NUTRITION IN SPORT

tarians who consume only plant proteins could
become protein deficient unless they balance
their amino acids. Plant proteins are incomplete
and lack one or more essential amino acid. While
it was once thought that all the amino acids must
be consumed in one meal, it is now agreed that
timing of the amino acid intake is less critical and
that amino acid intake must be balanced over
days rather than hours. Research now shows that
the liver monitors the amino acid composition of
proteins consumed in a meal: if the meal is low in
an essential amino acid, the liver can break down
its own proteins to supply it. When the amino
acid is once again plentiful, the liver will replen-
ish its protein source.
Because individual plant foods do not contain
all the essential amino acids, it is recommended
that vegetarians complement their proteins. For
example, cereals are very low in the essential
amino acid lysine, while legumes are slightly
deficient in the sulphur-containing amino acids.
By combining these two groups of foods (i.e.
refried beans and corn tortillas), a vegetarian
could provide a mixture of amino acids similar to
that of a complete or high-quality protein food.
Figure 33.1 illustrates different combinations of
incomplete proteins to make a complete protein
as well as demonstrating the fact that when an
animal protein is combined with an incomplete

446 special considerations

protein, the result is a complete, high-quality

protein.

Protein requirements for vegetarian athletes

A major concern for vegetarian athletes is to

make sure that they have consumed enough food

so that their protein requirement will be met. The

total protein intake of athletes consuming a vege-

tarian diet may have to be increased slightly to

take account of the lower digestibility, lower

energy density and lower protein quality of plant

foods consumed. This may lead to problems with

the volume of food to be consumed, because ath-

letes with high energy requirements may find it

difficult to consume sufficient volume of foods to

maintain energy balance on a purely vegetarian

diet. Generally, if vegetarian athletes consume

between 0.8 and 1.7 g protein · kg–1body mass ·

day–1and maintain energy balance, they should

meet their protein requirement for exercise and

health.

Vitamin B 12

Another nutrient that may be low in a vegetarian

diet is vitamin B 12 , especially for those individu-

als on a strict plant-based diet (vegans). Rauma et

al.(1995) studied the vitamin B 12 status of long-

Grains

Rice with sesame seeds Nuts

and

seeds

Milk Milk in legume soups

products Legumes

Peanut butter sandwich

Rice and bean casserole

Sunflower seed

peanutsand

Macaroni and cheese

cereal with milk Fig. 33.1The concept of mutual

supplementation, which is the strategy of

combining two incomplete sources of

protein so that the amino acids in each food

make up for those lacking in other foods.

Such protein combinations are sometimes

called complementary proteins.