Weight change over 12 months was not significantly different between those who followed a healthy low-fat diet (HLF) and those choosing a healthy low-carbohydrate (HLC) diet, found researchers from the Stanford University School of Medicine.
Additionally, the scientists found that there were no significant diet-insulin or diet-genotype interactions.
This finding conflicted with previous research, which had suggested that individuals may respond better to a particular type of diet depending on their genetic make-up. In other words, those with a low-fat-responsive genotype might lose more weight when following a low-fat, rather than a low-carb, regime.
“In this 12-month weight loss diet study, there was no significant difference in weight change between a HLF diet vs a HLC diet, and neither genotype pattern nor baseline insulin secretion was associated with the dietary effects on weight loss,” wrote first author Professor Christopher Gardner. “In the context of these 2 common weight loss diet approaches, neither of the 2 hypothesised predisposing factors was helpful in identifying which diet was better for whom.”
Compared with baseline, both groups lowered blood pressure, insulin and glucose levels. However, blood lipid results showed some differences between the two groups.
“The 12-month changes in low-density lipoprotein cholesterol (LDLC) concentrations significantly favoured a healthy low-fat diet,” commented the researchers.
LDLC was found to increase in the low-carb group over 12 months.
“High-density lipoprotein cholesterol (HDLC) concentrations increased significantly more and concentrations of triglycerides decreased significantly more for the HLC diet group than for the HLF diet group. The decrease in the prevalence of the metabolic syndrome was not significantly different between the diet groups,” observed the researchers.
The inter-group differences in outcome in each individual type of cholesterol was about 5%, while the difference in triglycerides between HLC and HLF groups was 15%.
The key significance of the study findings is the approach to food. Irrespective of following the low-fat or low-carb route, the choices should be healthy, emphasised the researchers.
“Dietary modification remains key to successful weight loss. Yet, no one dietary strategy is consistently superior to others for the general population.
"On both sides, we heard from people who had lost the most weight that we had helped them change their relationship to food, and that now they were more thoughtful about how they ate," said Gardner.
The study included 609 overweight individuals (roughly similar numbers of men and women) who were randomised into HLF or HLC groups. The researchers performed genetic analysis at baseline to identify possible profiles which might be linked to preferential fat or carbohydrate metabolism. They also measured baseline insulin secretion.
In the first two months, participants in the individual groups were limited to 20 grams/ day (g/d) of fat or carbohydrate. (The balance of their calories were obtained from their non-restricted macronutrient). They subsequently added back successive increments of 5-15 g/d of fat or carbs until they reached a level they felt was sustainable over the very long term.
At the end of the 12 month trial, the HLF group reported an average fat intake of 57 g/d, while HLC diet followers were consuming an average of 132 g/d of carbohydrates. (This compared favourably with pre-study intakes of 87 g/d and 247 g/d respectively).
Changes in weight, anthropometric measures, blood lipids, blood pressure, insulin, glucose and markers of metabolic syndrome were calculated at the end of the 12 months.
The average 12 month weight loss was 5.3 kg in the HLF group and 6.0 kg in the HLC group. There were large variations in weight change (between -30 kg and +10 kg) within each group. However, these ranges were similar between groups.
Long-term weight management
Sustainability and avoiding/ minimising processed foods is critical, Gardner reemphasised in conclusion.
"We made sure to tell everybody, regardless of which diet they were on, to go to the farmer's market, and don't buy processed convenience food cr**. Also, we advised them to diet in a way that didn't make them feel hungry or deprived -- otherwise it's hard to maintain the diet in the long run," said Gardner. "We wanted them to choose a low-fat or low-carb diet plan that they could potentially follow forever, rather than a diet that they'd drop when the study ended."
Volume 319, issue 7, pp 667-679. DOI: 10.1001/jama.2018.0245
“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”
Authors: Christopher D. Gardner et al.