Study Sheds Light on How Low-Carb Diets Can Boost Metabolism and Help With Weight Loss

When it comes to weight loss, the conventional approach has been to eat fewer calories to drop more pounds. But a study published in November 2018 in the journal BMJ seems to challenge that notion and finds that the body may not process all calories the same way.

Calories from carbs have often been thought to promote weight gain more than other calories. Many current diets emphasize a low-carb approach, including the popular ketogenic diet.

“Our study adds to the literature that suggests metabolic benefits for reducing processed carbohydrates in our diets,” says David Ludwig, MD, principal investigator in the study and an endocrinologist at Harvard Medical School and Boston Children’s Hospital in Massachusetts. “We found that diet had a major impact on metabolism — people consuming the lower-carbohydrate diet in our study burned 250 calories more than those on the high-carb diet. That increase in metabolism could spontaneously cause a 20-pound weight loss over three years without eating any less. If the low-carb diet reduced hunger and food intake as well, the weight loss would be greater.”

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Dr. Ludwig and his colleagues followed 164 overweight volunteers from Framingham State University who were between ages 18 and 65 with a body mass index (BMI) of 25 or more. Researchers divided them into groups and assigned them one of three diets: one with 60 percent of their calories from carbohydates and 20 percent fat, one with 40 percent from carbs and 40 percent fat, or one with 20 percent from carbs and 60 percent fat. Each group received 20 percent of their calories from protein.

Ludwig says the team kept each diet as healthy as possible. For instance, the low-carb diet eliminated grains and potatoes for the most part and substituted them with healthy high-fat foods, like nuts, avocados, full-fat dairy, and olive oil, while mashed cauliflower might have replaced high-carb mashed potatoes. Meanwhile, the high-carb diet contained lots of whole grains, fruits, and vegetables.

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For five months of the test, scientists made sure subjects maintained the same weight so they could measure their metabolisms according to the different diets.

Ludwig and his team found that all calories are not alike to the body. “Restricting carbohydrates makes for a better strategy than restricting calories for maintaining weight loss,” he says.

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The Relationship Among Insulin, Carb Intake, and Weight Gain

Insulin is a hormone produced by the pancreas that helps keep blood sugar levels steady. Ludwig says insulin plays a role in weight gain when a person eats too many carbs. After eating processed carbs from sources of refined carbs, such as white bread, cookies, or cake, your insulin levels spike. Next, insulin traps more calories in your fat cells, causing weight gain. But the rest of the body still needs calories, so you remain hungry. This cascade of events can make you more likely to overeat and slow your metabolism.

Ludwig says that study participants who had been producing the most insulin (the high insulin secretors) saw the biggest effect on the low-carb diet. The theory is that on their normal diets, the participants’ bodies worked in overdrive to produce insulin, leading to extra stored calories and a slower metabolism. On the low-carb diet, these high insulin secretors were burning 400 more calories per day compared with those on the high-carb diet.

The Health Benefits of a Low-Carb Diet for People With Diabetes

Although this study didn’t examine diabetes, the researchers found that high insulin secretors may benefit the most from the low-carb and high-fat diet.

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“We have found that the low-carb approach works with diabetes patients who have insulin resistance,” says Jan Rystrom, MD, a registered dietitian and diabetes educator at Swedish Medical Center in Seattle, who was not involved in the study. “Insulin resistance tends to cause excess insulin secretion. Insulin, a growth-like hormone, stores fat. Excess insulin production is required to normalize blood glucose following a high-carbohydrate meal, and results in fat storage. A low-carb approach can be very effective for weight loss for the person who has some metabolic disorder,” including diabetes.

With diabetes, glucose is not getting into the cells properly so it can be used as energy. The hormone insulin normally acts to unlock the cells so the cells can use the glucose. With diabetes, too much glucose remains in the blood (called hyperglycemia), which can cause numerous health complications when uncontrolled. Previous studies have found that low-carb diets can improve blood sugar levels.

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Why More Research Is Needed on the Effects of a Low-Carb Diet for People Without Diabetes

It’s well known that a low-carb diet can help people with diabetes, but Kelly Kennedy, RD, dietitian for Everyday Health, says a study longer than 20 weeks would be needed to know how the diet may benefit an overweight person without the disease.

“The results of this study are certainly interesting,” Kennedy says. “For those looking to lose weight, though, I'd advise to take these results with a grain of salt.”

As anyone who’s tried a low-carb diet knows, sticking to it long-term can be challenging. The participants in the current study had the benefit of close medical supervision that helped them adhere to their assigned diet.

It’s also unclear what health effects may result from being on a low-carb diet when followed for the long haul. In the current study, participants in the low-carb diet saw lower levels of leptin, a hormone that inhibits hunger. Over time, Kennedy says, “having lower levels of it could potentially lead to an increase in hunger in the low-carb group, and increased calorie intake.”

Another possible risk of going low-carb is taking in too much saturated fat, which can increase the risk of heart disease, according to a study published in November 2016 in the journal BMJ. In the current study, 35 percent of the participants’ fat came from saturated fats, and 15 percent of the carbs came from unhealthy added sugar, Kennedy says. “In the low-carb and high-carb groups, respectively, this can add up to quite a bit of these unhealthy nutrients,” Kennedy says.

Another good reason for more research: Past research shows there are conflicting findings regarding the overall health benefits of a low-carb diet. For example, a small study published in August 2015 in the journal Cell Metabolism found that a low-fat diet resulted in more body fat loss in people with obesity than did a low-carb diet.

Separately, Kennedy adds that low-carb diets may in fact damage the gut microbiome, which is the collection of bacteria that a review published in January 2015 in the journal Current Opinion in Gastroenterology revealed may play a role in weight, immunity, and chronic disease risk. A review published in September 2015 in the journal Current Opinion in Clinical Nutrition & Metabolic Care suggested a high-fat, low-carb diet may alter gut bacteria in a way that possibly affects the risk for obesity and chronic disease.

“There are not enough studies on the health outcomes of following a low-carb diet for a long period of time in order to determine its potential effects,” Kennedy says.

Ludwig says that’s on the horizon. Next, his team plans to study how a low-carb diet may influence weight loss over a longer period of time.

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Additional reporting by Melinda Carstensen.