Nutrition - Prevention

Carbohydrates and the glycemic Index

The glycemic index measures how fast and how high blood sugar rises after eating food that contains carbohydrates. White bread, for example, is digested almost immediately to sugar and causes the blood glucose to rise rapidly, and thus is classified as having a high glycemic index. Brown rice is digested more slowly to sugar, has a low glycemic index, and results in a lower and more sustained rise in blood glucose. Diets rich in high-glycemic-index foods have been linked to an increased risk for both diabetes and heart disease. Conversely, lower glycemic index foods have been shown to help control type 2 diabetes.

  • High glycemic index foods include sugar, potatoes, and refined foods such as white bread, white rice, white spaghetti and cereals (i.e., Corn Flakes, Cheerios).
  • Low glycemic index foods include legumes, whole fruits, oats, bran, and whole-grain cereals.

One of the most important factors that determine a food's glycemic index is how highly processed its carbohydrates are. Processing carbohydrates removes the fiber-rich outer bran and the vitamin-rich and mineral-rich inner germ, leaving mostly the starchy endosperm. Finely ground grain is more rapidly digested and has a higher glycemic index than coarsely ground grain. Other factors that influence how quickly the carbohydrates in food are digested, release sugar and raise glucose include:

  • Fiber content. Fiber protects the starchy carbohydrates in food from rapid digestion, slowing the release of carbohydrate sugar molecules into the bloodstream.
  • Ripeness. Ripe fruits and vegetables tend to have more sugar than unripe ones, and therefore tend to have a higher glycemic index.
  • Type of starch. Starch comes in many different (linear and branched) forms, and some release sugar molecules more easily than others. The starch in potatoes, for example, is digested and absorbed into the bloodstream relatively quickly.
  • Fat and acid content. The more fat or acid a food contains, the slower its carbohydrates are converted to sugar and absorbed into the bloodstream.

There is one important item that is not disclosed in a food's glycemic index: the relative amount of carbohydrate in a given food. For example, watermelon is a sweet-tasting fruit with a high glycemic index. However, a slice of watermelon has only a small amount of carbohydrate per serving and (as the name suggests) is made up mostly of water. Thus, the glycemic index alone may not tell us everything about a food's impact on blood sugar (glucose) levels. A new classification of food, called the glycemic load, takes into account both the amount of carbohydrate in the food and the impact that carbohydrate has on the blood glucose level. A food's glycemic load is determined by multiplying its glycemic index by the amount of carbohydrate it contains. Although seemingly complicated, the basic message is simple: whenever possible, replace highly processed grains, cereals and sugars with minimally processed whole-grain products.

When Sugar Management Goes Away

Digestible carbohydrates are broken down in the intestine into their simplest form, sugar, which then enters the bloodstream as glucose. Insulin is a hormone secreted from the pancreas in response to the rising blood glucose and helps lower blood glucose by depositing it into body cells for energy or storage. In some people, this cycle doesn't work properly. People with type 1 diabetes do not make enough insulin to respond to the rising blood glucose. Individuals with type 2 diabetes have tissue cell insulin resistance, which prevents lowering of the blood glucose. Genetic makeup, a sedentary lifestyle, being overweight, and eating an unhealthy diet can all promote insulin resistance. Data from the Insulin Resistance Atherosclerosis Study suggests that substituting whole grains for refined grains in the diet can improve insulin sensitivity.

Health Effects of Eating Fiber

Fiber appears to reduce the risk of developing various conditions, including heart disease, diabetes, colon diverticular disease and constipation. However, fiber may have little, if any, effect on preventing colon cancer.

Fiber and colon cancer

Well-designed studies have failed to show a link between fiber and prevention of colon cancer. One of these - a Harvard study - followed over 80,000 female nurses for 16 years and found that dietary fiber was not strongly associated with a reduced risk for either colon cancer or colon polyps (a precursor to colon cancer).

Fiber and heart disease

Dietary fiber intake has been linked to a lower risk of heart disease in a number of large studies. In one study - a Harvard study of over 40,000 male health professionals - a high total dietary fiber intake was associated with a 40% lower risk of coronary heart disease. Cereal fiber seemed particularly beneficial. A related Harvard study of female nurses produced similar findings.

Fiber and type 2 diabetes

Type 2 diabetes is the most common form of diabetes. It is characterized by sustained high blood sugar (glucose) levels and resistance to the effects of insulin to lower glucose. Harvard studies of male health professionals and female nurses both found that a diet low in cereal fiber and rich in high glycemic index foods more than doubled the risk of type 2 diabetes compared to a diet high in cereal fiber and low in high glycemic index foods. Fiber intake has also been linked with the metabolic syndrome, a constellation of factors that increases the chances of developing heart disease and diabetes. Factors of the metabolic syndrome include high blood pressure, high insulin levels and elevated blood glucose, excess weight (especially around the abdomen), high levels of triglycerides and low levels of HDL cholesterol. Several studies suggest that consuming a higher intake of fiber may help prevent metabolic syndrome and the risk for diabetes.

Fiber and diverticular disease

Diverticulitis is an inflammation of the intestine and is one of the most common age-related disorders of the colon. In North America, this painful disease is estimated to occur in one-third of individuals over age 45 and in two-thirds of those over age 85. Among male health professionals in a long-term follow-up study, eating dietary fiber, particularly insoluble fiber, was associated with about a 40% lower risk of diverticular disease.

Fruits, Vegetables and Health

There is compelling evidence that a diet rich in fruits and vegetables can lower blood pressure and cholesterol, and decrease the risk of heart disease and stroke. There is also data to suggest that eating fruits and vegetables likely provides protection against certain types of cancer.

Fruits and Vegetables and Cardiovascular Disease

The largest and longest study to date, done as part of the Harvard-based Nurses' Health Study and Health Professionals Follow-up Study, found that the higher the average daily intake of fruits and vegetables, the lower the chances of developing cardiovascular disease. This study included almost 110,000 men and women whose health and dietary habits were followed for 14 years. Compared with those in the lowest category of fruit and vegetable intake (less than 1.5 servings a day), those who averaged more than 8 servings a day were 30% less likely to have had a heart attack or stroke.

Fruits and Vegetables, Blood Pressure and Cholesterol

Hypertension is a primary risk factor for developing heart disease or stroke. One of the most convincing associations between diet therapy and lower blood pressure was found in the Dietary Approaches to Stop Hypertension (DASH) study. The study diet was rich in fruits, vegetables and low-fat dairy products and also restricted the amount of saturated and total fat consumed. The upper (systolic) blood pressure fell by about 11 mm Hg and the (lower) diastolic blood pressure declined by almost 6 mm Hg with diet therapy; these are decreases similar to that achieved by medications.

In the National Heart, Lung, and Blood Institute's Family Heart Study of 4,466 men and women, those with the highest consumption (more than 4 servings/day) of fruits and vegetables had significantly lower levels of LDL cholesterol. Eating more fruits and vegetables can help lower cholesterol, but how this occurs is still unknown. It is possible that eating more fruits and vegetables (1) results in eating less meat and dairy products, and thus less cholesterol and saturated fat, and (2) soluble fiber in fruits and vegetables may help block the absorption of cholesterol ingested.

Fruits and Vegetables and Cancer

Numerous early case-control studies revealed what appeared to be a strong link between eating fruits and vegetables and protection against cancer. However, it is possible that these results may have been skewed by problems such as recall bias and selection bias. Data from cohort studies that follow large groups of initially healthy individuals for many years have not consistently shown that diets rich in fruits and vegetables prevent cancer. For example, the 14-year Nurses' Health Study and Health Professionals Follow-up Study reported men and women with the highest intake of fruits and vegetables (8or more servings/day) were just as likely to have developed cancer as those who ate only 1.5 servings daily.

It may be possible that fruits and vegetables protect against certain types of cancer. The International Agency for Research on Cancer, of the World Health Organization, recently reported that there is limited evidence for a cancer-preventive effect of total consumption of fruit and vegetables for cancers of the mouth and pharynx, esophagus, stomach, colon-rectum, larynx, lung, ovary, bladder and kidney. The International Agency for Research on Cancer acknowledges that specific fruits and vegetables may also protect against certain types of cancer. For example, lycopene (red pigment) in tomatoes may help protect men against prostate cancer (Health Professionals Follow-up Study).


Are dietary supplements necessary in otherwise healthy adults? The answer depends on eating patterns, lifestyle habits and other factors such as age. A standard multivitamin supplement doesn't come close to making up for an unhealthy diet. A multivitamin provides a dozen or so of the vitamins known to maintain health, a mere shadow of what's available from eating a healthy diet. However, there's good evidence that taking a daily multi-purpose vitamin may be reasonable even in otherwise healthy adults. A standard multivitamin can ensure the Recommended Daily Intake (RDI) of vitamins and minerals needed for health. Vitamin supplementation should not exceed more than 100% of the RDI value. Dietary supplements may be appropriate if any of the following exist:

  • Poor dietary habits - Many people don't receive all of the nutrients they need from their diet because they either cannot (or do not) eat enough, or an adequate variety, of healthy foods. Eating less than five total servings of fruits and vegetables daily may not provide all of the vitamins and minerals the body needs. Also, eating only once or twice a day may limit the number and variety of servings from the various food groups.
  • Consuming less than 1,200 calories a day - Low-calorie diets limit the types and amounts of foods, and nutrients, consumed. Unless monitored by a doctor, a low-calorie diet isn't usually recommended.
  • Vegetarian diets - Vegetarians may not consume enough calcium, iron, zinc, vitamin B-12 or vitamin D. However, these nutrients are naturally present in non-meat sources, such as fortified soy products, green leafy vegetables, legumes, whole-grain products and nuts.
  • Medical conditions affecting nutrient absorption - Food allergies or intolerance (such as protein or dairy products) may limit nutrient intake. Diseases of the liver, gallbladder, intestines or pancreas may cause malabsorption or maldigestion of nutrients.
  • Postmenopausal women - After menopause, women are at an increased risk of bone loss due to estrogen deficiency. Postmenopausal women should ingest 1000-1500 milligrams elemental calcium in divided doses daily. Vitamin D guidelines are 400 international units (IU) for young postmenopausal women, 600 IU for women over 70 years, and 800 IU in the setting of osteoporosis.
  • Heavy menstrual bleeding - Additional iron may be needed to treat anemia and replace iron depleted by blood loss.
  • Pregnancy - Requirements for calcium, folate and iron all increase during pregnancy. Folate is needed very early in pregnancy to help protect the baby against neural tube birth defects, such as incomplete closure of the spine (spina bifida). Iron can help prevent fatigue by treating iron deficiency anemia. The National Academy of Sciences recommends 1000 mg/day elemental calcium for women 19-50 years and lactating women. It's important to take these supplements during planning of pregnancy, before becoming pregnant.
  • Smoking - Tobacco decreases the absorption of many vitamins and minerals, including vitamin C, folate, magnesium and calcium. Dietary supplements won't make up for the major health risks caused by smoking, and it is safest to avoid all tobacco products.
  • Excessive alcohol intake - Long-term excessive alcohol intake, as a substitute for food calories, may result in a diet lacking in essential nutrients. Excessive drinking is defined as more than two drinks a day for men and more than one drink a day for women. Taking dietary supplements will not make up for the major health risks caused by excessive alcohol consumption.

Choosing and using supplements

There are upper limits for intake of vitamins and minerals, and more is definitely not always better. Randomized trials of vitamin C, vitamin E, and beta-carotene have not revealed much in the way of protection from heart disease, cancer or age-related eye diseases. In addition, over-consumption of one vitamin has the potential to mask a deficiency of another. For instance, (1) if excess folic acid is taken, it can mask a vitamin B12 deficiency, and (2) excess zinc supplementation can lead to a deficiency in copper and can cause problems with red or white blood cell production. When taking a vitamin or mineral supplement, consider these factors:

  • Look for “USP” on the label. This ensures that the supplement meets the standards for strength, purity, disintegration and dissolution established by the testing organization U.S. Pharmacopeia (USP).
  • Read labels carefully. Product labels list the serving size and the active ingredients and nutrients per serving (for example, capsule, packet or teaspoonful).
  • Avoid supplements that provide “megadoses” of individual vitamins or minerals. In general, choose a brand name multivitamin-mineral supplement (i.e., Centrum, One-A-Day, or others) that provides about 100% of the Daily Value (DV) of all the vitamins and minerals. The exception to this is calcium as most calcium-containing vitamin supplements don't provide 100% of the DV for calcium.

Beware of gimmicks. Synthetic vitamins are usually the same as so-called "natural" vitamins, but "natural" vitamins usually cost more. Added herbs, enzymes or amino acids mostly add only cost, and some herbs can interact negatively with certain prescribed medications.