Q: Thanks for
your very informative column on the nutritional value of
beans. One of the things you said was that beans are high in
protein. Many years ago I read that beans were an incomplete
protein, so you should eat them with corn, and then the two
together made a complete protein.
Some time later
I read that you didn’t have to eat them at the same meal; if
you had one of them at one meal, and the other at a different
meal the same day, that still made a complete protein.
Does either of
these ideas hold in current thinking, or are beans now
considered a complete protein?
— Thanks, Ann
Ann. Beans are incomplete proteins because they are low in one
key amino acid (amino acids are used by the body to build
protein) called methionine. Cereal grains — including corn,
rice and wheat — are high in methionine yet lack another
amino acid found in beans.
So if you eat
beans with a grain such as corn or rice, your body gets all
the essential amino acids it needs to build quality protein.
It’s almost romantic ... two incomplete proteins become
complete when they get together.
And it’s also
true that we don’t necessarily have to eat these
complementary proteins at the same meal to get the benefit.
What’s most important, especially for vegetarians, is to eat
a variety of foods throughout the day. Our bodies are smart
enough to pick and choose the essential amino acids they need
— even if they aren’t provided at the same time.
On the other
hand, animal-based foods such as eggs, milk, cheese, fish,
poultry and meat are “complete” protein foods because they
provide all the essential amino acids to build protein in the
body. Let’s say you have eggs for breakfast and a bowl of
beans for lunch. Your body can use the complete protein (eggs)
to enhance the protein quality of the beans, even if they are
eaten at different times.
Beans may vary
in flavor, size, color and shape, yet their high nutrient
content is remarkably similar, says Susan Raatz, PhD Research
Nutritionist for the USDA Human Nutrition Research Center. One
bean, however, stands out as the only complete protein in the
And don’t be
afraid of the carbohydrates in beans; almost half their carbs
are in the form of dietary fiber. Because of this, beans are
considered a low glycemic food — they don’t tend to spike
blood sugars (unless you eat the whole pot). One cup of cooked
beans, for example, provides more than half the dietary fiber
we need for the whole day.