Effect Of Reducing Total Fat Intake On Body Weight: Systematic Review And Meta-Analysis Of Randomised Controlled Trials And Cohort Studies, British Medical Journal, 6 December 2012
It analyzed 33 randomised controlled trials (73,589 participants) and 10 cohort studies (107,624 participants). Not a small undertaking. Note that the bulk of the data was from controlled intervention trials, the gold standard of studies, "the best scientific way of testing cause and effect," say the authors.
They found that a low-fat diet was associated with:
- Lower body weight
- Less weight gain
- Lower body mass index (BMI)
- Smaller waist circumference
- Lower LDL cholesterol
- Lower total cholesterol
- Lower systolic and diastolic blood pressure
"Meta-analysis of 33 randomised controlled trials in adults suggested that diets lower in total fat on average reduced body weight by 1.6 kg, body mass index by −0.51, and waist circumference by 0.3 cm. These effects were from randomised controlled trials in which weight loss was not an intended outcome, suggesting that they occur in people eating normal diets and the direction of effect on weight was consistent regardless of subgroups or sensitivity analyses."They excluded studies where weight loss was the goal, making their findings all the more compelling:
"Populations recruited specifically for weight loss studies and interventions intended to result in weight loss would be excluded. This was because they were potentially confounded by the implicit objective of reducing calorie intake to produce weight loss."There was criticism that the effect was minimal. The authors said:
"We disagree that loss of 1.6kg is pointless. The effect on health of an individual reducing his or her body weight by 1.6 kg is likely to be small, but the effects of a whole population doing so would be noticeable. In a man of average height (1.75 m) and weighing 80 kg a loss of 1.6 kg will reduce body mass index from 26.1 to 25.6, a reduction of 0.5. Moving the whole population distribution (remember, we are talking about a sensible way to eat for the whole population, not an individual diet) to the left by 0.5 BMI units would have a serious impact on risks of weight-related illnesses including respiratory problems, infertility, diabetes, some cancers, some forms of arthritis and high blood pressure."But that 1.6 kg was an average. Many experienced greater losses. The lower the fat intake, the lower the weight:
"There was evidence of a dose-response gradient between total fat intake and change in weight."The mechanism? It may be that eating less fat leads to eating fewer calories. May be:
"Although further metabolic studies may reveal a mechanism of action, most studies that reported energy intake suggested lower energy intake in the low fat group than in the control or usual fat groups, and subgroups suggested that a greater degree of energy reduction in the low fat group (compared with control) was related to greater weight loss. This suggests that weight reduction may be due to reduced energy intake in those on low fat diets, rather than a specific effect of the macronutrient composition of the diet."There's a lot of good literature that says macronutrients do matter. I think they do. These authors say that those who ate low-fat ate more carbohydrates (and a little more protein). There is a profound and complex impact on the body from carbohydrates. The effects of resistant starch and fiber are just two carbohydrate-based lines of study that might have contributed to the the weight loss seen above.