Study: No Big Difference Among Popular Diets

When it comes to choosing among popular diets, patients might as well choose the one they like. The diets all result in modest weight loss and their differences generally disappear at 12 months, scientists said.

The meta-analysis of 121 randomized clinical trials comparing 14 popular diets found that the top three in terms of mean weight loss and blood pressure reductions at 6 months were Atkins (5.5 kg, 5.1 mm Hg systolic); Zone (4.1 kg, 3.5 mm Hg); and DASH (3.6 kg, 4.7 mm Hg), reported Bradley Johnston, PhD, of McMaster University in Hamilton, Ontario, and colleagues.

However, as shown in the study online in The BMJ, the other popular diets were associated with some degree of weight loss and blood pressure improvement at 6 months, on the order of 2-3 kg for body weight and 3-4 mm Hg for systolic blood pressure.

By 12 months, the effects had diminished to the degree that the differences between diets were negligible, the team said.

None of the popular diets were associated with significant reductions in lipids, with the notable exception of the Mediterranean diet, for which there was a mean 4.59 mg/dL reduction in low-density lipoprotein (LDL) cholesterol at 6 months. The Mediterranean diet was also notable in that this effect did not diminish at 12 months, the researchers said.

When Johnston's team grouped the popular diets according to their macronutrient patterns, there were similar patterns of weight loss and blood pressure reduction for low-fat versus low-carbohydrate diets. "Network meta-analyses showed that although there were statistically significant differences between some dietary patterns, these differences were generally small at 6 months and negligible at 12 months," the researchers wrote. "The same small differences between diets at 6 months and even smaller and uncertain differences at 12 months apply to the popular named diets."

The results suggest that healthcare providers can advise patients to choose a diet based on personal preferences, the investigators said. "Differences between diets are ... generally trivial to small, implying that people can choose the diet they prefer from among many of the available diets without concern about the magnitude of benefits."

Writing in an accompanying editorial, Helen Truby, PhD, and Terry Haines, PhD, both of Monash University in Melbourne, Australia, agreed with the researchers' conclusion: "These different approaches can be considered by individuals in the secure knowledge that reducing dietary energy intake -- regardless of macronutrient composition -- will result in modest weight loss after 6 months," Truby and Haines wrote.

"The extensive range of popular diets analyzed provides a plethora of choice but no clear winner," the editorialists added. "So conversations should shift away from specific choice of diet and focus instead on how best to maintain any weight loss achieved."

Maintaining weight loss involves incorporating weight monitoring and physical activity into one's daily routine, Truby and Haines continued. "So, following the initial weight loss period, the narrative needs to change from quantifying weight loss to maintaining a lower body weight and associated health benefits through long-term improvements in food and activity choices."

The systematic review and meta-analysis included 121 randomized clinical trials that enrolled adults (age 18 or older) who were overweight (body mass index [BMI] 25-29) or obese ( BMI ≥30) to a popular named diet or an alternative diet. The trials included a total of 21,942 participants.

Key outcomes of the analysis included change in body weight, LDL and high-density lipoprotein cholesterol, systolic blood pressure, and diastolic blood pressure.

Two reviewers independently extracted data on study participants, interventions, and outcomes, assessing the risk of bias and the certainty of evidence using the GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) approach. In addition, the researchers used a Bayesian framework and a series of random effects network meta-analyses to estimate the relative effectiveness of the diets.

A key limitation of the analysis, Johnston and co-authors said, was that participant adherence to diets was generally not reported in the trials. "If this is the case, our results describe what is likely to happen for average adherence by patients," the team wrote. "Full adherence would probably yield larger effects in improvement of weight loss and cardiovascular risk factors. If the weight loss achieved at 6 months continued at 12 months, it is uncertain whether the improvements in cardiovascular risk factors would also be maintained. Future studies, therefore, could usefully examine how to achieve longer-term adherence to diets."

Comment