Nutrition
- The Role of Meat
1.
In the typical Western diet, meats such as beef, lamb, pork, veal, poultry,
and fish are the predominant sources of protein, B vitamins, iron, and zinc.
2.
Iron and zinc are the two nutrients most often deficient in vegetarian or
modified-vegetarian diets. Also, iron and zinc are the most cited nutrients
that may be deficient in the diet of athletes.
3.
Athletes who choose to exclude meat from their diets must carefully plan
diets to enhance nutrient availability, particularly for iron and zinc.
Generations
of athletes have consumed a diet centered around meat. In the 6th century
B.C., a famous Greek athlete, Milo of Crotona, was the wrestling victor
in five Olympian games and in many other sacred festivals. In what is probably
an apocryphal account, he applied progressive resistance training by lifting
a growing calf daily. When the calf was four years old, he carried her the
length of the Olympian stadium, killed, roasted, and ate her. It was reported
that Milo's normal daily intake of meat was about 20 pounds (Ryan, 1981).
Times
have changed. As scientific evidence has solidified the link between high-fat,
meat-based diets and increased risks of chronic diseases such as cancer
and heart disease, more people are turning to a vegetarian style of eating
for health reasons, rather than moral or ethical reasons. Almost seven percent
of the American public, or about 12.4 million people, consider themselves
vegetarians (Havala, 1994). Athletes, too, are becoming attracted to a more
plant-based style of eating. Although most athletes do not eliminate all
animal foods from the diet, increasing numbers of athletes avoid beef and
other red meats on a fairly regular basis. (Snyder et al., 1989; Raben et
al., 1992; Lyle et al., 1992). A survey of nationally competitive female
runners showed that more than 40 percent avoided red meat for "health
reasons" (Clark et al., 1988). Other reasons for avoiding meat include
fat content, calorie content, and financial cost (Steen, 1991). In some
cases athletes limit animal foods based on misconceptions, such as the erroneous
association of milk with fluid retention (Kleiner et al., 1994).
Can
an athlete achieve peak performance on a meatless diet? Can plant foods
supply the right nutrients in adequate amounts to replace nutrients depleted
during intense physical activity? The purpose of this review is to summarize
the factors that influence the adequacy of a meatless diet for athletes
and to provide practical guidelines to assist with the healthy planning
of such diets.
Typical
Western diets are made up of the following food groupings:
Vegetables;
Fruits;
Breads, cereals, rice, and pasta;
Milk, yogurt, and cheese;
Meats, poultry, fish, dry beans and peas, eggs, nuts, and seeds;
Fats, oils, and sweets.
In
the typical Western diet, meats (including beef, lamb, pork, veal, poultry,
and fish) are the primary staple around which meals are designed, and are
the predominant sources of protein, B vitamins, iron, and zinc.
Just
as no single vegetable or fruit can provide all of the critical nutrients
common to its food group, no single type of meat can provide all of the
protein, B vitamins, iron, and zinc necessary for a healthy and well-balanced
diet. It is the variety of types and cuts of meats that provide the total
array of nutrients necessary for an adequate diet.
For
example, beef is only an average source of niacin, riboflavin, thiamin,
and vitamin B6 (1 serving provides 1024 percent of the RDA for adults and
children over 4 years of age). But most cuts of beef are excellent sources
of zinc (1 serving provides 40 percent of the RDA for adults and children
over 4 years of age). Likewise, pork is an excellent source of thiamin and
iron, a good source of niacin (1 serving provides 25-39 percent of the RDA
for adults and children over 4 years of age), and only an average source
of riboflavin, vitamin B6, and zinc. Because vitamin B12 is a byproduct
of animal metabolism, virtually all types of meats are good or excellent
sources of vitamin B12 (USDA, 1990).
Generally
speaking, red meats like beef and the dark meat of poultry are better sources
of iron and zinc than are white meats like fish and light meat of poultry.
However, there are some exceptions. Pork is an excellent source of iron,
as are clams and oysters. Oysters are also an excellent source of zinc (USDA,
1990).
The
elimination of some or all meats from the diet does not mean that a wellbalanced
and adequate diet is impossible. Dry beans and peas (legumes) and nuts are
somewhat similar to meats in providing protein and most vitamins and minerals.
But there are some significant nutritional differences between plant and
animal food sources of proteins.
The
quantity of protein in the diets of athletes is rarely a concern, regardless
of whether they are meat eaters or nonmeat eaters. For example, an average
of 21-25% percent of the energy in legumes comes from protein calories (Geil
& Anderson, 1994), and protein constitutes 34% of the energy in soybeans.
There is, however, a limitation to the quality of the protein from most
legumes. With the exception of soybeans, legumes do not contain a full complement
of the essential amino acids required for the efficient manufacture of protein
by the human body. Wellprocessed soybean protein is equal in quality to
animal protein (Young, 1991).
Previous
vegetarian dietary guidelines recommended that a variety of plant protein
sources (such as grains and beans) be combined simultaneously at one meal
to complement each other and provide a complete protein source. Current
research supports the notion that by eating a variety of legumes, as well
as all other food groups throughout the day, one can obtain the full array
of essential amino acids required for efficient protein metabolism.
According
to World Health Organization (1985) recommendations, protein digestibility
is reduced by about 10% in a vegetable-based diet due to the high fiber
content of the diet. Accordingly, it is suggested that those who eat such
diets should consume 110% of the calculated protein requirement to ensure
adequate protein intakes (WHO, 1985).
In
spite of the fact that meats are a major source of B vitamins in the Western
diet, whole and enriched grain products, eggs, legumes, nuts, seeds, fruits,
vegetables, and dairy products are good and reliable sources of B vitamins
and can fully supply the dietary requirements for B vitamins.
An
exception to this rule is vitamin B12, which is available only from animal
products. A B12 supplement must be used if animal products are completely
eliminated from the diet.
In
absolute amounts, it is surprising to note that most meats are only average
sources of iron when compared to many grains and legumes. However, the bioavailability
of iron from meat versus vegetable foods makes a significant difference
in the value of meat as a source of iron in the diet.
There
are two forms of dietary iron, heme iron (from animal tissue) and nonheme
iron. Heme iron is absorbed with the iron still contained within hemoglobin
or myoglobin molecules. Absorption of heme iron is affected by the body's
stores of iron, but it is not affected by intestinal factors or by meal
composition. However, absorption of nonheme iron is very dependent on iron
stores, intestinal factors and meal composition. Furthermore, heme and nonheme
iron are absorbed from the intestine at differing rates. In the ironreplete
individual, as little as 15% of heme iron that reaches the intestine is
absorbed, whereas up to 35% may be absorbed in an individual with little
or no iron stored in the body. Absorption of nonheme iron can range from
2% in the iron-replete individual consuming a meal of low iron availability
to 20% in the person with minimal stores of iron who consumes a meal that
contains highly bioavailable nonheme iron (Monsen & Balintfy, 1982).
Intestinal factors and meal composition factors that affect absorption of
nonheme iron are discussed later under "Practical Considerations."
Meats,
particularly red meats and oysters, are good or excellent sources of zinc
and are the major sources of zinc in the Western diet. The bioavailability
of zinc varies with the food source; some foods contain factors that inhibit
zinc absorption. Factors that inhibit zinc absorption include fiber, phytic
acid, oxalic acid, ethanol, tannins, iron, calcium, and tin. These constituents
are found in varying amounts in soy protein, whole wheat, tea, coffee, celery,
milk, cheese, corn tortillas, and beans (Shils & Young, 1984). Zinc
from animal sources is generally regarded as more bioavailable than zinc
from vegetable sources (Mares-Perlman et al., 1995).
Potential
iron and zinc deficiencies are the two most noted drawbacks of vegetarian
or modified-vegetarian diets, and are the most common dietary deficiencies
among athletes (Dallongeville et al., 1989; Lamanca & Haymes, 1992;
Nutter, 1991; Pate et al., 1993; Snyder et al., 1989; Telford et al., 1992,
1993; Williford et al., 1993).
Iron
is an essential trace element required for the formation of hemoglobin,
myoglobin, the cytochromes, and iron-containing enzymes critical in immune
function (Haymes, 1987). Numerous recent studies have documented a prevalence
of iron deficient conditions in both male and female athletes, but more
commonly among women (Dallongeville et al., 1989; Lamanca & Haymes,
1992; Nutter, 1991; Pate et al., 1993; Snyder et al., 1989; Telford et al.,
1992, 1993; Williford et al., 1993).
Iron
depletion, the first stage of iron deficiency, is the most common iron deficiency
condition documented among athletes, and is indicated by low serum ferritin
values (<12 ug/dL). The next two stages of iron deficiency, iron-deficient
erythropoiesis and irondeficiency anemia, are much less commonly observed.
Plasma volume expansion (which reduces the concentration of iron in plasma),
low dietary iron intake, low bioavailability of dietary iron, and increased
rates of iron excretion (Clarkson & Haymes, 1995) are suggested as possible
reasons for the high prevalence of iron depletion in athletic populations.
The
Recommended Dietary Allowance (RDA) for iron is 15 mg/d for women and 10
mg/d for men. The average iron intake among U.S. women is only 6 mg/1000
kcal or 10.6 mg/day. Most males consume more than the RDA for iron (Clarkson
& Haymes, 1995).
The
results of studies investigating iron status of athletes and exercising
individuals indicate that athletes with decreased iron stores generally
consume less dietary iron and fewer servings of meat on a regular basis.
Subjects with low iron stores eat significantly less heme iron than do those
with normal iron stores. However, no performance decrements have been demonstrated
in these subjects (Lyle et al., 1992; Dallongeville et al., 1989; Pate et
al., 1993; Snyder et al., 1989; Williford et al., 1993). Some studies that
indicated lower iron intakes among subjects with decreased iron stores failed
to demonstrate a correlation between heme iron intakes and serum or plasma
ferritin values (Lamanca & Haymes, 1992; Telford et al., 1993).
Several
researchers have studied the influence of the dietary source of iron on
iron stores. Lyle et al. (1992) studied the effect of oral iron therapy
versus increased consumption of meat in women participating in a moderate
exercise program for 12 weeks. The additional meat was more effective in
protecting hemoglobin and ferritin status than was iron supplementation.
In contrast, a similar study repeated by the same researchers failed to
find improved iron stores with extra meat consumption but did show increased
iron stores with a daily supplement of 50 mg ferrous sulfate supplement
(Rajaram et al., 1995). Thus, there is no clear choice between these two
approaches to increasing iron intake.
Although
it is certain that iron-deficient erythropoiesis and iron-deficiency anemia
will impair physical performance, whether or not iron depletion affects
performance is uncertain. Most studies (Dallongeville et al., 1989; Dressendorfer
& Sockolov, 1980; Dressendorfer et al., 1982; Haralambie, 1981; Janelle
& Barr, 1995; Lyle et al., 1992; Pate et al., 1993; Singh et al., 1990;
Snyder et al., 1989; Williford et al., 1993;) have shown no negative performance
effects due to decreased iron stores; however, Telford and colleagues (1992)
showed that improving low plasma ferritin levels (<30 ng/mL) in males
was associated with an increase in performance during a 10s maximal exercise
test.
Zinc
is one of the most widely distributed metals in the body and is an important
co-factor for more than 100 enzymes involved in metabolic pathways, endocrine
function, and immune integrity (Clarkson & Haymes, 1994). Carbonic anhydrase
III, AMP-deaminase, and lactate dehydrogenase are zinc-dependent enzymes
that serve important functions in energy metabolism during exercise.
Zinc
status is difficult to measure. Although most studies measure levels of
zinc in the serum, this is a relatively poor indicator of overall zinc status
in the body. For example, a recent study suggested that prolonged, vigorous
physical activity increases the content of zinc in mononuclear cells, but
does not change the content of zinc in serum and red blood cells (Dolev
et al., 1995).
At
least one study suggested that training status may not affect bodily zinc
stores. Deuster et al. (1989) reported no differences between fasting concentrations
of plasma zinc, serum albumin, alpha2-macroglobulin, and erythrocyte zinc
content between highly trained and untrained women. The highly trained women
did, however, have significantly higher urinary zinc excretions after a
25 mg oral zinc load.
On
the other hand, many studies of the zinc status of athletes have reported
lower than normal levels of serum or plasma zinc (Dressendorfer & Sockolov,
1980; Dressendorfer et al., 1982; Haralambie, 1981; Singh et al., 1990).
Possible reasons for this hypozincemia in athletes include low dietary zinc,
low bioavailability of dietary zinc, excessive zinc loss during exercise,
dilution of zinc by plasma volume expansion, and redistribution of zinc
in the body (Clarkson & Haymes, 1994).
The
RDA for zinc is 12 mg/day for women and 15 mg/d for men. The average zinc
intake of both the sedentary and the athletic populations of U.S. women
is approximately 10 mg/d, and that for men slightly exceeds the RDA (Clarkson
& Haymes, 1994). In a recent study of vegan and lactovegetarian women
zinc intakes were lower than recommended (8.5 mg/d and 8.2 mg/d, respectively)
(Janelle & Barr, 1995).
The
lack of meat sources of zinc in the diet may contribute to or increase the
potential for the development of hypozincemia in athletes. Among the 25
major sources of zinc in the U.S. diet, meat or dishes containing meat comprise
the top 10 (Mares-Perlman, 1995). Zinc bioavailability from some plant sources
is limited by their contents of fiber and/or phytate. Although fractional
absorption of zinc from plant-based diets can be similar to that from animal
sources, the low zinc content of plant foods tends to result in a low net
absorption (Janelle & Barr, 1995).
A
true clinical deficiency of zinc would certainly impede physical performance.
Due to the critical role of zinc in regulating lactate dehydrogenase activity,
among other clinical symptoms, zinc deficiency results in decreased muscle
strength and endurance (Krotkiewski et al., 1982). Whether or not mild hypozincemia
is an impediment to peak performance is uncertain, but appears unlikely.
Zinc supplementation is common among athletes, but other than in cases of
correcting a long-standing zinc-deficient diet, there is little evidence
of a performance benefit from zinc supplementation. In fact, diet supplementation
with 50 mg of zinc interferes with copper status in men. Furthermore, zinc
intake in amounts 10 times the RDA can significantly reduce lymphoctye function
and phagocytosis of bacteria by polymorphonuclear leukocytes, decrease HDL
cholesterol levels, and increase LDL cholesterol. It is therefore recommended
that zinc supplementation not exceed 15 mg/d. (Clarkson & Haymes, 1994).
Whether
to include or exclude meat in the diet of an athlete is obviously a matter
of personal choice; however, if the choice is made to decrease the amount
of meat in the diet, then careful dietary planning is necessary to enhance
nutrient availability, particularly for iron and zinc.
Include
heme iron sources in the diet. All
types of meat contain this more-easily-absorbed form of iron. If only red
meat has been eliminated from the diet, heme iron is still available from
poultry and fish.
The
"MFP Factor". Meat, fish, and poultry also contain a special
quality called the "MFP Factor" that helps the body absorb more
nonheme iron. When meat and vegetables are eaten together at the same meal,
more nonheme iron is absorbed from the vegetables than if the vegetables
had been eaten alone.
Include
vitamin C sources. Fruits, vegetables and other foods that contain vitamin
C help the body absorb nonheme iron. For example, if citrus fruits are eaten
along with an ironfortified cereal, more iron will be absorbed from the
cereal than if it had been eaten alone.
Avoid
constituents that block iron and zinc absorption. Some food con-stituents,
e.g., tannins, polyphenols, phytates, and oxalates, can block the absorption
of iron and zinc by the intestine. Coffee and tea (regular and decaffeinated),
whole grains, bran, legumes, spinach, and a high fiber intake in general,
are a few examples of foods that contain ironand zincabsorption blockers.
These foods are best eaten with heme iron sources and/or vitamin C sources
to help the body absorb more iron.
There
are good meatless sources of iron and zinc, as shown in the table on the
following page. Because iron and zinc intake may be low or marginal in a
completely plant-based diet, an extra effort must be made to include these
sources in the diet on a daily basis.
Due
to the rigorous demands of athletic participation, those who choose to completely
eliminate meat from the diet may find it difficult to plan, prepare, or
consume the quality and quantity of food required to meet recommended guidelines.
Despite the fact that dietary supplements do not entirely replace food,
when key nutrients are insufficient in the diet it is wise to use a supplement
rather than face a potential nutrient deficiency. Daily supplementation
of iron and zinc at the level of 100% of the RDA is a safe method of ensuring
adequate intake of these nutrients.
As
vegetarian styles of eating become more popular among athletes, the risk
of poorly planned diets leading to nutrient insufficiencies and deficiencies
increases. Suboptimal dietary intakes of iron and zinc resulting in decreased
nutritional status have been observed in athletes who have eliminated meat.
Marginal iron or zinc status may negatively affect exercise performance.
Full-blown iron or zinc deficiency will definitely have a negative effect
upon exercise performance.
It
is possible to obtain all essential nutrients by eating a completely plantbased
diet. However, the planning and execution of the diet is critical to both
the health and performance of an athlete. Practically speaking, because
vegan diets are also typically high in fiber, it may be difficult for an
athlete to consume enough food to satisfy nutrient and energy needs without
feeling so full that exercise performance is inhibited. Athletes must learn
that it is not sufficient to merely cut meats out of the diet; these foods
contain essential nutrients that must be carefully replaced by adding other
foods to the diet. If the decision to consume a meatless diet is not based
upon moral or ethical principles, it may be more practical to encourage
the athlete to include some meat in their diet. It is also important that
athletes base their dietary decisions on scientific evidence, rather than
on myths and misconceptions.
References
Susan M. Kleiner, Ph.D., R.D.
High Performance Nutrition
Mercer Island, Washington
