Buzz
When 600 individuals were guided to increase their vegetable intake and cut down on added sugar for one year, they experienced weight loss. The DIETFITS study published today offers no shocking revelations, but it provides important insights into what factors don’t impact dieting.
Here are the key takeaways from the study, published today in JAMA:
Low-carb and low-fat diets showed similar results.
Both groups reduced their calorie intake without any guidance on counting or cutting calories.
Individuals’ insulin response (the amount of insulin their body releases in response to sugar) was not a predictor of which diet worked better for them.
Three genetic variations thought to forecast dieting success... proved irrelevant.
While the study itself is behind a paywall, Examine.com offers a free analysis of the study's design and outcomes. This study is notably large—300 participants per group—and well-constructed. It’s a randomized controlled trial, where participants were assigned specific diets. The researchers used more reliable methods than simply asking participants about their food intake, and focused on answering key questions upfront. The results summarized above stem from this solid approach, as opposed to casting a wide net to find inconclusive data.
The Diets That Were Effective
Participants following a low-fat diet were instructed to limit their fat intake to just 20 grams per day. This is roughly the amount in four teaspoons of olive oil or three-quarters of an avocado. To put it into perspective, a single McDonald's double cheeseburger would quickly exceed this limit.
Low-carb dieters were also told to limit their carbohydrate intake to 20 grams, which pushes them into a ketogenic state, where the body shifts its metabolism to burn fat more effectively. For comparison, half a cup of rice contains 22 grams of carbs.
Both groups attended weekly classes led by registered dietitians, initially for one hour per week, with frequency decreasing as the year went on. During the first eight weeks, participants were taught how to drastically reduce their intake of fat or carbs. Afterward, they gradually increased their intake of fat or carbs to discover how much they could tolerate while still losing weight and enjoying their meals. Here's what was covered in the classes, as described in a study released last year:
Those in the Healthy Low-Fat group were encouraged to opt for whole grains (like steel cut oats, farro, barley, quinoa, brown rice, and wild rice) instead of refined products like whole wheat flour. They were also urged to incorporate a variety of legumes, fresh fruits, low-fat dairy, and lean meats into their meals. On the other hand, the Healthy Low-Carb group was advised to select high-quality fats and oils, avocados, hard cheeses, nut butters, and a variety of nuts and seeds. As part of the Titrate phase (after the initial eight weeks), both groups gradually reintroduced small amounts of their respective nutrient—fat for the Healthy Low-Fat group and carbs for the Healthy Low-Carb group—using these high-quality foods throughout the remaining 12-month program.
Since high-quality foods often cost more than their lower-quality counterparts, the recommendation to choose higher-quality options was framed as a spectrum rather than an absolute. For example, for the Healthy Low-Fat participants, organic wheat berries were considered the highest quality, followed by conventional wheat berries, then whole wheat bread with minimal ingredients and no additives, followed by a more conventional whole wheat bread containing additives, and finally, refined white flour bread with many ingredients and additives was considered the lowest quality. In other words, participants were encouraged to choose the highest quality foods they could realistically find, afford, and enjoy.
Most participants didn't fully reach the 20-gram target. The low-carb dieters averaged 96.6 grams of carbs after eight weeks, which increased to 132 grams by the end of the year. The low-fat dieters consumed 42 grams of fat by the end of the first stage, gradually rising to 57 grams per day by the year's end.
Although they didn't meet their goals, the majority of participants still lost weight, regardless of their group. On average, both groups shed around 12 to 13 pounds over the year, though there was considerable individual variation. The folks at examine.com created this graph to illustrate the findings. In each group, a few individuals lost more than 55 pounds, while others saw no weight loss or even gained up to 22 pounds.
What Doesn’t Matter
This study doesn’t bring us closer to understanding why some people respond better to certain diets than others. Since each participant only tried one diet, we also don’t know if those who gained weight on one diet would have had better results with the other.
The researchers had hoped that genetics would provide clarity. Earlier studies had pointed to three genes, PPARG, ADRB2, and FABP2, which might influence whether individuals perform better on one diet over another. They examined SNPs (gene variants) known as rs1801282, rs1042714, and rs1799883. If a person’s genetic makeup aligned with all three variants linked to success on a low-fat diet, and they were assigned to it, they were considered to have genetics that suited the diet. The same logic applied to low-carb dieters. Anyone whose genes suggested a blend of low-fat and low-carb preferences was labeled mismatched to their diet.
Unfortunately, this approach didn't work at all. The lead researcher shared with STAT, 'We were so excited and thought this would work,' but upon reviewing the results, they had to admit, 'We didn’t even come close.'
Can I just say, as a science enthusiast, it's refreshing that a study with negative results is still getting attention and being published? The researchers aren’t trying to spin the findings into something they’re not, which is commendable. In today’s world of questionable genetics-based diet advice, knowing what doesn’t work is just as valuable as understanding what does.
Insulin secretion failed to predict success on either diet. When we eat, especially carbs, our pancreas releases insulin into the bloodstream, prompting other cells to absorb the nutrients. Those with diabetes have either insufficient insulin production (type 1) or their cells don’t respond to the insulin (type 2).
They also tested insulin levels 30 minutes after consuming a large dose of sugar. Given that type 2 diabetes is often linked to obesity, the researchers wanted to explore whether people who produce excess insulin might perform better on a low-carb diet, which triggers less insulin release. Unfortunately, this hypothesis didn’t hold up either.
What does this imply for me?
Firstly, this research was conducted with a specific group of people, so we can’t say for certain whether the results apply to everyone. The participants were from the Stanford and San Francisco Bay areas of California, mostly educated, and generally had enough financial resources to afford healthy food without difficulty.
The study only included individuals with a BMI between 28 and 40, aged 18 to 50, excluding those with diabetes, heart disease, cancer, uncontrolled high blood pressure, or those who were pregnant or breastfeeding. This leaves out a significant number of people who may want to lose weight, and in fact, excludes many reasons for wanting to lose weight, such as managing heart disease, diabetes, or post-pregnancy weight loss.
Both diets emphasized consuming lots of vegetables and reducing sugar intake, but the study doesn’t confirm if that alone is sufficient; it’s possible that a particular nutrient restriction is also necessary. I would have appreciated seeing a control group that wasn’t given any specific low-carb or low-fat target, but was simply provided with general advice on maintaining a high-quality diet.
Ultimately, we still don’t understand why some participants lost 55 pounds while others gained more than 20. The researchers are conducting additional analyses to uncover more answers. However, our best assumption at this point is that flipping a coin may give you similar chances of success with a low-fat diet as it would with a low-carb one.
Updated at 7am on 2/21/2018 to fix the journal name and adjust the weight loss figures (individuals lost as much as 25 kilograms, not pounds—a significant distinction!)
