Want to lose body fat? At least initially, a diet low in fat may be better than one that limits carbohydrates, a US study suggests.
To test the effects of cutting fat or carbs head-to-head, researchers put 19 obese adults in the hospital twice, each time reducing their calorie intake by about 30 per cent and putting them all on identical exercise routines.
During one stay, they reduced calories by reducing the proportion of carbs, and for the other one they cut a similar number of calories by reducing fats.
Eliminating fat calories for six days during the experiment led to about 89 grams (3.1 ounces) a day of body fat loss, compared with 53 grams (1.9 ounces) a day of fat loss from restricting carbs, the study found.
"Our data and our model suggest that the body doesn't care that fat calories were cut," Kevin Hall, a researcher at the National Institutes of Health, said by email. "It just kept burning the same amount of fat as it did before which led to a substantial imbalance between the fat eaten and burned and therefore body fat loss."
But when researchers calculated the long-term impact of the two approaches, they estimated that differences in fat loss would diminish over time.
Globally, 1.9 billion adults are overweight or obese, according to the World Health Organisation. Obesity increases the risk of heart disease, diabetes, joint disorders and certain cancers.
Previous research has found obese people often struggle to shed excess pounds or keep weight off when they do lose it. Lifestyle changes such as following a healthy diet and getting regular exercise can often help in the short-term but fail to produce lasting results, particularly among people who have more than 100 pounds to lose before reaching a healthy weight.
For the current study, Hall and colleagues set out to test a popular belief that drives many people to try low-carb diets - that reducing foods like pasta, white bread and sugary treats can curb supplies of the hormone insulin, which in turn limits the body's ability to accumulate fat. With less insulin, the body may also burn existing fat stores for energy and lose weight.
Ten men and nine women were hospitalised twice for two-week stays, which included five days to establish a baseline diet before starting the experiment restricting calories through fat or carbohydrate reduction. In addition to prepared meals, they all exercised on a treadmill for one hour a day at a set pace and incline.
In one respect, the study lent credence to popular beliefs about low-carb eating. Cutting carbs did indeed decrease insulin production in the study. It also led to an increased breakdown in stored fats that can be used for energy, a process known as fat oxidation.
By contrast, the low-fat diet didn't significantly change insulin production or fat oxidation.
It's hard to say how these results might translate into weight loss in the real world, given how tightly controlled the food intake and energy output was for study participants during their hospitalised diet test, the authors point out in the journal Cell Metabolism.
But the results do offer solid evidence that low-carb diets aren't superior for fat loss, Susan Roberts, a nutrition researcher at Tufts University, notes in an editorial accompanying the study.
"Basically what the results say is that overall energy balance is the biggest factor - how do you cut calories in what you eat," Roberts said by email.
To help answer this question, more research is needed that explores how consumption of fat, carbohydrates and protein influences how many calories people eat, she said.
"Moderation is not a cool word these days, but if it actually works better we should be working out how to make that the case," Roberts said.