There is evidence that ultra-low-fat diets are the most effective for weight loss. A
National Institutes of Health study put people in a metabolic ward where they controlled the composition and calorie content of meals. The short-term study compared an ultra-low-fat diet (8% of calories from fat) and a low carbohydrate diet.
Each diet contained an equal number of calories. According to the lead author,
“Calorie for calorie, reducing dietary fat results in more body fat loss than reducing
dietary carbohydrate when men and women with obesity have their food intake
strictly controlled.” The reduced-fat diet led to a roughly 67% greater body fat loss.
The authors of the study did caution that “We did not address whether it would be
easier to adhere to a reduced-fat or a reduced-carbohydrate diet under free-living

Evidence supporting the effectiveness of low-fat diets for weight loss comes from
a meta-analysis of 32 feeding studies with substitution of the same number of carbohydrate calories for fat calories. It found that fat loss was greater with lower fat diets. A similar meta-analysis by Hooper and colleagues concluded that lower
total fat intake does lead to small (3.5 lb.), but statistically significant and clinically
meaningful sustained greater weight loss than other diet interventions. Ornish
notes that the patients in his randomized controlled trial achieved an average weight
loss of 24 pounds in the first year. In a larger study of almost 3,000 patients on
the Ornish lifestyle program in 24 hospitals and clinics, BMI decreased by 6.6%.
Ornish notes that President Bill Clinton lost and kept off more than 20 pounds since
following his whole foods, plant-based diet.

Many additional studies support the effectiveness of low-fat diets for weight loss.
The BROAD study featured semi-weekly classes that encouraged participants to eat
unrestricted amounts of whole, low-fat plant-based foods, fruits, vegetables, whole
grains, and legumes. The overweight study subjects lost an average of 19 pounds at
the end of the three-month study and were down by 27 pounds six months later.

The ultra-low-fat Hawaii traditional food diet featured low-calorie density eating
of whole grains, fruits, vegetables, and beans. The study participants could eat
as much food as they wanted. They actually ate more, but different, food that cut
calories by 40% and lost an average of 17 pounds over three weeks. Other studies
have concluded that a plant-based dietary pattern is the most effective approach for
weight loss. A review of 12 randomized controlled trials (RCTs) found that compared
to non-vegetarian weight-loss diets, those on the vegetarian diets lost the most

Another study of five different diets, vegan, vegetarian, pesco-vegetarian, semi-vegetarian,
and omnivorous, found that those following the vegan and vegetarian diet
plans lost the most weight over the course of the 6-month study. The authors
concluded that vegan diets may result in greater weight loss than other diets.

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