MEAT IN HUMAN NUTRITION
For meat and meat products there are extensive and comprehensive nutrient databases available for reference for particular products of interest. Thus, this article will provide a sampling of the data available for representative meats and meat products. One of the best and most comprehensive listings of the nutrient values of all meat, poultry, and other meat products is the nutrient database developed and maintained by the US Department of Agriculture. In this database, complete nutrient profiles are listed for more than 700 beef, 200 pork, 195 lamb, 85 veal, 140 poultry, and 130 turkey products. This database is updated as new data become available for various food products.
For another extensive listing of the nutrient values of many meat and meat products, including some by brand name, the reader is referred to the publication 'Bowes & Church’s Food Values of Portions Commonly Used' (18th edn.). This reference, although not as extensive in terms of products listed, provides data directly in common serving sizes and provides available data on some additional nutrient and nutrient-related components of meat products (e.g., values for !-3 and trans fatty acids, glutathione, vitamin D activity, and other vitamin-like compounds).
Nutritional Value
The nutritional value of foods, including meat and meat products, can be defined in a number of different ways, from simply listing the quantities of various nutrients contained in the foods to consideration of biological factors that affect the utilization of these nutrients by the body. Some foods may contain nutrients in forms that the body cannot readily utilize. Thus, nutrient bioavailability, or availability, becomes important.
The nutritional value of meat and meat products is related to the quantity and utilization of nutrients and the potential for these products to either enhance or restrict nutrient utilization by the body. There are five major classes of nutrients: protein, lipid, carbohydrate, vitamins, and minerals.
The nutrient content of meat (muscle foods) is fairly similar among the various mammals, birds, and fish. However, differences in the levels of the various nutrients may result from differences in the carcass composition among species and within species as a result of different fat-to-muscle ratios in the edible portion. As fat percentage increases, nutrient concentration of the muscle portion decreases. Also, to a certain extent, the fat profile/composition and other nutrient content levels may be modified or affected by the animal’s diet and/or genetic makeup.
In general, cooking or heat processing has only minimal effects on the nutritional value of muscle foods. In most cases, cooking usually decreases moisture content and concentrates other nutrients, including fat content, especially in lower fat products. This is due to moisture loss. However, in some intensely heated meat products, fat content may also be reduced significantly with negligible loss of other nutrients.
Classes of Nutrients and Meat Products
Protein
Proteins comprise the structural unit of all muscle cells and connective tissue. As such, meat and meat products (muscle foods) are major protein sources. Furthermore, muscle foods, as a group, are excellent sources of high-quality protein that supplies all the essential amino acids in desirable proportions for human consumption. Amino acids are the building blocks of protein, and those provided by meat match or exceed the profile required by humans.
The protein content of most muscle foods, on a wet basis, is between 15 and 35%. This percentage will change due to the moisture and lipid content of the specific product. On a raw weight basis as purchased at a store, the protein content is generally less than 20%. However, people do not eat muscle foods raw, and visible fat in red meat products and skin in poultry products are usually trimmed away. Therefore, muscle foods, as consumed, have a much higher protein content, in the range of 30%.
Lipids
The lipid component of meat and meat products includes a diverse group of substances, such as glycerides (glycerol with fatty acids attached), phospholipids, and sterols. The basic component of most meat lipids is the fatty acids, which can be saturated, monounsaturated, or polyunsaturated.
The relative amount of lipid in muscle foods is probably the most variable aspect of the nutritional profile. Within the lipid component, the relative amount of the different forms of fatty acids present is another variable among meat products. Despite the common reference to animal fats (and especially meat and meat products) as ‘saturated,’ less than half of all the fatty acids of meats are saturated. The largest proportions of fatty acids in meats are monounsaturated, followed by saturated and then polyunsaturated fatty acids. Among meat products, poultry has a higher proportion of polyunsaturated fatty acids and slightly less saturated fatty acids compared to other meat sources.
The fat in meat products provides much of the flavor associated with these foods and also contributes to the palatability and overall acceptability by consumers. In addition, the fats in meat and meat products also contain several essential fatty acids (linoleic and linolenic acid), and they contain the fat-soluble vitamins A, D, E, and K.
Carbohydrates
Meat and meat products are not significant sources of dietary carbohydrates. Almost all dietary carbohydrates come from plant sources. The only naturally occurring carbohydrate in muscle foods is glycogen. In some processed meat products, such as those that are ‘sugar-cured,’ there may be additional sucrose or glucose added.
Vitamins
Meat and meat products are especially good sources of most of the water-soluble vitamins. In general, meat is the major dietary source of vitamin B12 and is an excellent source of many of the other B vitamins, such as pyridoxine (B6), biotin, niacin, pantothenic acid, riboflavin, and thiamin. For vitamin B12, red meat products such as beef and lamb are especially good sources. Pork products are one of the very best sources of thiamin. Although present in muscle foods, the fatsoluble vitamins are less abundant than in plant foods. Vitamins E and K are present, but at lower levels.
Vitamin D activity may be present in some meat products, but at extremely low levels. This is reflected in the USDA nutrient database, in which vitamin D activity is not listed for beef, pork, lamb, veal, and chicken/turkey products; however, it is listed for some processed meat products. In recent years, there has been production research on beef, pork, and lamb to determine if added vitamin D3 or its metabolites, fed to the animal for a brief period of time prior to slaughter, can result in improved meat tenderness.
Although the results are inconsistent, and commercial application is premature, there is some indication that tenderness may be improved with relatively low levels of vitamin D supplementation, which seems to leave very little residual vitamin D3 or its metabolites in the muscle. Research in Denmark notes that the more biologically active 25-OHD is present at low levels in meat; however, there is no consensus on the conversion factor for 25-OHD to calculate vitamin D activity. Also, there are very few data on the vitamin D and 25-OHD levels in most meat products. This represents a potential future area of research regarding the nutrient composition of meat.
Minerals
Meat and meat products are good to excellent sources of most minerals. Among the macrominerals, calcium is not high in muscle foods, although phosphorus and potassium are prominent. In natural meat products, sodium is present but not a significant contributor to the diet. However, processed meat products may contain significantly higher levels of sodium (added as part of curing, preserving, or flavor-enhancing ingredients).
Some of the microminerals (trace elements) are especially abundant in meat and meat products. Iron is of greatest significance from meat sources because it is present in the ‘heme’ form, which is more bioavailable than the non-heme form. Of meat products, beef is an especially rich source of iron in this bioavailable form. Muscle tissue is a very rich source of minerals, such as phosphorus, potassium, magnesium, iron, copper, zinc, and selenium. For instance, pork, poultry, and beef are especially good sources of selenium.
Bioavailability of Nutrients
Muscle foods have been shown to contain ‘intrinsic’ factors that improve the bioavailability of a variety of nutrients. Moreover, the bioavailability of these nutrients from muscle foods is high, often exceeding the availability for the same nutrients in foods derived from plants. Heme iron is one example. Zinc and copper have been shown to be more available from meat sources than from plant sources.
Several of the B vitamins may also be more bioavailable from meat sources than from plant sources. Another interesting aspect of meat products is their ability to promote the bioavailability of nutrients in nonmuscle foods when the two are eaten together. This has been referred to as the ‘meat factor.’ Perhaps the best example of this is the positive effect of meat in the diet on non-heme iron sources, also in the diet.
Nutrient Density of Meat and Meat Products
The nutrient density of meat is high. Muscle foods have high levels of essential nutrients per unit of weight and per amount of calories provided. Meat and meat products (muscle foods) provide significant amounts of essential nutrients at levels/concentrations higher than those of most other foods relative to the caloric content provided.
The US Food and Drug Administration food labeling guidelines allow a food to be designated a ‘good’ source of a nutrient if it contributes 10% or more of the Daily Value (DV) and an ‘excellent’ source if it contributes 20% or more of the DV, for that nutrient, per 3-oz. serving. Most meat products are good or excellent sources of many nutrients. It is generally recognized that in diets that lack muscle foods, greater care is required in diet/menu selection to ensure that adequate levels of essential nutrients are present and bioavailable.
Meat Sources and Nutritional Values
Beef is an excellent source of high-quality protein, and provides significant contributions of many B vitamins and minerals. In macronutrient terms, the lean-to-fat ratio of the particular beef product influences the calorie and nutrient composition. In general, as the fat content decreases, the concentration of other nutrients (especially protein, B vitamins, and minerals) in beef tends to increase. Most beef products available to the consumer are much leaner than they were 20 or 30 years ago. This is a result of changes in feeding and genetics, producing leaner animals, and also due to closer trimlevels on the products that consumers see in the meat case. Whereas in the past, beef cuts with 1/4 in. of fat trim were common, now the same products have only 1/8 in. fat trim or, in some cases, even 0 in. fat trim. In the case of ground beef products, 10 or 20 years ago 17% fat ground beef was considered as ‘extra lean.’
Today, ground beef is commonly available at fat levels as low as 5 or 10%. Other common fat levels for ground beef are 15, 20, and 25%; however, a large proportion of current ground beef sales are in the 5–15% fat level range.
The fat content of beef contains a varied fatty acid profile, with the largest proportion being contributed by monounsaturated fat, followed by saturated fat and polyunsaturated fatty acids. In addition, because it is a ruminant product, beef is an excellent source of the naturally occurring fatty acid conjugated linoleic acid (CLA), which has been demonstrated to provide anticarcinogenic properties among other health benefits.
Pork, like beef, is an excellent source of high-quality protein and contributes significant amounts of many B vitamins and minerals. As for other muscle foods, pork’s nutrient composition is greatly affected by its fat and water content. As fat percentage decreases, the concentration of other nutrients increases. In addition, as pork is cooked and moisture is removed, the concentration of nutrients also increases. Pork is an excellent source of minerals, such as selenium, iron, zinc, phosphorus, and potassium. Compared to other muscle foods, the contribution of pork to selenium in the food supply is especially significant.
Pork is an excellent source of the B vitamins. Pork is an especially good source of thiamin (vitamin B1), being the single best source of this vitamin among commonly eaten foods. The fat profile of pork can be influenced by feeding regimes such that it is moreor less saturated or firm. However, overall the fatty acid profile of pork is largely monounsaturated, followed by saturated and then polyunsaturated fatty acids.
Although it represents a smaller portion of overall muscle food consumption, lamb still provides anutrient profile with significant benefits for the human diet. In addition to being a source of high quality protein, lamb is also a good source of many minerals and B vitamins. Vitamin B12 is especially abundant in lamb. It is also a good source of the minerals iron and zinc. In addition, as a ruminant, lamb is another naturally occurring dietary source of CLA, a unique fatty acid with anticarcinogenic and other health benefits (from animal model studies).
Veal
Although representing a smaller proportion of overall meat consumption, veal still provides a nutrient profile that is very beneficial. As with all meat sources, veal provides high-quality protein in a product that may be slightly leaner (in terms of fat) than other red meat sources. Compared to other meat sources, veal has a lower iron content.
The nutrient composition of poultry (chicken and turkey) is similar to that of red meat animals with a few exceptions. Poultry is lower in iron content, and thus heme iron, than beef. Turkey is slightly higher in several minerals (Ca, Fe, P, K, Zn, and Cu) than chicken. As in red meats, there are significant amounts of several B vitamins (e.g., niacin, B6, and pantothenic acid) compared to other meat sources, and these are not significantly reduced during cooking.
The fat content of poultry is predominantly monounsaturated fat, followed by saturated fat and polyunsaturated fat. Poultry fat, like pork fat, is somewhat more unsaturated than beef fat. Poultry is significantly higher in polyunsaturated fat compared to beef, pork, lamb, and veal.
Processed Meats
Processed meats represent a diverse array of products that have undergone additional treatment from the fresh meat form to the point of consumption, including curing with other ingredients added and the addition of salt or other flavor or preservative mixtures. Also, these products often represent combined meat sources.
Summary
By P A Lofgren in the 'ENCYCLOPEDIA OF HUMAN NUTRITION'- SECOND EDITION. Editor-in-Chief BENJAMIN CABALLERO, Editors LINDSAY ALLEN & ANDREW PRENTICE, Elsevier Ltd. Oxford, UK, 2005 , p.230-237. Adapted and illustrated to br posted by Leopoldo Costa.
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