Karen Collins, MS, RD, CDN
American Institute for Cancer Research
After years of being unsure whether cutting back on dietary fat is the key to weight loss, the public is now more confused by the recently released results of the Women's Health Initiative (WHI). This study seems to suggest that fat in the diet doesn't matter at all. Justifiably, many people are wondering how much fat consumption does affect weight control.
One part of the WHI study looked at how the weight of 19,000 post-menopausal women was changed when they were told to follow a lowfat diet. The women were supposed to reduce their dietary fat to only 20 percent of their calories. Most women did not get very close to that low goal, but they did progress from diets that averaged 38.8 percent fat (rather high) at the beginning to diets with 29.8 percent fat (which is considered a lowfat diet) at the end. To see if fat consumption alone can influence weight, these women were instructed to eat more carbohydrates as they cut back on fat to try to maintain their initial weight. At the beginning of the study, 74 percent of the women were overweight or obese.
Because of its setup, the study was able to answer two separate questions. First, does eating less fat for better health lead to weight gain if you eat more carbohydrates? The answer turned out to be no. You can cut back on fat without fear it will lead to weight gain. However, to achieve health benefits you should focus on decreasing saturated and trans fat consumption.
Second, can people lose weight by cutting back on fat without changing their exercise habits or trying to reduce calories? For the group of women over age 50 in this study, the answer was no. Like these women, if you cut fat in your diet but substitute other foods to maintain the same calorie intake, you shouldn't expect to lose weight.
Women in the WHI study on the lowfat diet reportedly reduced their calorie intake by an average of about 360 calories a day, which should have led to a significant weight loss. However, these women lost less than two pounds in seven years. Perhaps, the women inaccurately reported what they ate. Or, perhaps, their drop in calorie intake was only enough to prevent weight gain. To induce weight loss, they may have needed to do more, since we all tend to become less active with age and our metabolisms tend to slow.
For people already at a healthy weight who want to avoid the usual adult weight gain that increases the risk of several cancers, the modest fat and calorie reduction that women on a lowfat diet achieved in this study may be enough. However, other studies show that many adults need 45 to 60 minutes of exercise a day to avoid overweight. The women in the WHI study had a wide range of activity levels, but they averaged only the equivalent of walking for 30 minutes at a moderate pace, six days a week.
For overweight people who want to control their weight, this study suggests you should do more than adopt a lowfat diet. Working in blocks of activity that add up to an hour a day seems to help.
However, creating enough of a change in your balance of calories consumed and burned to produce weight loss almost always requires eating fewer calories. Eating less fat, without increasing calories from other sources, is one way to do this. If you currently eat few vegetables, fruits, whole grains and beans, eating more of these foods may help fill you up enough that you can cut down on foods more concentrated in calories from fat and sugar. But in today's world of enormous servings and constant food availability around the clock, studies suggest that without consciously cutting back on your portion sizes and unnecessary eating, simply eating more healthy food like vegetables and fruits is unlikely to cause weight loss.
Future studies can test whether the conclusions of the WHI study apply to men and younger women. But it's reasonable to assume that increasing activity and eating fewer calories will help everyone.