It's a Draw! Low-Fat or Low-Carb Diets Equally Good, Finds Study

Marlene Busko
February 20, 2018

In a large, randomized trial, 1-year weight loss was similar with a healthy low-fat diet or a healthy low-carbohydrate diet.

Moreover, neither genotype patterns nor baseline insulin secretion predicted which type of diet would be better for any one individual.

These findings, from the Diet Intervention Examining the Factors Interacting With Treatment Success (DIETFITS) trial by Christopher D Gardner, PhD, from Stanford University, California, and colleagues, were published in the February 20 issue of JAMA.

"In the context of these two common weight-loss diet approaches, neither of the two hypothesized predisposing factors was helpful in identifying which diet was better for whom," the researchers report.

A small preliminary trial had suggested that overweight people would have greater weight loss success if they followed a low-carb or low-fat diet, based on a certain genotype pattern, but this was not the case.

The current work also shows that individuals with high insulin secretion did not have greater weight-loss success with a low-carb diet.

However, for both diets participants were instructed to minimize or eliminate refined grains and added sugars, and maximize vegetable intake, the researchers stress.

"We conclude that when equal emphasis is given to high dietary quality for both low-fat and low-carbohydrate eating plans, it is not helpful to preferentially direct an individual with high insulin secretion status who is seeking weight loss to follow a lower-carbohydrate eating plan instead of a lower-fat eating plan," say Gardner and colleagues.

Do Certain Diets Fit Certain People Better?

Some early studies had suggested that individuals might have greater weight-loss success with a low-fat or low-carb diet, depending on their genotype pattern or insulin levels.

"We've all heard stories of a friend who went on one diet — it worked great — and then another friend tried the same diet, and it didn't work at all," said Gardner in a Stanford University press release.

"It's because we're all very different, and we're just starting to understand the reasons for this diversity. Maybe we shouldn't be asking what is the best diet, but what's the best diet for whom?"

To investigate this, DIETFITS recruited and randomized 609 overweight or obese adults from the local area to a healthy low-fat or healthy low-carb dietary intervention between 2013 and 2015. Participants had a relatively high education level and were able to afford high-quality foods.

They were 18 to 50 years old (mean age 40 years) and had a BMI of 28 to 40 kg/m2 (mean 33 kg/m2), and 57% were women.

Women who were postmenopausal or individuals who had uncontrolled hypertension or metabolic syndrome, diabetes, or other criteria were excluded.

Participants received 22 small-group behavior modification sessions given weekly for 8 weeks, then every 2 to 3 weeks until 6 months, and then monthly until 12 months.

Healthy Foods, But Decreased Carbs or Fat

Nutritionists instructed participants in the low-fat and low-carb diet groups to reduce their respective intakes of total fat (especially edible oils, fatty meats, whole-fat dairy, and nuts) or digestible carbohydrates (especially cereals, grains, rice, starchy vegetables, and legumes) to 20 g/day for the first 8 weeks.

Then they could slowly add back 5 to 15 g/day of fats or carbs per week, until they reached the lowest intake levels they believed they could maintain indefinitely.

Both groups were instructed to eat lots of vegetables; minimize intake of foods with added sugars, refined flour, and trans fats; and focus on eating minimally processed, nutrient dense, whole foods that were home-cooked whenever possible. They were not explicitly instructed to cut calories.

They were, however, encouraged to be physically active and also taught behavior modification strategies to help them adhere to their diets.

What's key, says Gardner, was emphasizing that these were healthy low-fat and low-carb diets: a soda might be low-fat, but it's certainly not healthy. Lard may be low-carb, but an avocado would be healthier.

"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 crap. 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."

"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," he explained.

Participants also had a blood test to determine the presence of single nucleotide polymorphisms of three genes (PPARG, ADRB2, and FABP2) related to fat and carbohydrate metabolism, and they were classified as having a genotype believed to respond to a low-fat diet, a low-carbohydrate diet, or neither.

And they had an oral glucose tolerance test to determine blood insulin levels 30 minutes after a glucose challenge (a proxy for insulin secretion).

Participants in both groups attended an average of 14 of the 22 sessions, and 79% completed the study.

Both groups achieved well-differentiated diets. The diet in the low-fat group was 48% carbs, 29% fat, and 21% protein, and that in the low-carb group was 30% carbs, 45% fat, and 23% protein.

Door Closes on One Hypothesis, but Opens on Others

At 12 months, on average, those in the healthy low-fat diet group had lost 5.3 kg (11.7 pounds) and those in the heathy low-carb diet group lost 6.0 kg (13.2 pounds), which was not a statistically significant difference.

Weight change in each group ranged from a 30-kg loss to a 10-kg gain.

This represents immense weight loss variability, say the researchers. But, contrary to the study hypotheses, there were no associations between the genotype pattern or baseline insulin levels and a propensity to succeed on either diet.

"With the large sample size, good retention, substantial weight loss and weight loss variability, and good adherence to and differentiation of diets, the study was well positioned to detect significant interactions by the primary variables of interest if they existed," say Gardner and colleagues.


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"However, no such effects were observed. Differences in weight loss between the two groups were nonsignificant and not clinically meaningful."

"This study closes the door on some questions — but it opens the door to others," noted Gardner, adding that they now have lots of data they can use in secondary exploratory studies.

He and his team are continuing to delve into their databanks, now asking if the microbiome, epigenetics, or a different gene expression pattern can explain why there's such drastic variability between dieting individuals.

The study was supported by grants from the National Institute of Diabetes and Digestive and Kidney Diseases, Nutrition Science Initiative, and National Heart, Lung, and Blood Institute, and by the Stanford Clinical and Translational Science Award. The authors have reported no relevant financial disclosures.

SOURCE: Medscape, February 20, 2018. JAMA. Published online February 20, 2018.

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