Experts Reveal the Most Important Macronutrient for Weight Loss—and It's Not Carbohydrates

Reviewed by Craig Primack, MD, FACP, FAAP, FOMA

Written by Maxwell Barna

Updated 01/16/2024

One of the schemes that unfolded in the original Mean Girls was new girl Cady Heron’s attempts to sabotage the uber-popular Regina George by feeding her lies about nutrition. That element of their feud led to some of the 2004 movie’s most memorable quotes. 

There was the nearly Shakespearean inquiry about the essential quality of a common food: “Is butter a carb?” And, of course, who could forget the brutal rebuke Regina issued after another friend suggested they go on a fun outing to a big fast-food chain: “I can't go to Taco Bell, I'm on an all-carb diet. God, Karen, you're so stupid!” 

These scenes reflected common attitudes about dieting: Regina was dumb, the movie implied, for not realizing that Cady’s suggestion to try “an all-carb diet” would obviously make her gain weight, not “lose three pounds” like she hoped to do. 


But how much do carbs really affect our bodies? A new study attempts to shed light on that. 

The research, conducted by the Human Research Committees of Brigham and Women’s Hospital and the Harvard T.H. Chan School of Public Health, used data from three large ongoing studies to evaluate the effects of low-carbohydrate diets on weight loss and management. 

Researchers concluded that protein intake ultimately mattered more. In their findings, people who consumed low-carb diets (LCDs) with a heavy focus on animal protein gained more weight over time than those who followed low-carb diets with an emphasis on plant-based proteins.

But however tempting it might be to toss out the chicken breasts in your freezer after reading that, you might want to exercise some caution when it comes to applying the study’s conclusions to your own life. 

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Craig Primack, MD, FACP, FAAP, FOMA, a physician specializing in obesity medicine, noted that the study has some important limitations. 

For starters, it measured changes in self-reported body weight over four-year intervals, rather than weighing respondents in a lab or instituting a rigorous method for tracking their actual food intake. 

“It’s really hard to determine what people are doing when you only ask them something every four years,” Dr. Primack, Senior Vice President of Weight Management at Hims & Hers, says. “If I asked you to recall your diet from even a month ago, like, ‘What did you eat over a given week?’ You'd be like, ‘A month ago—what date was that?’”

Tiffani Bachus, RDN, a dietitian and certified personal trainer, co-founded the wellness resource U Rock Girl with fellow RDN Erin Macdonald. The two dietitians share Dr. Primack’s concern about the methodology. Food frequency questionnaires like the one used in the study are susceptible to inaccuracies, she noted in an email. And despite its large sample size—a total of 123,332 respondents—the grouping of participants also isn’t as straightforward as it might seem. 

The study claims to evaluate five distinct kinds of low-carb diets: a “total LCD” focused on overall lower carb intake; an “animal-based LCD” focused on animal-sourced protein and fat; a “healthy LCD” focused on less refined carbs, more plant protein and healthy fat; and an “unhealthy LCD” focused on less healthy carbs, more animal protein and unhealthy fat. 

Bachus points out that the researchers lumped all animal protein in with their “unhealthy LCD” group, not just processed animal proteins. That makes the data harder to parse. We don’t know if those respondents were actually consuming healthy animal proteins, meaning any weight gain measured in the group can’t be meaningfully attributed to their animal protein intake.

As a general rule, experts agree, blanket statements about diets usually aren’t the answer. 

But protein does have a uniquely important role in our diets—including weight loss and management.

“Protein, which is made up of amino acids, are the building block and repair mechanism for every tissue in the body—muscle, bone, cells, digestive system,” Bachus explains. “Protein levels in the body, often regarded as lean mass, are directly correlated with metabolism. The higher the lean mass, the higher the metabolism.” 

Consuming a variety of protein sources throughout the day is one of the easiest ways to maximize these benefits. Some of the best sources of protein, Bachus notes, include a mix of animal and plant-based foods: organic poultry, pasture-raised eggs, wild-caught fish, raw nuts and seeds, organic soy and grass-fed/finished beef. 

The crucial role protein plays in building muscle also helps explain why just measuring weight loss isn’t an ideal way to evaluate the impact of diet on our health. 

As Dr. Primack notes, the new study doesn’t account for the fact that the total average weight loss of its “healthy LCD” group could very well be loss of muscle. And, of course, any diet that revolves heavily around restriction simply isn’t healthy—or even viable—for most people. 

Our bodies need many different kinds of nutrients, which diets such as keto make harder to attain. Completely restricting specific foods, especially palatable ones like refined carbohydrates or fats, just isn’t a sustainable way of approaching diets. 

Jessica Yu, Ph.D., a clinical psychologist and Sr. Director of Patient Experience at Hims & Hers, explains that this psychological pressure makes it more likely for people to have a hard time sticking to their rigid diets. And even if they do adhere to the diet for a set time, or lose the desired weight, people who are prone to all-or-nothing thinking tend to experience worse mental health outcomes

“If you restrict the types of foods that you eat, it may build up physical hunger, but it also builds up psychological hunger,” Dr. Yu says. “You start to crave the things that you used to enjoy without limitation.” 

Dr. Yu added that the most sustainable approach to weight loss is thinking flexibly about diets, regimens and what your unique body needs to succeed. That could mean working with a dietitian or consulting with a physician if you’ve spent considerable time struggling to lose significant amounts of weight. 

Whatever your plan, it’ll need to work for your lifestyle in the long-term. That’ll likely require changing your approach to both diet and exercise. Or, to put it a little more bluntly, odds are pretty high that you won’t achieve your goals by jumping on the latest iterations of fad diets.

As Dr. Yu explains, “I always encourage people, as you're hearing about diets that may be popular, look at the diets and carefully do your research, but make sure to also understand your own behavior, history and preferences to decide what is the right diet for you.”

7 Sources

  1. Liu, B., Hu, Y., Rai, S. K., Wang, M., Hu, F. B., & Sun, Q. (2023). Low-carbohydrate diet macronutrient quality and weight change. JAMA Network Open, 6(12).
  2. Kipnis, V., Midthune, D., Freedman, L., Bingham, S., Day, N. E., Riboli, E., Ferrari, P., & Carroll, R. J. (2002). Bias in dietary-report instruments and its implications for nutritional epidemiology. Public health nutrition, 5(6A), 915–923.
  3. Moon, J., & Koh, G. (2020). Clinical Evidence and Mechanisms of High-Protein Diet-Induced Weight Loss. Journal of obesity & metabolic syndrome, 29(3), 166–173.
  4. Calton, J. B. (2010). Prevalence of micronutrient deficiency in popular diet plans. Journal of the International Society of Sports Nutrition, 7(1).
  5. Harvard Medical School. (2022, August 9). Ketogenic diet: Is the ultimate low-carb diet good for you?. Harvard Health Publishing .
  6. Gillen, M. M., Markey, C. N., & Markey, P. M. (2012). An examination of dieting behaviors among adults: Links with depression. Eating Behaviors, 13(2), 88–93.
  7. Khawandanah, J., & Tewfik, I. (2016). FAD diets: Lifestyle promises and health challenges. Journal of Food Research, 5(6), 80.
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