Friday December 14 2012
On average, people who ate a diet with less fat lost 1.6kg
A recent headline in the Daily Mail tells us to ‘Ditch the fancy diets, just cut down on fat: Eating healthily is more reliable way to lose weight’. Apparently, cutting back on fatty foods such as butter, cheese and crisps will help us lose weight.
In recent years we have be bombarded with supposedly science-based diet plans, ranging from the Atkins diet to the Dukan diet and even the caveman diet. But according to the Mail, this new research appears to cut through complicated diet rules and regulations to provide a single clear message: that if you want to lose weight, you should eat less fat.
The news was based on a large well-conducted review that tested the effect on body fat of reducing how much fat people ate. The review excluded studies that specifically aimed to make people lose weight, as researchers didn’t want to include only people who were already overweight or obese. They also excluded trials where overall calorie intake was being reduced as well as fat intake.
The pooled results found that people who were supposed to eat a diet lower in total fat had 1.6kg lower body weight by the end of the trial than those who continued to eat normally. This effect was independent of age, sex, starting weight, or health of the participants.
Overall, the review finds good evidence that having a lower intake of dietary fat results in sustained lowering of body weight in adults.
Where did the story come from?
The study was carried out by researchers from the University of East Anglia and Durham University in the UK, and the University of Otago in New Zealand. Funding was provided to Durham University by the World Health Organization (WHO), which wanted to update their guidance on the relationship between fat intake and levels of body fat.
The study was published in the peer-reviewed British Medical Journal, where research articles are open access.
The media coverage of this study was generally representative. It is worth taking the Mail’s claim that eating less fat is a more reliable way of losing weight than following “fancy diets” with a hint of caution. While this may be the case, this reserarch did not compare eating less fat with any particular alternative diet pattern, and the researchers did not look at studies where the specific aim was weight loss.
With that in mind, simply cutting down the amount of fat in your diet would appear to be an easier method to lose weight than some of the more complicated diet plans of recent years, including:
- eating a low-carbohydrate, high-protein diet (such as the Dukan and Atkins diets)
- replacing meals with milkshakes (such as the Slim-Fast diet)
- having cold baths and drinking lots of black coffeee (such as the OMG diet)
To find out more about the pros and cons of popular diet plans, read the Top 10 most popular diets review.
What kind of research was this?
This was a systematic review which aimed to identify all trials and cohort studies that had examined the association between total dietary fat intake and levels of body fat (fatness) in adults and children.
However, the researchers specifically excluded any trials where the purpose of the trial was weight loss. This was because they wanted to look at general population samples, rather than have a predominance of overweight and obese participants.
The assumption being, reasonably enough, that people who want to lose weight would be overweight or obese.
They also considered that in such trials, reduction of calorie intake may be confounding the results (this was important because it may have been the overall lower number of calories that the participants were eating that was responsible for any change seen, rather than the low fat intake specifically).
A systematic review which has identified all relevant trials examining the effect of low fat dietary intake upon body fat is the best way of examining the effects of low dietary fat upon the body.
However, such a review may contain inherent limitations due to differences in the designs of the individual trials, the dietary interventions used, measurement of weight outcomes and duration of follow-up.
In this instance, the systematic review looked at both cohort studies and randomised controlled trials (RCTs).
Cohort studies are less reliable than RCTs for examining the effect of dietary fat consumption on body fat. In these observational studies, people are choosing how much fat they consume, so the people who choose lower fat diets may have other healthy lifestyle behaviours that are influencing their body weight, such as taking more physical activity.
What did the research involve?
The researchers searched medical literature databases to identify RCTs and cohort studies conducted in apparently healthy adults or children.
Trials were included if they compared a lower total fat (in grams per day) intervention with a control group consuming their normal fat intake, and included a follow-up time of at least 26 weeks.
Eligible interventions could include dietary advice or provision of foods, but could not include any other supplementary lifestyle or medical interventions (such as physical activity advice or weight loss tablets), unless identical supplementary advice or interventions had been given to the control group.
The researchers excluded studies where the specific aim of the intervention was weight loss. The trials had to have assessed some measure of body fat, such as body weight, body mass index (BMI), or waist circumference.
Cohort studies had to have looked at the relationship between total fat intake at study start and change in body fat over the duration of follow-up (at least one year) to be included.
The quality of the identified studies was assessed, and where possible the results were pooled. Where it was able, the review also took into account any potential confounders that could be having an influence (such as alcohol intake). In their analyses the researchers included a measure of how different the study results were from each other (heterogeneity).
What were the basic results?
The researchers identified 33 eligible RCTs covering 73,589 adults. Twenty of the trials were conducted in North America, 12 in Europe, and one in New Zealand. The duration of the trials varied from six months to more than eight years. Four trials included only men, 15 trials included only women, and 14 included both sexes. Average age of participants and their health status (such as their risk of cardiovascular disease) varied between trials.
The pooled results of all 33 RCTs found that the low fat diets resulted in 1.57kg lower total body weight than in the control arms (95% confidence interval 1.16 to 1.97kg).
However, there was significant heterogeneity between the trials, meaning that although the trials all generally showed that low dietary fat was associated with lower weight, the size of this weight difference varied quite considerably between trials. Analyses suggested that greater reductions in fat intake and having lower fat intake at the start of the study were associated with greater weight loss.
The researchers conducted subgroup analyses according to:
- study duration
- participant gender
- study year
- fat intake in the intervention or control groups
- energy intake in the intervention or control groups
- health status or BMI at the start of the study
None of these factors affected the significance of the relationship, and all analyses by subgroup still resulted in significantly less weight in the low dietary fat intervention group.
In the nine trials that assessed BMI, lower fat intake was also associated with lower BMI (0.51kg/m2 difference between the intervention and control groups, 95% confidence interval 0.26 to 0.76 kg/m2 difference). One trial looked at waist circumference in women and similarly found that lower fat intake reduced waist circumference (0.3cm difference between the intervention and control groups, 95% confidence interval 0.02 to 0.58 cm difference).
The researchers identified 10 cohort studies in adults including 107,624 people. Seven of the cohorts had looked at the effect of dietary fat upon weight change and these trials were reported to have carried out 16 assessments, 11 of which found no significant effects and five (31%) found that low dietary fat was associated with a significant reduction in weight.
The researchers identified one trial that had been conducted in children (aged 12-13) and this trial also found that those in the low fat group had a reduction in their BMI by the end of the study.
Three cohort studies in children were also identified.
Two of these studies also found that a low fat diet was associated with significantly lower weight, but the researchers found problems with the quality of all three cohorts in children.
How did the researchers interpret the results?
The researchers conclude that there is high quality, consistent evidence that reduction of total fat intake leads to ‘small but statistically significant and clinically meaningful, sustained reductions in body weight in adults’. They also say that the evidence supports a similar effect in children and young people.
This is a well-conducted and valuable review that demonstrates, perhaps unsurprisingly, that if people eat consistently lower fat food (with trial periods ranging from six months to eight years) they will end up at a slightly lower weight than people who continue to follow their normal dietary patterns.
This effect found was independent of the age, sex, starting weight or health of the people involved in the study.
However, although the results of the individual studies all showed a similar trend, the actual amount of weight difference between intervention and control groups in the individual trials did differ quite a lot. This was mostly due to variations in how much dietary fat intake was reduced by, and how much dietary fat the participants consumed at the start of the study.
Nevertheless, the trials do provide reliable evidence that having a lower intake of dietary fat does result in weight loss, even when this is not the specific aim. As they were RCTs, the randomisation process would be expected to have balanced out any other lifestyle factors between the groups that could be influencing the association.
For example, although a third of the 10 observational studies in adults also supported an association between lower fat and weight loss, we can have less confidence in these results because, people choosing to eat less fat could also be choosing other healthy lifestyle behaviours (such as exercising more) which could be influencing the weight loss.
It is also notable that none of the trials were conducted with the specific aim of weight loss and were conducted in apparently healthy populations, rather than among a predominance of overweight or obese individuals.
The evidence for children is less firm, as only one trial was available, although again, this trial did show that lower dietary fat was associated with weight loss compared to normal fat.
Overall, the review finds good evidence that having a lower intake of dietary fat results in sustained reductions in body weight in adults.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.