For years, researchers have debated which diet is best for weight loss: low-fat or low-carb. Now, a robust long-term study has settled the question once and for all. The answer? There’s no one-size-fits-all, but any weight loss diet should emphasize real food. Read on to learn more.
Obesity is a major public health epidemic. Forty percent of adults are already obese, and if current trends continue, more than half of adults will be obese by 2030 (1). Obesity is associated with an increased risk of heart disease, diabetes, and cancer and presents a massive $147 billion burden on the healthcare system each year (2).
Researchers have long debated whether low-fat or low-carbohydrate diets are best for weight loss. Regardless of the macronutrient content, however, most long-term studies have reported little success in achieving and maintaining significant weight loss. In 2016, I wrote an article called “Carbohydrates: Why Quality Trumps Quantity,” in which I argued that the answer to obesity and metabolic disease lies not in how much carbohydrate we eat, but rather what types of carbohydrate we eat.
A new study found that people who cut back on added sugar, refined grains, and processed food lost weight over 12 months—regardless of whether they were low-carb or low-fat
A landmark study recently published in the Journal of the American Medical Association supports this argument and suggests that the same principles apply to fats. The researchers found that on average, people who cut back on added sugar, refined grains, and processed food lost weight over 12 months—regardless of whether the diet was low-carb or low-fat (3). In this article, I’ll break down the methods and findings of the study.
Treading where few researchers have gone before
Nutrition science research is already fraught with problems, and weight loss studies bring their own unique difficulties. In order to produce a robust weight loss trial, an aspiring research group must:
- Have enough funding to support a large scale randomized trial
- Recruit and retain a very large group of subjects
- Collect data over a long-term period
- Ensure subjects are complying with the dietary intervention
Ideally, they would also use weight loss strategies that can be applied and sustained by free-living people. Unfortunately, few, if any, studies published in the literature have succeeded in meeting all four of these criteria—until now, that is. In February, the results of a long-term, large-scale, randomized clinical trial led by Dr. Christopher Gardner, Director of Nutrition Studies at the Stanford Prevention Research Center, were published in JAMA.
The study design: whole foods low-fat vs. low-carb
The scientists recruited 609 adults to participate in the 12-month study. The subjects were both male and female, were between 18 and 50 years old, and had an average body mass index of 33 (class I obesity). Those with uncontrolled metabolic disease were excluded from the study. The researchers randomly split them into two diet groups: “healthy low-carb” and “healthy low-fat.”
During the first eight weeks, participants in the low-fat and low-carb groups were instructed to reduce intake of total fat or digestible carbohydrates, respectively, to 20 grams per day. They then slowly added fats or carbohydrates back into their diets until they reached the lowest level of intake that they believed they could sustain indefinitely. Additionally, both diet groups “were instructed to 1) maximize vegetable intake; 2) minimize intake of added sugars, refined flours, and trans fats; and 3) focus on whole foods that were minimally processed, nutrient dense, and prepared at home whenever possible” (3).
For example, foods like fruit juice, pastries, white rice, white bread, and soft drinks are low in fat, but were not recommended to the low-fat group. Instead, the dietitians encouraged participants to eat whole foods like lean meat, brown rice, lentils, low-fat dairy products, legumes, and fruit. Meanwhile, the low-carb group was instructed to focus on foods rich in healthy fats, like olive oil, avocados, salmon, cheese, nut butters, and pasture-raised animal products.
The best part? The participants were told not to worry about counting calories or limiting portion sizes, but to simply eat enough to avoid feeling hungry.
Both groups attended 22 instructional classes led by registered dietitians over the course of the 12 months to help support them in these changes. Overall, 79 percent of the participants completed the trial. At the end of the year, average macronutrient breakdown by energy was as follows:
Low-fat group: 48% carbohydrate, 29% fat, 21% protein
Low-carb group: 30% carbohydrate, 45% fat, 23% protein
This was determined by random 24-hour dietary recalls, but was also confirmed with more objective measures of compliance to the diet, like respiratory exchange ratio.
Weight loss without calorie counting or restriction
The results? Both diet groups eating a whole-foods, nutrient-dense diet spontaneously reduced caloric intake and lost significant weight in the year-long study. However, there were no significant differences between the low-carb and low-fat groups in regard to weight loss, body fat, or waist circumference. On average, the low-carb group lost 13.2 pounds (6 kg), while the low-fat group lost 11.7 pounds (5.3 kg).
This might not seem like a dramatic amount of weight loss, but according to the National Heart Forum, even a modest (5 percent) reduction in body mass index could spare the lives of millions of Americans and save billions of dollars in healthcare costs (4).
Indeed, both groups also experienced improvements in other health markers. After 12 months, participants had reduced fasting glucose, insulin, and triglycerides and improved systolic and diastolic blood pressure. In fact, 36 participants in each group that had metabolic syndrome at baseline had improved their health so much that they no longer fit the diagnostic criteria for metabolic syndrome.
Just to reiterate, there were no differences between low-carb and low-fat. When the subjects focused on real, whole foods and cut refined grains, sugars, and processed foods out of their diet, they lost significant weight, without having to count calories or restrict energy intake. However, this was based on averages, and does not mean that an individual might not respond better to a low-carb or low-fat diet.
Open questions and future directions
This study represents an incredible effort by the researchers and fills an important gap in the scientific literature. Still, any study worth its marbles will inevitably create some open questions:
Why did some people lose weight and others didn’t? While participants on average lost weight in both groups, there was a huge variability in individual responses: some lost up to 50 pounds, while a few gained weight. The researchers hypothesized that individual responses would depend on genetics or insulin response to carbohydrates, but the data didn’t support this idea. Thus, the elusive factor that determines success in weight loss is still unknown. (I heard through the grapevine that they also collected fecal samples in this study, so analysis of the gut microbiome may offer some clues.)
Was the low-carb group truly low-carb? Despite starting off at a carbohydrate intake of about 20 grams per day in the first two months, the low-carb group was already consuming 97 grams per day by three months, 113 grams per day by six months, and 132 grams per day by 12 months (including 22 grams of added sugar). In other words, while they started the trial on a very-low-carb diet, by month 12, they were consuming more of a moderate-carb diet. It’s conceivable that sustained ketosis could have sparked greater weight loss in the low-carb group. However, the subjects were instructed to eat the lowest amount of carbs they could sustain over time, and most found that ketosis was simply unsustainable.
Will both groups keep it off? It will be interesting to see the five-, 10-, and 15-year follow-ups of this study, to see how many people in each group were able to successfully keep off the weight they lost. If they stick to a whole-foods way of eating, my guess is that they might!
“Western” diets vs. ancestral diets
Not too surprisingly, the findings from this study align with what we see in populations eating a more ancestral diet. Among traditional cultures, fat and carbohydrate consumption vary widely, yet obesity is essentially nonexistent. The Inuit, Masai, Turkhana, and Kavirondo consume up to 58 percent of their daily energy from animal fat, yet are lean and have excellent metabolic health (5). Likewise, the Kitavan Islanders of Melanesia consume 60 to 70 percent of their daily energy as carbohydrates from fruit and tubers (6), yet boast healthy levels of insulin and blood glucose (7, 8) and have a virtual absence of obesity (9).
These tribes don’t just have superior genetics. Inuit that have left their traditional lifestyle for a Western diet and lifestyle also left their protection against cardiovascular disease behind (10). Similarly, Kitavan Islanders who leave for the mainland and begin to eat a Western diet quickly become overweight (11).
What we can learn from this study
Altogether, this new research adds to anthropological data to clearly show that the quality of food is far more important than the macronutrient composition. Here are the overall takeaways from this article:
- Eat real food for weight loss. Focus on fresh, whole foods that are minimally processed, and eat mindfully, stopping when you’re full.
- Self-experiment. The study described here found that on average, there were no differences in weight loss between the low-fat and low-carb groups. However, it doesn’t mean that a single individual won’t fare better on one diet versus another.
- Consider life stage and underlying conditions. This study was performed on obese adults without major underlying conditions. If you are pregnant, lactating, or have poor thyroid function, you probably still need a moderate to high carb intake. If you have diabetes, a lower-carb diet may help manage the condition.
- Get support. This study also provides evidence that access to regular support from dietitians or health coaches can help people make lasting behavior change and improvements in their health.
Now I’d like to hear from you. Have you tried low-carb or low-fat for weight loss? Which ancestral tribe does your diet look most like? Let us know in the comments!