An End to the Low-Fat vs. Low-Carb Debate?

For years, dieters have been trying to decide whether to follow a low-fat or low-carb diet. Now they are likely to find themselves at a dead-end. The superiority of either for weight loss may be nothing more than a moot point, with a much more powerful dieting framework rising to take its place.

Researchers at Stanford University School of Medicine recruited over 600 adults to see if they could find a link between a person’s genetics and what type of diet would work best for them to promote weight loss. Study participants were between the ages of 18 and 50; approximately half men and half women. One group followed a low-fat diet for one year while the other group ate a low-carbohydrate diet for the year.

After a year, people in both groups had lost an average of 13 pounds, but there was a big range individually when it came to how much weight was lost.

Prior to beginning their diets for the study, participants took part in genome sequencing and a fasting insulin test that were used as predictors of weight loss. Genome sequencing enabled researchers to search for gene patterns that might influence carbohydrate or fat metabolism. The fasting insulin test measured insulin secretion, which is believed to affect how the body responds to certain diets.

People in the study were told to limit their carbohydrate or fat intake to 20 grams a day during the first eight weeks of the study. That amounts to consuming about a slice and a half of whole wheat bread or a substantial handful of nuts for each person daily, depending on which diet they followed. They were allowed to gradually and incrementally add back from 5 to 15 grams of fat or carbohydrates after the second month, until they reached an amount they felt like they could live with.

People on both the low-fat and low-carb diets were told to eat in a way that didn’t make them feel deprived or hungry and to follow a diet they could stick with forever.

After a year, the people eating the low-fat diet reported eating an average of 57 grams of fat per day, down from 87 grams before the study. Those on the low-carb diet were taking in roughly 132 grams of carbohydrates each day, down from about 247 grams of carbs a day before the study began; so there was an improvement in both groups.

Researchers tracked the dieters’ weights, body composition, fasting insulin levels, and the amount of fat or carbs they ate daily over the course of the study. After a year, people in both groups had lost an average of 13 pounds, but there was a big range individually when it came to how much weight was lost. Some people lost 60 or more pounds, while others gained 15 to 20. And as to the question of the role of genetics or insulin secretion on weight loss success, none was found.

What’s important to note here is that the people in both groups of the study were eating healthy — low-fat and low-carb diets. They weren't eating “low-carb” pork rinds and salami or drinking “low-fat” soft drinks. They were urged to shop at farmers’ markets and to avoid buying processed, convenience foods. They were told to eat in a way that didn’t make them feel deprived or hungry and to follow a diet they could stick with forever.

The takeaway from this study is that the basic strategy for losing weight is not low-fat versus low-carb, but eating less sugar and refined flour, less trans fats and more whole foods, and loading up on vegetables. There was no calorie counting in this study. Cutting back on either fats or carbohydrates automatically reduces calories.

So it turns out the answer as to which diet is the best for weight loss is whatever you can live with for the rest of your life.

The study is published in JAMA.