An estimated 45 million Americans go on a diet every year and spend billions on weight-loss products, yet more than a third remain obese. Diet experts have notoriously flip-flopped between advocating for low-carb and low-fat diets, but finally after 12 years of research, the National Institutes of Health reveals the scientific truth. The findings, published in the journal Cell Metabolism, are the first to demonstrate how carbohydrate and fat restriction diets work in the body.

"A lot of people have very strong opinions about what matters for weight loss, and the physiological data upon which those beliefs are based are sometimes lacking," said the study’s lead researcher Kevin Hall, a metabolism scientist at the National Institute of Diabetes and Digestive and Kidney Diseases, in a press release. "I wanted to rigorously test the theory that carbohydrate restriction is particularly effective for losing body fat, since this idea has been influencing many people's decisions about their diets."

First, Hall and his research team studied data from dozens of dietary studies on how nutrition played a part with an individual’s metabolism and body weight. He found when a person limited their carb intake, it would lead to change in the amount of fat burned by the body. However, when fat intake was limited, it led to a greater overall bodily fat loss.

Next, Hall needed to prove his theory with actual humans. For two weeks, the researchers monitored 19 obese people by confining them to a ward where every single piece of food they ate was carefully controlled. In the first stage, 30 percent of the calories from carbs were limited. Every day the amount of fat each participant ate and burned was measured.  In the second stage, 30 percent of the calories from fat were limited, and each participant’s levels of fat loss were measured. The participants' weight loss results reflected his theories that it was cutting fat, not carbs, that would provide the greatest results. 

The Great Carb-Fat Debate

When Robert Atkins came across a study written in 1958 discussing the benefits of carb-restrictive diets, he quickly designed a diet plan boasting the low-carb benefits. By 1972, the diet spread like wildfire as a revolutionary one-trick pony for losing weight. “Eat hamburgers and throw away the bun,” Atkins told his readers.

The diet was high in fat and protein, and because millions of people began religiously following the low-carb diet, the Center for Science in the Public Interest took a closer look into Atkins’s theories. They said it failed to mimic the real world because it didn’t measure the impact of how balanced fat, low carbs, and proteins interacted with the human body. Despite Atkins’s resistance to scientific scrutiny, the diet became one of the greatest fads to hit America.

People became confused. Was it the fat they should be cutting instead? The focus turned back on the U.S. Department of Agriculture, which recommended limiting the tippy top of the food pyramid: fats. It was 1982 and the public demanded an answer for how to lose weight. Fat became the target, blamed for heart disease and said to be high in calories and low in essential nutrients and fiber. To reduce fat, they recommended to “cut down on fatty meats.” Losing out on vital levels of protein and iron on a long-term basis was difficult for many dieters. Today, dieticians have a better understanding on the difference between good dietary fat and bad, but unraveling the decades of misconceptions will continue to be a battle as the public struggles to keep up with scientific discovery.

The NIH’s new study will help alleviate some of that confusion. Although it was a small short-term study, it was thorough enough to prove different diets lead to different types of weight loss. The results rule out popular misconceptions that say all calories were created equal and another says carbs are fattening, so cutting them should lead to weight loss.

According to Hall, cutting fat may work on a short-term basis, but committing to a diet that cuts out a major nutrient like fat may be difficult. In the end, commitment to a diet for long periods of time is what’s going to prove successful. Their next step is to see how the two diets affect the brain’s reward region of the brain in order to figure out why certain individuals respond to restrictive diets and nutrients.

"We are trying to do very careful studies in humans to better understand the underlying physiology that will one day be able to help generate better recommendations about day-to-day dieting," Hall said. "But there is currently a gap between our understanding of the physiology and our ability to make effective diet recommendations for lasting weight loss."

Source: Hall, et al. Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity. Cell Metabolism. 2015.