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Carbohydrates
There is probably no greater
debate today than the one over the role of carbohydrates in our diets. Low-carb
diets such as the Atkins diet have captured the public's attention to an
extent that few other weight-loss plans have. As a result, more and more
people have come to believe that carbohydrates are inherently bad. But
that's not entirely the case. Starches and sugars are our major source of
energy. Fiber, another form of carbohydrate, also has significant health
benefits.
Almost all of the starches and
sugars that humans burn for energy come from plants; the only major
exception is lactose, the sugar in milk. In effect, each plant is a complex
food factory that takes water from the soil, carbon dioxide from the air,
and energy from the sun to make glucose, a simple sugar that is later
converted into starch. As the plant develops and grows, it also makes
various vitamins, minerals, and other phytochemicals, as well as some fat
and protein. Consequently, we can get our carbohydrates and most of the
other nutrients needed to sustain us from the thousands of different grains,
seeds, fruits, and vegetables that can be grown.
Carbohydrates are classified
according to their chemical structure and digestibility; they are divided
into two groups: simple and complex. Simple carbohydrates, or sugars, can
generally form crystals that dissolve in water and are easily digested.
Naturally occurring sugars are found in a variety of fruits, some
vegetables, and honey. Processed sugars include table sugar, brown sugar,
and molasses.
Complex carbohydrates have a
range of textures, flavors, colors, and molecular structures. Composed of
complex chains of sugars, these carbohydrates are further classified as
starches or fiber. Our digestive system can break down and metabolize most
starches, which are found in an array of grains, vegetables, and some
fruits. Our digestive system, however, lacks the enzymes that are needed to
break down most fiber, including cellulose and other woody parts of the
plant skeleton, and pectin and other gums that hold plant cells together.
But dietary fiber is still important because it promotes smooth colon
function and may help prevent some types of cancer, heart attacks, and other
diseases.
Energy food
Our body metabolizes simple
carbohydrates and starches into glucose, or blood sugar, the body's primary
source of fuel. Carbohydrates are high-quality fuels because – compared to
proteins or fats – little is required of the body to break them down in
order to release their energy.
Glucose, the only form of
carbohydrate that the body can use immediately, is essential for the
functioning of the brain, nervous system, muscles, and various organs. At
any given time, the blood can carry about an hour's supply of glucose. Any
glucose that is not needed for immediate energy is converted into glycogen,
a large molecule composed of a chain of glucose units, which is store in the
liver and muscles; when necessary, the liver turns the glycogen back into
glucose. the body can store enough glycogen to last for several hours o
moderate activity.
Glycemic index (GI) researchers are learning that the rate at which
carbohydrate-rich foods are digested and absorbed into the bloodstream also
affects health. The rate at which a food causes blood sugar to rise can be
measured and assigned a numerical value. This measure is referred to as the
food's glycemic index. Foods with a low GI such as pumpernickel bread, rye
bread, brown rice, bulgur, oatmeal, lentils, yams, apples, pears, and yogurt
take longer to digest and cause a slower, more gradual rise in blood sugar.
This means that energy is released more slowly, leading to more consistent
energy levels. Low-GI foods are better for blood sugar control in diabetics
and may help with weight loss. The carbohydrates in high-GI foods such as
white bread, white rice, mashed potato, corn flakes, and watermelon are more
quickly absorbed and so provide a quicker source of energy. for active
people high-GI foods can be a source of quick energy to aid short-duration
sports performance and recovery, while lower-GI foods are better for
endurance.
When glucose reserves run low, the body turns first to protein and then to
fat for conversion into glucose. Burning protein, however, robs the body of
lean muscle tissue. In addition, if the body has to burn fat in the absence
of carbohydrates, toxic by-products called ketones are released; these can
lead to a potentially dangerous biochemical imbalance.
Complex carbohydrates The
human diet worldwide is based on complex carbohydrates. Populations that eat
a higher-carbohydrate, low-fat diet generally enjoy good health. Vegetarian,
Mediterranean, and Asian diets typically provide a high percentage of
calories from complex carbohydrates in foods such as whole grains, lentils,
beans, fruits, and vegetables. In North America, too much of the
carbohydrate intake is in the form of simple sugars due to the high
consumption of refined and processed foods. Another factor has been the
proliferation of low-fat foods. Consumers often assume that these are also
low-calorie foods. In many cases, fats have been replaced by carbohydrates
with no great saving in calories. The many low-fat, high-carbohydrate foods
available, including low-fat cookies, cakes, muffins, baked chips, and
oversized bagels are all adding to expanding waistlines.
How much do you need ? In
2002, a joint American-Canadian expert report provided a set of reference
values for nutrient intakes for healthy Americans and Canadians. It suggests
that adults get 45 to 65 percent of their caloric intake from carbohydrates.
This translates to roughly 225 to 325 g of carbohydrates each day for a
2,000-calorie diet. Both children and adults should consume at least 130 g
of carbohydrates a day. This is based on the minimum amount needed to
produce enough glucose for the brain to function.. This amount is easily
exceeded in the average North American diet. The problem is that the excess
usually comes from refined carbohydrates.
Although refined carbohydrates, such as white flour and white rice, are just
as good energy sources as whole-wheat flour and brown rice, processing
removes many essential nutrients, including the B vitamins, iron, and other
minerals, as well as dietary fiber. The best approach is to build a diet
around whole or lightly processed grains, legumes, beans, and raw or
slightly cooked vegetables and fruits.
Carb loading Nutrition can
have a significant impact on athletic performance and vice versa. Regular
exercise increases the body's ability to utilize glucose efficiently and to
store glycogen in muscle tissue. Thus, the fitter you are, the greater your
ability to store the extra glycogen that is needed for endurance events,
such as running a marathon or cross-country skiing. that's why carbohydrates
are the preferred fuel for most sports.
Special concerns
Carbohydrates can be worked into almost any diet, but certain diseases may
need adjustments. Diabetics must manage the total amount and type of carbs
eaten at each meal and snack. Contrary to popular belief, sugar does not
cause diabetes, nor do diabetics have to completely avoid sugar.
Those with heart disease need to emphasize high-fiber complex carbohydrates
in their diet. Soluble fiber, found in oat bran and fruit pectin, helps
lower cholesterol and plays an important role in preventing atherosclerosis,
the buildup of fatty deposits in coronary arteries and other blood vessels.
Cancer patients are often advised to increase their carbohydrate intake and
decrease fat intake, especially if they have cancers of the breast, colon,
uterus, prostate, or skin. Evidence suggests certain fats may encourage
tumor growth. |
