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. Reductions in total fat intake were associated with small but statistically significant reductions in total cholesterol, low density lipoprotein, and ratio of total to high density lipoprotein cholesterol, and in systolic and diastolic blood pressures, suggesting a lack of harm on other major cardiovascular risk factors.
Metaregression suggested that greater reduction in total fat intake and lower baseline total fat intake were associated with greater relative weight loss in studies with a total fat intake at baseline of between 28% and 43% of energy, but that any reduction in total fat will be reflected in some weight reduction relative to control. Longer study duration was not associated with a reduction in the degree of weight loss in studies of 0.5 to over eight years. Data on weight were taken from the latest reported time in each trial, which was at seven year follow-up for the single largest study, the Women’s Health Initiative, which included over half of the participants in the systematic review.
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.
Cohort data in adults suggested either no relation between percentage of energy from total fat at baseline and weight change over one or more years, or a positive relation (in a third of comparisons). Given the strength of evidence from randomised controlled trials of a consistent effect of reducing total fat intake on weight, the general lack of association found in cohort studies is surprising. However, this may be due to the relative insensitivity of the instruments used to measure total fat intake (most studies used food frequency questionnaires, two used some form of dietary recall, although one of these was a single 24 hour recall, and one used a seven day weighed intake), the small size of the relation being sought, and the confounding effect of dieting behaviour that is common in the populations studied.
The small amount of data from the randomised controlled trials (one from Greece, 191 participants) and cohort studies (three cohorts from the United States, 1337 participants) in children and young people was confirmatory of a relation between total fat intake and subsequent weight change.
Strengths and weaknesses of the review
Strength of evidence is discussed according to the GRADE headings of risk of bias, inconsistency, indirectness, imprecision, and other factors (including dose-response).15
Although most of the randomised controlled trials in adults were unblinded and randomisation was rarely well enough described to assess allocation concealment, results from these trials were remarkably consistent. Sensitivity analyses removing randomised controlled trials without clear allocation concealment did not lose the statistically significant weight loss in the low fat arm, and neither did running fixed (rather than random) effects meta-analysis or removing randomised controlled trials with attention bias favouring those in the low fat arm, or those with other interventions alongside the reduction in fat intake. The consistent weight loss was despite the fact that none of the randomised controlled trials included intended to alter weight in either arm, and reporting bias seemed unlikely. Given the consistency and strength of the data from the randomised controlled trials in adults, the assessment of effect size and risk of bias for GRADE assessment are based on data from randomised controlled trials alone, and the risk of bias for effects in adults was low. In children the risk of bias was based on only one randomised controlled trial (191 young people aged 12-13 years) and three cohort studies (1337 people aged 3-19 years), all with flaws, so that the risk of bias was moderate.
The effects in the randomised controlled trials in adults were remarkably consistent—in almost every trial, participants with lower total fat intakes were lower in weight (on average) at the study end than participants eating a higher percentage of total fat (or gained less weight or lost more weight). The only inconsistency (where heterogeneity arose) was in the size of this effect. The heterogeneity was partly explained in subgroups and metaregression by the degree of reduction of fat intake and by the level of fat intake in the control group, together explaining 58% of the variance between studies. The reduction in weight in those on lower fat diets was seen in very different populations and from six months to several years. It was still present when we excluded trials that gave additional support, time, or encouragement to the low fat arms and included trials that delivered additional dietary interventions (on top of the change in dietary fats). Inconsistency was not considered a problem.
In children and young people the single randomised controlled trial and two of three cohort studies suggested that higher total fat intake was associated with more weight gain or higher body mass index over 1-2 years. However, the number of included studies were insufficient to assess the possibility of small study bias or to formally assess heterogeneity.
All the randomised controlled trials (in adults and young people) directly compared (and randomised participants to) lower versus higher fat intake and measured absolute or changes in body fatness outcomes. There was only indirectness in extrapolating effects to developing or transitional countries.
Imprecision in the data from adults was unlikely, as over 14
000 participants were included in trials of at least six months’ duration and effect sizes were highly statistically significant. Although there was no imprecision for adults, imprecision was high in data from the children (although not quantifiable), and pooling was not possible.
In adults there was evidence of a dose-response gradient between total fat intake and change in weight and little evidence of publication bias. In children the number of studies were insufficient to assess either. Metabolic studies would be required to determine mechanisms of action.
Comparison with other studies
The question of whether dietary fat intake affects body weight has been investigated in several non-systematic reviews83
as well as some systematic reviews. One systematic review (37 trials, 9276 participants to 1997) that assessed a different set of randomised controlled trials, including those of only three weeks or more duration and some that aimed to reduce weight, found that for each decrease of 1% of energy from total fat there was a 0.28 kg reduction in body weight.5
Another systematic review assessed randomised controlled trials that compared ad libitum low fat diets with usual or moderate fat intake for at least two months to 1998.10
The review included 16 trials (1728 participants) for up to one year, some of which aimed at weight loss, and found that each reduction of 1% energy as total fat resulted in a 0.37 kg reduction in body weight. Trials that assessed short term effects or aimed to reduce weight in the low fat arms may well have overstated any effect size. Our review only included unconfounded randomised controlled trials of at least six months’ duration (33 trials and 73
589 participants) and excluded trials that aimed to reduce body weight. We found slightly more modest but clear effects on body weight, a reduction of 0.19 kg for each 1% energy from total fat intake. We found no systematic reviews that assessed effects in children or young people.
Meaning of the findings
Lowering total fat intake in adults compared with not lowering fat intake was associated with reductions in body weight, body mass index, and waist circumference. These effects were found in studies of more than eight years’ duration, with baseline total fat intakes of 28% to 43% of energy, and in healthy adults and those with risk factors or current illness. However, these relations have not been tested in low or middle income countries. Although the evidence was slightly less strong in children, diets higher in total fat seem to be associated with higher body weight, body mass index, and waist circumference in both adults and children than diets lower in fat.
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. A systematic review of 57 cohort studies found that over a body mass index of 25 kg/m2
(>60% of UK adults have a body mass index >25) each additional 5 kg/m2
was associated with 30% greater total mortality (with contributions from deaths related to vascular, renal, hepatic, and respiratory disease, cancer, and diabetes).85
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 26.12 to 25.60, a reduction of 0.52, which would be associated with a reduction in total mortality of 3%.
Implications for public health policy and research
Although it may be difficult for populations to reduce total fat intake, attempts should be made to do so, to help control weight, where mean total fat intake is 30% or more of energy. For populations where the mean total fat intake is below 30% of energy then interventions to restrict rises in total fat intake to over 30% of energy may help to avoid obesity. High quality trials are needed to examine the effect on body weight of reducing fat intake in developing or transitional countries with total fat intakes greater than 30% of energy, and of preventing total fat intake rising above 30% of energy in countries with total fat intakes of 25-30% of energy. High quality trials are also needed in children.
What is already known on this topic
- The ideal proportion of total fat in the human diet is unclear but it would help to understand the relation between total fat intake and body weight
- Two systematic reviews may have overstated relations in adults by including studies of short duration and those that encouraged weight reduction
- No systematic reviews have assessed the relation in children or young people
What this study adds
- This systematic review found that lowering the proportion of energy intake from total fat was associated with lower body weight (by 1.6 kg), body mass index, and waist circumference in adults
- Each 1% decrease in energy from total fat resulted in a 0.19 kg reduction in body weight, compared with not altering total fat intake, in populations with 28-43% of energy from total fat, and in studies of six months to over eight years
- Evidence in children and young people is more limited, but supports the relation in adults