When weight loss is the goal, is low-fat or low-carb better? This is a question that's had dieters scratching their heads for years, and which has plenty of die-hard believers on both sides. But, it turns out that the differences may not matter that much after all.
A recently concluded year-long Stanford University study gave the best effort yet at giving a definitive answer. It closely followed a full year of weight-loss attempts by 609 men and women, all of whom were in good health, with an average BMI of 33 (class 1 obesity) and an average age of 40 years. In other words, there were a lot of people in the study, they were tracked for a long time, and they were tracked carefully. As studies go, it was a huge effort and a well-designed approach.
Twelve months later, the two groups had lost a total of more than 6,500 pounds—though some people lost as much as 60 pounds, while others gained as much as 20 pounds. So, on average, which group lost more?
Hold on to your bacon, keto believers. The answer might surprise you.
Finding a Sustainable Level of Consumption
At the start of the study, members of each group were instructed to follow a specific diet for the first two months. People in the low-carb group were told to consume only 20 grams of carbs a day—which is low enough to meet the standard for most interpretations of ketogenic dieting. The low-fat group, on the other hand, consumed only 20 grams of fat per day. After the two months, people in the low-carb group were told to add more carbs back into their diet until they felt they could maintain the diet at that level. The low-fat group was instructed to do the same thing with their fat intake.
Crucially, the researchers never told the participants how many calories they could have every day. They just told them to "maximize vegetable intake...minimize intake of added sugars, refined flours, and trans fats; and...focus on whole foods that were minimally processed, nutrient-dense, and prepared at home whenever possible."
By the end of the third month, researchers found that fat consumption in the low-fat group had doubled to an average of 42 grams of fat per day, up from the original 20 grams. Meanwhile, carb consumption in the low-carb group shot up from an average of 20 grams of carbs per day to 96 grams per day.
And the Winner Is…
Despite this difference in consumption levels, at the end of the 12 months, results showed that it didn't matter at all whether people were focusing on their fat intake or their carb intake. On average, both groups consumed about the same number of calories each day and, even though some individuals gained or lost more weight than others, both groups lost about the same amount of weight.
No matter which group they were in, participants saw about the same improvements in measurements like:
- BMI numbers
- Body-fat percentage
- Waistline measurement
- Blood pressure
- Fasting insulin
- Blood glucose levels
There were some differences, though. For instance, the low-fat group saw a bigger drop in "bad" cholesterol (LDL). And while LDL levels rose more in the low-carb group, so did levels of "good" cholesterol (HDL).
The low-fat group lowered the amount of saturated fats they consumed while increasing the amount of fiber in their diets. The low-carb group consumed slightly less fiber, but saw an overall lower glycemic index of their diets. Both groups lowered their overall glycemic load, but the low-carb group lowered it more than the low-fat group.
As Kamal Patel points out in Examine.com's excellent in-depth analysis of the study, the difference (or lack thereof) in calories and protein intake may be the key determinant. Both groups ended up with nearly identical numbers in terms of caloric intake, and the low-carb group consumed only slightly more protein on average, to the tune of 12 grams per day more.
"The results of this study contribute to a large body of evidence indicating that, for weight loss, neither low-fat nor low-carb is superior (as long as there's no difference in caloric intake or protein intake)," Patel writes.
How to Explain the Results?
The researchers suggested that a key to the results may have been when, after the initial two months, they gave all the participants specific goals for their dietary behaviors, not specific daily caloric limits. Rather than saying "eat this much of that," they had them personally determine the lowest level of either fats or carbs they needed to not feel hungry, and crucially, to prioritize healthy, nutrient-rich foods when they did it.
The takeaway for you? Calories may still matter—although counting them exactly may not. Protein still matters. Food quality matters. But carbs and fats? Perhaps not so much. Many people find they simply prefer more of one or the other—i.e., they crave more carbs or more fat. As long as you have those first three priorities lined up, feel free to design your personal diet based on your preferences, and what you can sustain for the long haul.
- Gardner, C. D., Trepanowski, J. F., Del Gobbo, L. C., Hauser, M. E., Rigdon, J., Ioannidis, J. P., ... & King, A. C. (2018). Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA, 319(7), 667-679.