Meat and micronutrients 9111 Iron in meat

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Iron deficiency (Schrimshaw, 1991) and iron deficiency anaemia (Walker, 1998) remain the most common nutritional disorders in the world today. Iron deficiency is the only widespread nutrient deficiency occurring in both developed and developing countries. Iron deficiency affects between 20 and 50% of the world's population (Beard and Stoltzfus, 2001). There are many causes of iron deficiency, including hook worm infestation, low iron intakes, low bioavailability of dietary iron and increased demand due to physiological requirements. The most common result of iron deficiency is anaemia. Some of the liabilities associated with iron deficiency and anaemia are defective psychomotor development in infants, impaired education performance in schoolchildren, adverse perinatal outcome in pregnancy and diminished work capacity (Cook, 1999). All of the iron in our body comes from our diet, and meat is a rich dietary source. Concern about iron deficiency is one nutritional reason for recommending eating at least some meat (WHO, 1990; COMA, 1998).

Food iron can be classified as haem iron or non-haem iron. Haem iron is derived from haemoglobin and myoglobin and its chief food source is meat, whereas non-haem iron is derived mainly from cereals, fruits and vegetables. Meat is distinctive as it contains both types of iron, haem (50-60%) and non-haem. Our bodies readily absorb haem iron (20-30%) as it is not affected by other dietary factors. Meat positively influences the bioavailability of non-haem iron. Bioavailability of iron refers to the proportion of ingested iron that is absorbed and utilised by the body (O'Dell, 1989). Only two dietary factors enhance non-haem iron bioavailability, they are vitamin C (Hallberg et al, 1989) and meat (Cook and Monsen, 1976; Taylor et al, 1986; Hazell et al, 1978; Kapsokefalou and Miller, 1991, 1993, 1995; Mulvihill and Morrissey, 1998a, 1998b; Mulvihill et al, 1998). Absorption of non-haem iron from meat is typically 15-25%, compared with 1-7% from plant sources (Fairweather-Tait, 1989). The presence of meat in a meal enhances the bioavailability of non-haem iron contained in the other foods present such as cereals, fruits and vegetables.

The enhancing effect of meat on non-haem iron bioavailability is commonly referred to as the 'meat factor'. The exact mechanism by which the 'meat factor' works still remains unknown despite the fact that numerous efforts have concentrated on this topic. Research indicates that the mechanism of the 'meat factor' may not be due solely to a single factor but due to a number of contributing factors which work together promoting non-haem iron bioavailability. These factors include the release of cysteine-rich small molecular weight peptides during the proteolysis of meat; the ability of these peptides to reduce ferric iron to the more soluble ferrous iron; the chelation of soluble non-haem iron by these peptides; and the ability of meat to promote gastric acid secretion and gastrin release better than other food components do (Mulvihill, 1996).

Glutathione is a tripeptide containing cysteine, and this is considered to play a role in the 'meat factor'. However, reduced glutathione represents only 3% of total cysteine in meat and this is considered too low to have such a profound positive influence on non-haem iron bioavailability (Taylor et al, 1986). Elucidation of the mechanism(s) of the 'meat factor' is extremely important in the search for more effective ways to improve iron nutrition. Isolation of the 'meat factor' will allow the potential to produce stable non-haem iron absorption enhancers which can be added to other foods, thus improving iron bioavailability.

Meat and meat products provide 14% of iron intake (MAFF, 1999); within this, carcase meat and meat products supply 12.5% of total iron intakes. This figure grossly underestimates the value of meat for influencing iron status. Meat has an important influence on iron bioavailability and thus iron status due to its enhancing properties and overall greater absorption capacity.

Low iron intakes and status are common among certain subgroups of the population - toddlers (Gregory et al, 1995; Edmond et al, 1996), adolescents (Nelson et al, 1993; Nelson, 1996), pregnant women (Allen, 1997) and the elderly (Finch et al, 1998). Data from the National Diet and Nutrition Survey of children shows that 20% have low iron stores and 8% have iron deficiency anaemia (Gregory et al, 1995). Iron deficiency anaemia among toddlers is often associated with late weaning practices. A Spanish study showed that children who first ate meat before eight months of age showed a better iron status than those who were introduced to meat later than eight months (Requejo et al, 1999). Another study showed that low iron stores in one- and two-year old children is related to a low meat iron intake (Mira et al, 1996). The COMA report on Weaning and the Weaning Diet recommends that foods containing haem iron should be incorporated into the diets of infants by 6-8 months of age. Soft-cooked pureed meat can be introduced. This goes against the modern trend to delay introduction, the basis for which appears to be non-scientific.

Adolescents have high demands for iron to allow for muscle development, increased blood volume and the onset of menstruation in females, that makes them vulnerable to iron deficiency. Half the female population living in the UK aged between 15 and 18 years have iron intakes below the recommended level. This is reflected by the fact that 27% of that age group have low iron stores (Gregory et al, 2000). The prevalence of low iron stores among adolescent girls in the UK has been cited to be as high as 43% (Nelson et al, 1993). During pregnancy, more lac-tovegetarians (26%) reported suffering from iron deficiency than omnivores (11%) (Drake et al, 1999). Lyle et al (1992) has demonstrated that meat supplements were more effective than iron tablets in maintaining iron status during exercise in previously sedentary young women. Among the elderly, both low iron intakes and low iron status has been shown to increase with age (Finch et al, 1998).

Serum ferritin, the body's iron store, is strongly correlated with haem iron (Reddy and Sanders, 1990). Bioavailability of iron plays an important role in determining iron status. Studies have shown that despite the fact that vegetarians have either a similar or a higher iron intake than their omnivore counterparts, their iron status is lower (Nathan et al, 1996; Ball and Bartlett, 1999; Wilson and Ball, 1999). Vegetarians should consume iron-rich foods to compensate for the low bioavailability of non-haem iron from the foods they eat.

The importance of meat in iron nutrition cannot be over-emphasised. The effects of meat and meat products on iron nutrition are three-fold. Firstly, they are a rich source of iron. Secondly, they contain haem iron, which is readily absorbed. Thirdly, they promote the absorption of non-haem in the diet.

9.11.2 Zinc in meat

All meats, but in particular beef, are excellent sources of dietary zinc. It takes 41 oz milk, 15 oz tuna or 6V2 eggs to equal the amount of zinc in an average 4oz portion of beef (Hammock, 1987). On average, meat and meat products account for a third of total zinc intakes (MAFF, 1999). Zinc absorption is suppressed by inhibitors such as oxalate and phytate which are found in plant foods (Johnson and Walker, 1992; Zheng et al, 1993; Hunt et al, 1995). On the contrary, meat facilitates the absorption of zinc - 20-40% of zinc is absorbed from meat. For instance, one study showed that female omnivores who had a significantly lower zinc intake than their vegetarian counterparts had a higher zinc status (Ball and Ackland, 2000); such data highlights the role that meat plays in providing an assured source of dietary zinc. Because of the low bioavailability of zinc from plant foods, vegetarians should strive to meet or exceed their RDA for zinc to ensure adequate zinc intakes.

Zinc is necessary for growth, healing, the immune system, reproduction (Aggett and Comerford, 1995) and cognitive development (Sandstead, 2000). Low zinc intakes are becoming more prevalent, especially among adolescents. An NDNS survey showed that a tenth of 7-10 year old girls and a third of 11-14 year old girls have intakes of zinc below the recommended level (Gregory et al, 2000). Long-term, low zinc intakes leads to zinc deficiencies that may become a public health problem in the future (Sandstead, 1995). Iron and zinc deficiencies can often occur simultaneously, in particular among adolescents (Sandstead, 2000). Adolescents often avoid eating meat, in some incidences meat is providing up to just 25% of total zinc intakes compared to 40% of adult intakes (Gregory et al, 1995; Mills and Tyler, 1992; Gregory et al, 2000). Thus including meat in the diet of adolescents can aid in averting both iron and zinc deficiencies in concert, as these minerals in meat are in easily absorbable forms. Similarly, concern over low zinc status among infants prompted the DoH, in its COMA weaning report, to recommend increasing meat portion sizes for infants at the weaning stage (Department of Health, 1994a).

9.11.3 Selenium in meat

Selenium acts as an antioxidant and is considered to protect against coronary heart disease and certain cancers, such as prostate. Meat contains about 10 mg selenium per 100 g, which is approximately 25% of our daily requirement. Beef and pork contain more selenium than does lamb, which may be due to the age of the animal as selenium may collect in the meat over time. Bioavailability of selenium from plant foods was thought to be greater than that from animal foods, but recent data demonstrate that meat, raw and cooked, provides a highly bioavail-able source (Shi and Spallholz, 1994).

9.11.4 Other minerals in meat

Meat also contains phosphorus; a typical serving provides roughly 20-25% of an adult's requirement. Phosphorus has important biochemical functions in carbohydrates, fat and protein metabolism. Meat also provides useful amounts of copper, magnesium, potassium, iodine and chloride.

9.11.5 B vitamins in meat

Meat is a significant and an important source of many B vitamins. The B vitamins in meat are thiamin (vitamin Bj), riboflavin (vitamin B2), niacin, pantothenic acid, vitamin B6 and vitamin B12. B vitamins are water-soluble, hence lean meat contains more of these vitamins than does fattier meat. Some losses of B vitamins occur during cooking; the amount lost depends upon the duration and the temperature of the cooking method.

Thiamin and riboflavin are found in useful amounts in meats. Pork and its products including bacon and ham are one of the richest sources of thiamin. Pork contains approximately 5-10 times as much thiamin as do either beef or lamb. Thiamin aids the supply of energy to the body by working as part of a coenzyme that converts fat and carbohydrates into fuel. It also helps to promote a normal appetite and contributes to normal nervous system function. Typical servings of pork provide all the daily requirement of thiamin. Offal meats are good sources of riboflavin, for example, a single portion (100 g) of kidney or liver provides more than the daily requirement. Riboflavin, like thiamin, aids in supplying energy and also promotes healthy skin, eyes and vision.

Meat is the richest source of niacin. Half the niacin provided by meat is derived from tryptophan, which is more readily absorbed by the body than that bound to glucose in plant sources. Niacin helps to supply energy to the body as it plays a role in converting carbohydrates and fats into fuel. Meat and meat products supply more than a third of total niacin intakes in Britain (MAFF, 1999).

Liver and kidney are rich sources of pantothenic acid. Although most of this vitamin is leached into the drip loss associated with frozen meat, this is unlikely to be of any nutritional consequence as pantothenic acid is universal in all living matter.

A 100g portion of veal liver provides half our daily vitamin B6 needs and other meats provide around a third. Vitamin B6 is a necessary cofactor for more than 100 different cellular enzyme reactions including those related to amino acid metabolism and inter-conversion. Vitamin B12 is exclusively of animal origin as it is a product of bacterial fermentation that occurs in the intestine of ruminant animals such as cattle, sheep and goats. Vitamin B12 is required to produce red blood cells and acts as a cofactor for many enzyme reactions. Deficiency of vitamin B12 causes megaloblastic anaemia, neuropathy and gastrointestinal symp toms. Groups at risk of vitamin Bj2 deficiency include vegans and strict vegetarians, because vitamin Bj2 is exclusively of animal origin, and the elderly, because their ability to absorb this vitamin from the diet diminishes with age (Allen and Casterline, 1994; Swain, 1995; Baik and Russell, 1999; Drake et al, 1999a). In the past some vitamin B12 was provided from the soil of poorly cleaned foods. This may in part explain the apparent absence of deficiency in some vegan groups. Today, with the emphasis on good food hygiene practices, this source can no longer protect against deficiency in vulnerable individuals. Vegans are recommended to take vitamin B12 supplements since the quantity consumed from foods fortified with the vitamin is too low (Jones, 1995; Draper, 1991; Sanders and Reddy, 1994). The RNI for vitamin Bj2 among the elderly is 1.5 mg/day (Department of Health, 1991). A 100g portion of lean trimmed beef contains 2 mg vitamin B12, thus supplying all their daily needs for this vitamin. In Britain, meat and meat products supply more than a fifth of both vitamin B6 and B12 intakes (MAFF, 1999). The need for vitamin B12 has been a part of the rationale for recommending the consumption of animal foods among all age groups (WHO, 1990).

Raised homocysteine, an amino acid metabolite, is an independent risk factor for cardiovascular disease. It is estimated that 67% of the cases of hyperhomo-cysteinemia are attributable to inadequate plasma concentrations of one or more of the B vitamins namely folate, vitamin B6 and vitamin B12. Some enzymes that reduce homocysteine levels require vitamins B6 and B12 as cofactors. Vitamin B6 is a cofactor for two enzyme reactions which catabolise homocysteine to cys-teine via a transulphuration pathway, they are cystathionine b-synthase and cys-tathionase. Meanwhile, vitamin B12 is a cofactor for the remethylation enzyme, methionine synthase, which converts homocysteine to methionine. Research has shown that low levels of both vitamins B6 and B12 independently correlates with raised homocysteine. For instance, ovo-lactovegetarians or vegans who had significantly lower serum vitamin Bj2 levels than meat eaters had significantly higher levels of plasma homocysteine (Mann et al, 1999; Krajcovicova-Kudlackova et al, 2000; Mann, 2001b). Similarly, low doses of vitamin B6 can effectively lower fasting plasma homocysteine levels (McKinley et al, 2001). The role of meat in regulating homocysteine is intriguing and needs to be addressed further.

9.11.6 Meat and vitamin D

In the body vitamin D acts as a hormone, essential for the absorption of dietary calcium. Thus, vitamin D is essential for skeletal development and severe deficiency is associated with defective mineralisation of the bone resulting in rickets in children or its adult equivalent, osteomalacia (Fraser, 1995; Dunnigan and Henderson, 1997; De Luca and Zierold, 1998; Department of Health, 1998b). More subtle degrees of insufficiency lead to increased bone loss and osteoporotic fractures. Other functions of vitamin D include its role in the immune system, as well as possible protection against tuberculosis, muscle weakness, diabetes, certain cancers and coronary heart disease (Department of Health, 1998b).

It is well established that sunlight exposure on the skin is the main source of vitamin D. However, there are certain subgroups in the population who are more at risk of vitamin D deficiency, and these depend on diet in addition to sunlight in obtaining adequate vitamin D. Such subgroups include infants, toddlers, pregnant and lactating women, elderly and those who have low sunlight exposure, such as certain ethnic minorities and the housebound (Department of Health, 1998a). The prevalence of vitamin D inadequacies among these groups is widespread. For instance, 27% of 2 year old Asian children living in England have low vitamin D status (Lawson and Thomas, 1999), and 99% of elderly people living in institutions are not receiving enough dietary vitamin D (Finch et al, 1998). Vitamin D deficiency among the elderly will become much more apparent and a greater public health problem when we consider that we are living in an increasingly ageing population.

Liver aside, meat and meat products were considered poor sources of vitamin D. However, new analytical data for the composition of meat indicates that this is not true (Chan et al, 1995). Meat and meat products contain significant amounts of 25-hydroxycholecalciferol, assumed to have a biological activity five times that of cholecalciferol. In fact, the meat group is now recognised as the richest natural dietary source of vitamin D, supplying approximately 21% (Gibson and Ashwell, 1997). Vitamin D is present in both the lean and the fat of meat although its exact function in the animal is not yet known. Since interest in the role of meat in supplying vitamin D is a relatively new subject matter, there are certain areas that need to be researched such as the effect of cooking meat on vitamin D levels, the bioavailability of vitamin D from meat and the influence of seasonal variation on the vitamin D content of meat and meat products.

Low intakes of meat and meat products emerged as an independent risk factor for Asian rickets and absent intakes of meat and meat products emerged as an independent risk factor for Asian osteomalacia (Dunnigan and Henderson, 1997). It has been hypothesised by this research group that there may be a 'magic factor' in meat which is protective against rickets and osteomalacia. In Glasgow, at the beginning of the century, the incidence of rickets was high, whereas, between 1987 and 1991, only one case of rickets was reported. This may be explained by the fact that today infants are weaned onto an omnivorous diet from four months of age and this meat inclusion is offering protection against rickets (Dunnigan and Henderson, 1997). Obviously, much more research is required to improve our knowledge on this subject matter. It is also of interest to note that signs of both iron and vitamin D deficiency can occur simultaneously among toddlers (Lawson and Thomas, 1999). For instance, during the winter, half of the toddlers had both low vitamin D and low iron levels (Lawson and Thomas, 1999). Such evidence highlights the potential protective role that meat inclusion can play in a toddler's diet. It is important for toddlers and children to eat foods rich in both iron and vitamin D such as meat and meat products as well as playing out of doors to get sunlight.

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