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Overweight to obese adults who followed a low fat or low carbohydrate diet for 12 months both lost around 5 or 6kg in body weight. It made no difference whether they had a gene-type indicating that they break down fats or carbohydrates better.

Obesity is a major public health concern and there are many weight-loss approaches and fad diets that people try. Dietary trials tend to observe only modest weight loss. Given wide population variation, some suggest that knowing the genetic or metabolic characteristics of individuals might determine which diet is better for whom.

This US trial including 600 adults who followed strict dietary interventions found no evidence for this. The time to weight loss and range of loss among individuals was equivalent with either diet, regardless of genetics or blood glucose control.

The most important thing is that people adopt a balanced diet and lifestyle that they can maintain in the long-term, which includes regular physical activity.

Why was this study needed?

Latest data suggests that around 58% of women and 68% of men in England are overweight (BMI 25-30kg/m2) or obese (BMI 30kg/m2 or more).

Obesity is associated with many chronic diseases and places high demand on health and social care services. Obesity was recorded as a diagnosis for 525,000 hospital admissions in England in 2015/16. There were 6,438 bariatric (weight-loss) surgical procedures in the same period. The NHS in England spent £6.1 billion on overweight- and obesity-related illness in 2014/15. The cost to wider society is estimated at £27 billion per year.

Managing and preventing obesity is a major priority. This includes better understanding of effective and sustainable weight-loss strategies. Prior research suggested that variations in the DNA sequence on three genes (PPARG, ADRB2, and FABP2) differ between those who respond better to low fat or low carbohydrate diets. Other research indicated that people with greater insulin resistance may fare better on low carbohydrate diets.

What did this study do?

The DIETFITS randomised controlled trial recruited 609 US adults (57% female) with overweight to obese BMI 28-40 (average 33). Exclusions included diabetes, uncontrolled high blood pressure and heart disease.

Instruction on either a low fat or carbohydrate diet was provided through 22 diet-specific sessions over 12 months, scheduled weekly for two months, two-weekly to four months, three-weekly to six months, then monthly to completion.

Intake of fat or carbohydrates, respectively, was reduced to 20g daily in the first two months, then gradually added these back at 5-15g daily increments per week to the minimum intake that they could maintain indefinitely. Both groups maximised vegetables, minimised sugar and focused on whole foods. Exercise was encouraged and behavioural support available.

Seventy nine percent completed follow-up though all participants were analysed. Assessors were unaware of group assignment.

What did it find?

  • There was no difference between groups in weight loss by 12 months. People in the low fat group lost on average 5.3kg in weight (95% confidence interval [CI] -5.9 to ‑4.7 kg) compared with -6.0 kg (95% CI -6.6 to -5.4) in the low carbohydrate group (between-group difference 0.70, 95% CI -0.21 to +1.60). The range of weight loss among individuals in both groups was no different (-30kg to +10kg).
  • Thirty percent of all randomised participants had a gene type indicating they may be better suited to a low carbohydrate diet and 40% that they may be better suited to a low fat diet. When analysing weight loss by gene type in each group the researchers found it made no difference.
  • The person’s blood glucose control, as measured by blood insulin level 30 minutes after a glucose challenge, also made no difference to weight loss in either group.
  • Adverse effects potentially related to the intervention were evenly distributed between both groups.

What does current guidance say on this issue?

Multicomponent lifestyle interventions, incorporating dietary, physical activity and behavioural components, are central to the NICE guideline Obesity: identification, assessment and management.

For sustainable weight loss, NICE recommends that the person consumes 600kcal fewer per day than they would normally, in combination with intensive support and follow-up. NICE mention that lowering dietary fat content may be one approach to achieve this. Lowering carbohydrate is not mentioned. Very low-calorie diets are not routinely recommended.

General advice is to tailor dietary changes to preference, not to use restrictive or nutritionally unbalanced diets, and encourage a healthy balanced diet in the long term.

What are the implications?

There is widespread interest in different dietary strategies for weight loss, and whether certain characteristics may predispose people to weight gain. Contrary to such theories, this study finds no evidence that choosing low fat or low carbohydrate suits certain people better.

What it does highlight is that even within the context of a carefully implemented 12 month intervention; weight loss is possible if modest. Some people can achieve dramatic weight loss up to 30kg on either diet and it is not yet clear if there are personal characteristics (other than motivation, opportunity or capability) that are linked to this degree of weight loss.

What seems most important is to implement healthy and balanced dietary patterns combined with regular physical activity that can be maintained in the long term. This message is more helpful rather than viewing any particular “diet” as a short-term fix.

Citation and Funding

Gardner CD, Trepanowski JF, et al. Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial. JAMA. 2018;319(7):667-79.

The article was funded by the National Heart, Lung, and Blood Institute and the Nutrition Science Initiative.



NICE. Behaviour change: individual approaches. PH49. London: National Institute for Health and Care Excellence; 2014.

NICE. Obesity: identification, assessment and management. CG189. London: National Institute for Health and Care Excellence; 2014.

NICE. Weight management: lifestyle services for overweight or obese adults. PH53. London: National Institute for Health and Care Excellence; 2014.

NHS Digital. Health survey for England. London: NHS Digital; 2016.

NHS Digital. Statistics on obesity, physical activity and diet – England 2017. London: NHS Digital; 2016.

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