In shining the spotlight on body weight and diet last week, the British Medical Journal looked not just at new research on the links with sugar intake, which I blogged on Monday, but also at fat intake. Fair dos really. A research team did a systematic review and meta-analysis to investigate the relationship between total fat intake and body weight in adults and children. This was both a well-conducted review and one that was able to draw on high quality evidence – marvellous!
They looked for randomised controlled trials (RCTs) and cohort studies comparing lower versus usual total fat intake which assessed the effects on measures of body fatness – weight, body mass index (BMI) or waist circumference after at least six months for trials or a year for cohort studies. RCTs which aimed to reduce weight or using additional medical or lifestyle interventions were excluded. They were able to include 33 RCTs with over 75000 people and 10 cohort studies, all from developed countries. Here’s what they found:
- Meta-analysis of trial data from over 57000 people suggested that diets lower in fat were associated with lower relative body weight (by 1.6 kg)
- Lower weight gain in those having low fat diets was consistently found across trials but the size of the effect varied
- Greater reduction in fat intake appeared to be associated with greater relative weight loss
- When trials that spent more time and attention on low fat groups were removed from the analyses, the significant effects of a low fat diet remained
- Lower fat intake led to lower BMI and waist circumference
- There was only one RCT and three cohort studies on children/young people but these confirmed the effects seen in adult trials
The authors concluded:
There is high quality, consistent evidence that reduction in total fat intake has been achieved in large numbers of both healthy and at risk trial participants over many years. Lower total fat intake leads to a small but statistically significant and clinically meaningful, sustained reductions in body weight in adults in studies with baseline fat intakes of 28-43% of energy intake and durations from six months to over eight years. Evidence supports a similar effect in children and young people.
It’s encouraging to read that the reviewers note that reductions in weight weren’t only going to get the thumbs up from those of us who do a little dance when results are statistically significant but that they can be regarded as clinically meaningful (and sustainable) too. They give an example of what 1.6kg of weight loss means for a man of average height (1.75m) weighing 80kg; that would reduce his BMI from 26.12 to 25.60 and his risk of death by 3%. They recommend that attempts should be made to reduce total fat intake where it makes up 30% or more of energy in the diet. Where total fat intake is less than that, preventing a rise to above that threshold may help to avoid obesity.
Hooper L, Abdelhamid A’ Moore HJ, Douthwaite W, Skeaff CM, Summerbell CD. Effect of reducing total fat intake on body weight: systematic review and meta-analysis of randomised controlled trials and cohort studies. BMJ2012;345:e7666