Low-fat diets 'not the best way to lose weight'

Behind the Headlines

Friday October 30 2015

People may find it harder to stick to a low-fat diet

Low-carb rather than low-fat may be the way to go

"Low-fat diets are not the best way to lose weight," the Daily Mail reports. The findings come from a new review that analysed data from more than 50 previous studies on low-fat dietary interventions involving almost 70,000 adults.

The study aimed to see how low-fat diets measured up to comparison diets for achieving long-term weight loss, defined as a year or more.

The findings were mixed. There was evidence from a large number of trials that low-carb diets were better for weight loss than low fat – helping to achieve 1.15kg more weight loss after one year.

However, low-fat diets still worked, and consistently resulted in about 5.41kg more weight loss than if the person continued with their usual diet.

But a potential limitation of this latest research is that the individual trials varied greatly in their study populations (many had varied chronic diseases), as well as the constituents of the low-fat and comparison diets, and the ways these diets were encouraged or monitored. 

Also, it is difficult to know how compliant people may have been to the diets they were assigned to. This makes it harder to tease out the specifics of the best diet, as many different methods were pooled to find the overall pattern. 

Losing weight through diet alone can be difficult. Regular exercise can increase your chances, as well as bring about other health benefits. 

The NHS Choices Weight loss plan provides both dietary and exercise advice that can lead to sustainable long-term weight loss.    

Don't forget about fibre

In the thousands of words written about low-fat diets versus low-carb diets, it can be easy to overlook the usefulness of fibre for helping you lose weight.

 

The advantage of fibre is it can keep you feeling fuller for longer, so you are more likely to stick to your recommended calorie intake. Most people in the UK eat only about 18g of fibre a day, but should aim to eat at least 30g. 

 

You should increase your fibre intake gradually, as a sudden increase can cause cramps and constipation. Also make sure to drink plenty of water – aim for 1.2l a day – to avoid cramps and constipation.

 

Read more about how to increase your fibre intake

Where did the story come from?

The study was carried out by researchers from Harvard Medical School, Harvard School of Public Health, Brigham and Women's Hospital, and Boston Children's Hospital in the US. Funding was provided by the US National Institutes of Health and the American Diabetes Association, which had no role in the study.

One of the authors of the study reported receiving research support from the California Walnut Commission and Metagenics, a company that sells dietary supplements.

The study was published in the peer-reviewed medical journal, The Lancet Diabetes and Endocrinology.

While some of the headlines were somewhat simplistic, the UK media generally covered the new research accurately and gave good balance to the discussion. For example, the reporting included advice that guidance should perhaps focus on portion sizes and the need to limit processed foods, rather than focusing specifically on nutrient groups like fats, carbs or protein.

Coverage also included expert recommendations suggesting preventing weight gain in the first place by better informing people about healthy diet and exercise. Of course, you can do both, pursuing a long-term prevention strategy while doing your best to deal with the immediate consequences.  

What kind of research was this?

This was a systematic review that searched the literature to identify randomised controlled trials where people had been allocated to a low-fat diet or any comparison diet. The results of these studies were then pooled in a meta-analysis to look at the overall effects of low-fat diets.

The researchers discuss how the optimal nutrient balance of calories coming from fat, protein and carbohydrate to achieve long-term weight loss has been debated for decades.

Low-fat diets have been popular because of the far greater proportion of calories contained in fat, compared with the same weight of protein or carbohydrate.

However, the researchers say trials don't consistently show that low-fat diets actually achieve more long-term weight loss than other diets. This review therefore aimed to pool the evidence to see how different dietary interventions matched up against each other.

This review has strengths, as it only included randomised controlled trials, which are the best way of looking at the effectiveness of an intervention because the participants are randomly allocated to the diet. 

Studies of dietary patterns are often observational. While these can look at associations between diet and outcome, as the people choose the diet themselves, you can never be sure other health and lifestyle factors aren't influencing the outcome. 

What did the research involve?

This review searched literature databases for randomised controlled trials in adults comparing a low-fat diet with any diet of higher-fat contribution, including the person's usual diet. Only trials that measured long-term weight change over at least a year were included.

They excluded studies where the comparison arm was not a diet, such as exercise or weight-loss medication. They also excluded studies featuring dietary supplements or meal replacements, although studies that had additional dietary changes alongside the low-fat intervention (such as boosting fruit and veg intake) were allowed.

The main outcome examined was the average change in body weight from the study start to one year or greater.

A total of 53 trials, involving 68,128 adults, met the inclusion criteria, most of which (37) came from the US or Canada. Just over a third of the trials (20) included people with specific conditions or chronic diseases, including breast cancer, diabetes and heart disease. 

About two-thirds of the trials (35) had weight loss goals with the dietary intervention, but the remainder either had no weight loss target or were just aimed at weight maintenance. 

Most trials (27) were only one year in duration. However, it's not certain whether the interventions lasted this long, or just the follow-up.

The low-fat diets ranged from very low (≤10% calories from fat) to moderate fat intake (≤30% calories from fat). Comparison diets were varied and included moderate-to-high fat intakes, or other interventions, such as low carbohydrate.

The trials also varied in how they controlled the diets in their study. For example, some just gave instructions or information leaflets, while others actually provided the food. 

What were the basic results?

All 68,128 adults across all trial arms lost an average (mean) 2.71kg of weight after an average of one year follow-up. The average weight loss in the 35 trials that had weight loss goals was 3.75kg.

The pooled results of 18 trials found low-carbohydrate diets were more beneficial for weight loss than low-fat, resulting in an average 1.15kg greater weight loss (95% confidence interval [CI] 0.52 to 1.79kg). These were all diets with weight loss goals – no trials aimed at weight maintenance or no weight loss compared low-fat with low-carb diets.

Low-fat diets resulted in significantly greater weight loss compared with usual diet:

  • eight trials with weight loss goals found an average 5.41kg (95% CI 3.54 to 7.29) greater weight loss with low-fat compared with usual diet
  • 11 trials with no weight loss goal found 2.22kg (95% CI 1.45 to 3.00) greater weight loss with low fat
  • three trials aiming at weight maintenance found 0.70kg (95% CI 0.52 to 0.88) greater weight loss with low fat   

There was no significant difference when comparing the weight loss achieved with low-fat compared with high-fat diets, regardless of weight loss goal.

Overall, when pooling all the trials, regardless of comparator, there was no significant difference in weight loss between the low-fat diet and the comparison arms in trials aiming at weight loss.

However, for the trials with weight maintenance or no weight loss goals, low-fat diets did result in significantly greater weight loss than the comparator (1.54 and 0.70 kg, respectively).  

How did the researchers interpret the results?

The researchers concluded that, "These findings suggest that the long-term effect of low-fat diet intervention on body weight depends on the intensity of the intervention in the comparison group.

"When compared with dietary interventions of similar intensity, evidence from RCTs [randomised controlled trials] does not support low-fat diets over other dietary interventions for long-term weight loss." 

Conclusion

This review has aimed to answer the question of whether low-fat diets result in any greater weight loss compared with other diets, as has often been speculated. It showed they didn't. Most diets worked, and the low-fat ones weren't particularly better than the rest.

The systematic review design has many strengths. It has identified a large number of studies, with almost 70,000 participants, all of which were randomised controlled trials. This should balance out any non-diet-related health and lifestyle characteristics between participants. It also only included trials of at least one year's duration to look at longer-term effects on weight loss.

However, it is worth taking time to consider the results before potentially jumping to the conclusion that a low-fat diet is of no benefit and eating as much fat as you like is a healthy option.

The review found no difference in the effect of a low-fat diet compared with a high-fat diet. But it did consistently find changing to a low-fat diet resulted in significantly greater weight loss when continuing with the person's usual diet, regardless of whether the person was trying to lose weight or not.

The review did, though, find evidence from a large number of weight loss trials to suggest low-carbohydrate diets may be of more benefit than low fat. Unfortunately, there were no trials available to see whether this held when no weight loss goal was intended, but it is possible the same effect may be seen regardless of aim.

But interpreting this – particularly if trying to inform someone of their likely weight loss when following a particular diet – is difficult when variations across the trials included are considered.

These were all randomised controlled trials – which is a definite plus point – but they were still diverse in many ways. The study populations varied greatly. For example, some included men or women, some just overweight or obese people, and others had people with varied chronic diseases or health conditions.

The components of the low-fat and comparator interventions, and the ways these diets were encouraged or monitored, was also very different across trials.

There are many unknowns. For example, what were the other constituents of these diets – such as fruit and vegetable intake – particularly when it was the person's usual diet? Also, were there any specifications about what types of fat where being eaten, whether saturated fats or even trans fats, or "healthier" mono- or polyunsaturated fats?

Because of the variability in the trials, it is difficult to give a definitive answer about whether a low-fat or low-carb diet is going to help any individual lose more weight. It is likely to be overall components of the diet, and the total energy intake balanced against physical activity, that has an effect.

To lose weight, essentially we need to take in less energy, in the form of calories, than the energy we're using up in daily activity. We need fats, carbohydrates and protein in our diet, and following a diet that completely cuts out one of these groups is unlikely to be beneficial to our health or help sustain a healthy weight in the long term. 

An important goal is finding a reduced calorie diet that you actually enjoy eating. That way, you are more likely to stick to it. A healthy dietary pattern should include lots of fruit and vegetables and lower amounts of sugar, salt and saturated fats, combined with regular exercise.

If you're looking to lose weight, try the NHS Weight loss plan.

Analysis by Bazian. Edited by NHS Choices. Follow NHS Choices on Twitter. Join the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

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Media last reviewed: 14/07/2015

Next review due: 14/07/2017

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