradation, the amino acid building blocks must be consistently supplied.  Dietary leucine appears to have a particular function in that it enhances muscle growth by stimulating mammalian protein synthesis. Meat, poultry and fish are typically excellent sources of leucine, isoleucine and valine suggesting that they would enhance muscle protein synthesis. Meat and poultry products contain more than 2 g leucine/100g of food while most beans and cereal grains contain less than 1g leucine/100g.

Cognition, the ability to perceive, think, and remember, can be affected by deficiencies of specific nutrients, iron and zinc in particular, secondary to protein-calorie deficiency.  A positive correlation has been reported between diets containing animal protein source foods, iron, zinc and cognitive performance.  Animal protein also contributes choline and carnitine which appear to affect/maintain cognition.

Deliverable

This project compared the nutrient density, protein quality and glycemic response of various sources of protein, and evaluated the benefits of protein on bone health, immune function, inflammation, muscle wasting, and cognition.  The information contained in the white paper demonstrates the important nutritional role that meat plays in a healthy diet.