Most diets lead to modest weight loss and lower blood pressure, but effects largely disappear after a year, while the differences between popular diet programmes are typically small to trivial, according to a meta-analysis in The BMJ.
Reasonably good evidence suggests that most diets result in similar modest weight loss and improvements in cardiovascular risk factors over a period of six months, compared with a usual diet,
Weight reduction at the 12-month follow-up diminished, and improvements in cardiovascular risk factors largely disappeared, except in association with the Mediterranean diet, which saw a small but important reduction in 'bad' LDL cholesterol. As such, at least for short-term benefits, the researchers suggest that people should choose the diet they prefer without concern about the size of benefits.
Obesity has nearly tripled worldwide since 1975, prompting a plethora of dietary recommendations for weight management and cardiovascular risk reduction. But so far, there has been no comprehensive analysis comparing the relative impact of different diets for weight loss and improving cardiovascular risk factors, such as blood pressure and cholesterol levels.
To address this, a team of international researchers led by Bradley Johnson at McMaster University, Dalhousie University, Halifax, and the Texas A&M University, set out to determine the relative effectiveness of dietary patterns and popular named diets among overweight or obese adults.
Their findings are based on the results of 121 randomised trials with 21,942 patients (average age 49) who followed a popular named diet or an alternative control diet and reported weight loss, and changes in cardiovascular risk factors. The studies were designed differently, and were of varying quality, but the researchers were able to allow for that in their analysis.
They grouped diets by macronutrient patterns (low carbohydrate, low fat, and moderate macronutrient – similar to low fat, but slightly more fat and slightly less carbohydrate) and according to 14 popular named dietary programmes (Atkins, DASH, Mediteranean, etc).
Compared with a usual diet, low carbohydrate and low-fat diets resulted in a similar modest reduction in weight (between 4 and 5 kg) and reductions in blood pressure at six months. Moderate macronutrient diets resulted in slightly less weight loss and blood pressure reductions.
Among popular named diets, Atkins, DASH, and Zone had the largest effect on weight loss (between 3.5 and 5.5 kg) and blood pressure compared with a usual diet at six months. No diets significantly improved levels of 'good' HDL cholesterol or C reactive protein (a chemical associated with inflammation) at six months.
Overall, weight loss diminished at 12 months among all dietary patterns and popular named diets, while the benefits for cardiovascular risk factors of all diets, except the Mediterranean diet, essentially disappeared.
The researchers point to some study limitations that could have affected the accuracy of their estimates. But say their comprehensive search and thorough analyses supports the robustness of the results. As such, they say moderate certainty evidence shows that most macronutrient diets result in modest weight loss and substantial improvements in cardiovascular risk factors, particularly blood pressure, at six but not 12 months.
Differences between diets are, however, generally trivial to small, implying that for short-term cardiovascular benefit people can choose the diet they prefer from among many of the available diets without concern about the magnitude of benefits, they conclude.
The extensive range of popular diets analysed "provides a plethora of choice but no clear winner," say researchers at Monash University, Australia in a linked editorial. As such, they suggest conversations should shift away from specific choice of diet, and focus instead on how best to maintain any weight loss achieved.
As national dietary guidelines fail to resonate with the public, taking a food-based approach with individuals and encouraging them to eat more vegetables, legumes, and whole grains and less sugar, salt and alcohol is sound advice, they add.
"If we are to change the weight trajectory of whole populations, we may learn more from understanding how commercial diet companies engage and retain their customers, and translate that knowledge into more effective health promotion campaigns," they conclude.
Objective: To determine the relative effectiveness of dietary macronutrient patterns and popular named diet programmes for weight loss and cardiovascular risk factor improvement among adults who are overweight or obese.
Design: Systematic review and network meta-analysis of randomised trials.
Data sources: Medline, Embase, CINAHL, AMED, and CENTRAL from database inception until September 2018, reference lists of eligible trials, and related reviews.
Study selection: Randomised trials that enrolled adults (≥18 years) who were overweight (body mass index 25-29) or obese (≥30) to a popular named diet or an alternative diet.
Outcomes and measures: Change in body weight, low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol, systolic blood pressure, diastolic blood pressure, and C reactive protein at the six- and 12-month follow-up.
Review methods: Two reviewers independently extracted data on study participants, interventions, and outcomes and assessed risk of bias, and the certainty of evidence using the GRADE (grading of recommendations, assessment, development, and evaluation) approach. A bayesian framework informed a series of random effects network meta-analyses to estimate the relative effectiveness of the diets.
Results: 121 eligible trials with 21 942 patients were included and reported on 14 named diets and three control diets. Compared with usual diet, low carbohydrate and low fat diets had a similar effect at six months on weight loss (4.63 v 4.37 kg, both moderate certainty) and reduction in systolic blood pressure (5.14 mm Hg, moderate certainty v 5.05 mm Hg, low certainty) and diastolic blood pressure (3.21 v 2.85 mm Hg, both low certainty). Moderate macronutrient diets resulted in slightly less weight loss and blood pressure reductions. Low carbohydrate diets had less effect than low fat diets and moderate macronutrient diets on reduction in LDL cholesterol (1.01 mg/dL, low certainty v 7.08 mg/dL, moderate certainty v 5.22 mg/dL, moderate certainty, respectively) but an increase in HDL cholesterol (2.31 mg/dL, low certainty), whereas low fat (−1.88 mg/dL, moderate certainty) and moderate macronutrient (−0.89 mg/dL, moderate certainty) did not. Among popular named diets, those with the largest effect on weight reduction and blood pressure in comparison with usual diet were Atkins (weight 5.5 kg, systolic blood pressure 5.1 mm Hg, diastolic blood pressure 3.3 mm Hg), DASH (3.6 kg, 4.7 mm Hg, 2.9 mm Hg, respectively), and Zone (4.1 kg, 3.5 mm Hg, 2.3 mm Hg, respectively) at six months (all moderate certainty). No diets significantly improved levels of HDL cholesterol or C reactive protein at six months. Overall, weight loss diminished at 12 months among all macronutrient patterns and popular named diets, while the benefits for cardiovascular risk factors of all interventions, except the Mediterranean diet, essentially disappeared.
Conclusions: Moderate certainty evidence shows that most macronutrient diets, over six months, result in modest weight loss and substantial improvements in cardiovascular risk factors, particularly blood pressure. At 12 months the effects on weight reduction and improvements in cardiovascular risk factors largely disappear.
Long Ge, Behnam Sadeghirad, Geoff DC Ball, Bruno R da Costa, Christine L Hitchcock, Anton Svendrovski, Ruhi Kiflen, Kalimullah Quadri, Henry Y Kwon, Mohammad Karamouzian, Thomasin Adams-Webber, Waleed Ahmed, Samah Damanhoury, Dena Zeraatkar, Adriani Nikolakopoulou, Ross T Tsuyuki, Jinhui Tian, Kehu Yang, Gordon H Guyatt, Bradley C Johnston