The WHI Dietary Modification Trial was designed in 1990, when researchers were asking many questions about whether high-fat diets or low-fat diets were better for people’s health, said Barbara Howard, a senior scientist at the Georgetown University Medical Center in Washington D.C. In a study published in the Journal of the American Medical Association (JAMA) in 2006, Howard took data from the original WHI Dietary Modification Trial and looked to see whether the two diets had different effects on weight loss.
In the WHI, 40 percent of the participants were encouraged to reduce their fat intake so that only 20 percent of their total daily calories came from fat. (At the study’s start, all of the participants had reported that at least 32 percent of their daily calories had come from fat.) The women were encouraged to increase their daily servings of fruits, vegetables and whole grains, and attended group sessions where they got advice about cutting fat. However, because the trial wasn’t designed with weight loss in mind, these women were not encouraged to cut calories.
The other 60 percent of the women went into the control group. They were given a copy of the U.S. government’s Dietary Guidelines for Americans and some additional educational material, but that was it. [Low-Fat Diet: Facts, Benefits & Risks]
The researchers monitored the diets of all of the participants during the study through questionnaires, and measured the women’s height and weight at annual checkups.
It turned out that the women in the low-fat-diet group lost a little bit of weight over the course of the study period, and they maintained their weight loss, said Howard, who is also a scientist at the MedStar Research Institute, a non-profit healthcare system of hospitals and clinics in the Washington D.C. area.
The researchers found that the women in the low-fat-diet group lost about 5 lbs. (2.2 kilograms) during the first year of the study, and then maintained a lower average weight than the control group over the rest of the study period.
The researchers concluded that there was a “clear relationship” between the change in the women’s fat intake and their weight, they wrote in their study. The questionnaires showed that the women in the low-fat-diet group increased their daily intake of fiber, fruits, vegetables and whole grains, and decreased their daily intake of total fat, saturated fat and unsaturated fat. [Which States Are Eating Their Fruits and Veggies?]
In particular, the women who cut the most fat lost the most weight, the researchers found.
Also, the women in the low-fat-diet group who ate more fruits and vegetables also lost more weight than those whoate smaller amounts of fruits and vegetables, the researchers found. The same went for fiber: The women who ate more fiber lost more weight than the women who ate less fiber.
Another long-running clinical trial designed to compare the effects of different diets on weight loss was carried out in Israel: the Dietary Intervention Randomized Controlled Trial, or DIRECT, study.
But although that large, high-quality study found a relationship between a low-fat diet and weight loss, other studies conducted since have found that a low-fat diet is no more effective than other types of diets in helping people lose weight. It’s important to note that the women in the study who switched to a low-fat diet didn’t replace the fat in their diet with white bread and other refined carbohydrates, Howard said. In other observational studies, researchers have shown that when “high-carb” means sugar and refined carbohydrates, people don’t lose weight, she noted.