Karen Collins, MS, RD, CDN
American Institute for Cancer Research
Eating more vegetables and fruits, sources of so many antioxidant vitamins and phytochemicals, seems like a powerful step to lower risk of cancer and other health problems. However, large population studies have shown mixed results about such benefits. Now research suggests that this may be due to different parts of our diet interacting and altering the effect of any single change in our eating habits.
The vitamin C in fruits and vegetables is water-soluble, so it is unaffected by dietary fat. But beta-carotene and the other carotenoids, as well as vitamin E, are fat-soluble and require some fat for best absorption from the digestive tract. However, research suggests that only three to five grams of fat may be needed. That amount is in one teaspoon of oil, three ounces of poultry or other lean protein, or a tablespoon of nuts eaten along with the vegetables or fruit.
Researchers at the University of Michigan recently decided to see how results of boosting vegetable and fruit consumption might vary depending on level of fat consumption. About 100 women were followed for a year. Some made no changes at all in their eating. Others increased vegetables and fruits from less than four servings a day to about eleven, but kept dietary fat at their usual level of more than 30 percent of calories. A third group kept their usual low intake of vegetables and fruits, but reduced dietary fat to an average of about 16 percent of calories (quite low). A fourth group both decreased fat consumption and increased vegetables and fruits.
Eating more produce raised consumption of nutrients like vitamin C and beta-carotene substantially. More importantly, blood levels of vitamin C, beta-carotene and another carotenoid, alpha-carotene, also rose. These increases were not prevented by reduced dietary fat.
The study does suggest another possible problem with low-fat diets. Alpha-tocopherol is the form of vitamin E most widely studied for its possible antioxidant benefits. It is found in most vitamin supplements and fortified foods, and was not affected by reducing dietary fat. But consumption of gamma-tocopherol, a form that seems to be anti-inflammatory and may even stimulate self-destruction of cancer cells, dropped by more than 50 percent on the low-fat diets.
To get health benefits from low-fat diets, make sure that saturated fat is the type of fat you cut back. Also, be sure that you do include foods that provide both alpha- and gamma-tocopherol. We get alpha-tocopherol from olive and canola oil, fortified cereal, nuts, whole grains and dark green vegetables. We get gamma-tocopherol from canola oil, pistachios, pecans, walnuts and peanuts, as well as a small amount from avocados.
A general reminder is to make sure you are not eating too many calories if you are adding these foods to your diet. Increasing the amounts of fruits and vegetables you eat can promote weight control, but only if they replace higher fat foods like ice cream, meat and chips – to help you consume fewer calories. But if fruits and vegetables are simply added to what you already eat, total calories won?t significantly drop and weight loss should not be expected.