Calories in food come from three primary sources, known as macronutrients: carbohydrates (4 calories/gram), fat (9 calories/gram), and protein (4 calories/gram). Weight-loss theories have shifted over the years, alternating between recommending low-fat and low-carb diets. Low-fat diets gained popularity in the eighties based on the premise that, because fats are the most calorie-dense macronutrient, reducing their intake is the most effective way to reduce overall caloric consumption. Decades later, the pendulum shifted to low-carb diets with the popularity of the Atkins diet, which contends that carbohydrate depletion induces ketosis, causing the body to burn fat for fuel. The resulting ketotic state also theoretically suppresses appetite, leading to reduced caloric consumption.

So, which diet is better? Fortunately, numerous rigorous studies have compared low-fat and low-carb diets, providing scientific evidence to support a conclusion. Rather than discussing all the studies, we will focus on one long-term trial. A study published in 2010 randomized 307 overweight adults to either a low-carb diet (following Dr. Atkins' New Diet Revolution Plan) or a low-fat diet. After two years, average weight loss on the low-carb diet was 14 lbs (6.2%) compared to 16 lbs (7%) on the low-fat diet, a difference that was not statistically significant. [Study abstract]

This study found no significant difference in weight loss between a low-fat and low-carb diet over two years, and its results are in agreement with with other studies comparing the diets. A consistent finding across trials is that low-carb diets often induce greater weight loss during the initial 6-month period. A possible reason for this observation is that glycogen, the storage form of carbohydrates in muscle tissue, is hydrophilic, meaning it attracts and holds water. Low-carb diets deplete glycogen and the water associated with it; therefore, the enhanced early weight loss observed with low-carb diets may be due to water loss, not fat loss.

Another consideration when discussing dietary calories is alcohol. By itself, alcohol contains 7 calories/gram, second only to fat (9 calories/gram). Wine, beer, and mixed drinks also frequently contain other carbohydrates. Individuals trying to lose weight should be aware that alcoholic beverages can add a significant amount of calories to their diet. For example, a typical twelve-ounce light beer contains 100 calories, while IPAs and dark beers contain 200 to 300 calories. A five-ounce glass of wine contains around 130 calories. A shot of liquor contains about 100 calories, and mixed drinks can contain much more, depending on what is added. Alcohol can also induce hunger, causing people to eat more when drinking.

Finally, I want to mention satiety. A core principle of the Atkins diet is that fatty foods are more filling than low-fat foods; therefore, people eat less on a low-carb diet. Studies evaluating this notion have found that low-fat and low-carb foods containing the same number of calories have similar satiety. Because high-fat foods are more calorie-dense, the volume of food needed to induce satiety is less, but at the end of the day, an equal amount of carbohydrate calories will cause the same satiety. The same holds true for protein.

Ultimately, low-fat and low-carb dietary approaches yield similar long-term weight-loss results. For effective weight management, individuals should prioritize achieving a consistent caloric deficit rather than severely restricting any single macronutrient category. Diets with highly restrictive models, such as the Atkins diet, are challenging to maintain, compromising long-term adherence.

• Review of low-fat vs low-carb diets