Low-Fat Versus Low-Carb: Which Is Better for Weight Loss?

A new study suggests that when it comes to weight loss, high-fat diets may not be the way to go. REUTERS/Andres Stapff

In recent years, dieters have enjoyed gorging on copious amounts of bacon, steak and other high-fat foods after being told by experts that eating like cavemen could be the secret to staying slim. But Paleo and Atkins fans may want to reconsider their weight-loss plans after seeing a new study published Thursday in the journal Cell Metabolism that suggests maintaining a low-fat diet rather than one excruciatingly low in carbohydrates is more effective for dropping extra pounds.

This may seem surprising, but it's because the scientists sought to identify a clear distinction between the effects of these two common approaches to dieting by testing the theory that carbohydrate restriction changes the amount of fat burned off by the body, whereas sticking to a low-fat diet leads to a higher level of overall loss of body fat.

The head scientist, a former physicist turned metabolism researcher at the National Institutes of Health, has reviewed dozens of studies from over the past few decades and developed a mathematical model to explain the hypothesis that these two diets work in different ways and that one may ultimately be more effective. He came to see that no one had ever tested out the theory in humans.

So he and fellow researchers recruited 19 obese adults for a two-week inpatient trial (also known as a "metabolic ward" study, in which patients are locked up and under around-the-clock supervision). The study's design allowed the researchers complete control over what participants ate.

The researchers of the study found participants lost an average of 3.1 ounces of fat per day while on the low-fat diet for six days. When put on the low-carb diet, participants lost an average of 1.9 ounces of fat per day in the same amount of time.

"There is one set of beliefs that says all calories are exactly equal when it comes to body fat loss and there's another that says carbohydrate calories are particularly fattening, so cutting those should lead to more fat loss," Kevin Hall, lead author of the study and a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases at the NIH, said in a press statement. "Our results showed that, actually, not all calories are created equal when it comes to body fat loss, but over the long term, it's pretty close."

However, studies have shown there are a number of medical benefits of low-carb diets, as this eating plan lowers blood sugar and triglyceride levels, which significantly reduces the risk for diabetes and heart disease, and that is critical for obese patients. One study that involved 148 obese patients, published in 2014 in Annals of Internal Medicine, found that those on low-carb diets lost an average of 7.7 pounds more in a year than those who stuck with low-fat diets. The group also saw a significant improvements in levels of both good and bad cholesterol.

Researchers of the new study that favors low-fat diets admit their findings do have some limitations, since the study model relied on a strict control of participants' diets, which doesn't simulate real world experience. Additionally, the study didn't examine eating behaviors.

"We did not address whether it would be easier to adhere to a reduced-fat or a reduced-carbohydrate diet under free-living conditions," the researchers write in their study. "Since diet adherence is likely the most important determinant of body fat loss, we suspect that previously observed differences in weight loss and body fat change during outpatient diet interventions were primarily due to differences in overall calorie intake rather than any metabolic advantage of a low-carbohydrate diet."

Their conclusion then may be in line with another study published last year in JAMA that found the difference in the actual number of pounds lost on any of the popular commercial diet plans are negligible, regardless of whether it involves skipping pasta or steak for dinner.