Diet debates are nothing new. The diet wars have been around for decades, and at center stage you’ll find two heavyweights: Low-Carb vs Low-Fat. Which diet “works” better? Does either really work? Is there a magic metabolic advantage to one of the diets? Is a calorie still a calorie? What about protein?
You may have already picked your side and are diligently following one or the another. But keep an open mind as we find out answers to the above questions and more in this low-carb vs low-fat faceoff. I think you’ll be surprised.
What Is Low Fat?
For decades, a low-fat diet has been embraced by doctors, the government, the food industry, the media, and ultimately, people like you and me. The movement toward a low-fat diet stems from observational studies (most notably, Ancel Key’s Seven Countries study and the Framingham study) conducted in the 1940s and ‘50s. These studies suggested a strong correlation between diets high in saturated fats and cholesterol and increased risk of heart disease. They ultimately led to what’s referred to as the “diet-heart hypothesis.”1
Touted as being heart healthy and good for weight management, the low-fat movement, promoted by Ancel Keys and others, eventually transcended to ideological status. According to Virginia Tech professor Ann La Berge, “Many Americans subscribed to the ideology of low fat, even though there was no clear evidence that it prevented heart disease or promoted weight loss. Ironically, in the same decades that the low-fat approach assumed ideological status, Americans in the aggregate were getting fatter, leading to what many called an obesity epidemic.”2
The low-fat diets were intended to emphasize greater consumption of vegetables, fruits, whole grains, and legumes. Unfortunately, one of the biggest issues is that diet quality seemed to go to hell in a handbasket. In fact, many point the finger at the low-fat diet craze as the primary dietary culprit “contributing to the current twin epidemics of obesity and diabetes.”3 This is because it spurred an increase in the consumption of refined carbohydrates and added sugars (to replace the calories and palatability of fat).
That’s surely not what Ancel Keys had intended. But as I often am known to say, it is what it is. Case in point, consider the results of the U.S. National Health and Nutrition Examination Survey (NHANES). Results that showed carbohydrate consumption increased from 39% of total caloric intake in 1971 to 51% in 2011. During the same time period, the percentage of overweight Americans also increased dramatically, from 42% to 66%!4
Just how low is low fat? Surprisingly, it’s not as low as you might think. Piggybacking on Keys’ research, it’s generally accepted that a low-fat diet contains less than 30% of calories from fat. And a low-fat diet also typically emphasizes a low intake of saturated fat (<7% of calories). However, it’s important to point out that “low fat” encompasses a pretty broad range of fat intakes. Especially considering that some of the more extreme low-fat diets (such as the Ornish Diet) restrict fat intake to a measly 10% of calories.5
Of course, as alluded to above, a low-fat diet is synonymous with a higher carb intake (upwards of 55% of calories). And generally speaking, the lower the fat intake, the higher the carb intake. What’s more, many low-fat diets advocate limiting intake of red meat and other animal-based foods. In other words, low-fat diets tend to be synonymous with lower protein intakes. Make sure you tuck that in your pocket and hold onto it. You’ll see why it’s important in a minute.
As mentioned above, the intention of low-fat diets would be to emphasize minimally processed plant-based foods (e.g., veggies, fruits, whole grains, legumes). But in practice, we often see an increase in “foods” made with refined grains and added sugars along with a concomitant reduction in protein-rich foods and healthy fats (e.g., nuts, seeds, olives, coconut, avocados, etc.). Not good.
What is Low Carb?
Despite the stronghold low-fat diets have held on Americans, low-carb skeptics have been around for years. (The Atkins diet, for example, in the 1970s.) Certainly in the last couple of decades, we’ve witnessed a paradigm shift toward carbohydrate restriction. By and large, low-carb proponents argue that their approach targets the very causes of obesity: carbohydrates and insulin.6
Contrary to low-fat diets, which often lead to reductions in diet quality, low-carb diets frequently promote better food choices. For instance, with low-ish carb diets, you tend to see an increase in the consumption of vegetables, fruits, lean meats, poultry (including eggs), nuts, seeds, and healthier fats and oils.
Another typical benefit of low-carb diets: Addition by subtraction. Although certainly not always the case, by telling people what not to eat, low-carb diets can naturally lead to reduced consumption of ultra-processed, high-carb junk food (e.g., refined grains, added sugar).
I say “low-ish” because when it comes to defining “low carb,” the lines are quite murky. There’s no question there’s a lack of consistent definition for “low-carbohydrate diets,” whether you’re searching Google or Pubmed. And of course, this makes it very complicated to compare the effectiveness of different diets.
In some circles, anything less than 40% of calories from carbs would be considered low carb.7 Still others have set the low-carb bar as high as 45% of calories.8 Let’s be honest: That’s not low carb. At best, it’s moderate carb. On the other hand, some have set the cutoff definition for a low-carb diet at 26% of calories, or less than 130 grams of carbohydrate per day.9 And still others suggest that a true low-carb diet limits carbs to 30 – 70 grams per day—or less. If there’s one type of low-carb diet that is well-defined, it’s the very-low-carbohydrate ketogenic diet, which limits carbohydrate to less than 30 grams per day.
What’s more, the ketogenic diet is also at the highest extreme when it comes to fat content. Remember what I mentioned above: Fat intake is generally inversely proportionate to carb intake. Along those lines, low-carb diets are typically higher in fat. Also critical to note is that low-carb diets, which typically emphasize moderate to high consumption of animal products, tend to be significantly higher in protein than low-fat diets. Put that in your pocket too.
So, while “low fat” may be more clearly defined than “low carb,” both leave a tremendous margin for interpretation. And this may explain, at least in part, why it can be challenging to answer the following question.
Low-Carb vs Low-Fat: What Does the Research Say?
To sum it up in a single sentence, low-carb diets (with varying degrees of carbohydrate restriction) typically perform as well or better than low-fat diets. This is for weight loss and markers of heart health (e.g., blood lipids, blood pressure) and metabolic function (e.g., blood glucose, insulin).10
In a review of 23 weight-loss trials, for example, researchers from Tulane University found that both low-carb and low-fat diets led to weight loss, reduced waist circumference, and improved metabolic risk factors. And there were no significant differences between diets. They concluded, “These findings suggest that low-carbohydrate diets are at least as effective as low-fat diets at reducing weight and improving metabolic risk factors. Low-carbohydrate diets could be recommended to obese persons with abnormal metabolic risk factors for the purpose of weight loss.”8
In a recent systematic review study published in The Lancet, a group of researchers (including esteemed Harvard scientists Walter Willett and Frank Hu), examined the data from over 50 randomized controlled dieting trials. They found that both low-carb and higher-fat diets led to greater fat loss than low-fat diets. This led them to conclude that the evidence “does not support low-fat diets” for long-term weight loss.11
In a separate review published in the journal Nutrients, researchers from the University of Florida examined the evidence base for current popular diets. They looked at diets ranging from low-fat (e.g., Ornish, DASH) to low-carb (e.g., Atkins) and everything in between (e.g., Zone, Mediterranean). Interestingly, they found the Atkins Diet, which emphasizes carbohydrate restriction, “showed the most evidence in producing clinically meaningful short-term and long-term weight loss.”12 Having said that, a randomized controlled trial published in the Journal of the American Medical Association comparing the Atkins, Ornish, Weight Watchers (low-fat), and Zone (low-carb) Diets found that “each popular diet modestly reduced body weight and several cardiac risk factors” after 1 year.13
Of course, when we look at individual studies, we could certainly “cherry pick” well-controlled, long-term (i.e., ≥ 1 year) randomized diet trials showing that low-carb diets are more effective for weight loss and improving cardiovascular risk factors.14 One of the best and most reliable examples is the A TO Z Weight Loss Study. This is a randomized trial conducted by a group of Stanford researchers led by Dr. Christopher Gardner. In the trial, the researchers compared 4 popular weight-loss diets—Atkins (low-carb and high-fat), LEARN (low-fat), Ornish (low-fat), and Zone (technically considered low-carb). They found that women following the Atkins diet lost more weight and experienced more favorable metabolic effects after 12 months compared to the other diets.15
At this point, it should be apparent that there seems to be compelling evidence to suggest that a low-carb diet is better for weight loss. However, before we conclude that low-carb diets possess some sort of magical “metabolic advantage,” several studies have pointed out that low-carb diets may hold the edge in free-living conditions (i.e., real life). This is because they tend to be significantly higher in protein, which ultimately leads to greater satiety, and you guessed it, reduced caloric intake.16
In other words, decreased caloric intake—not necessarily carbo restriction—explains the weight loss.
Wait! All this talk about fat and carbs, and protein may be the answer? Indeed, when studies match protein and/or calories, there is no apparent weight-loss advantage to low-carb vs low-fat diets.17,18 There’s no question that protein is one important variable in the equation, but there’s also something to be said about addition by subtraction (e.g., removing junk food) and improved diet quality. These may be additional factors helping to control appetite and caloric intake.
For what it’s worth, a tightly-controlled, short-term in-patient study (performed in a metabolic ward), a research group led by Kevin Hall at the National Institutes of Health found that, calorie for calorie, fat restriction led to greater fat loss than carbohydrate restriction. In other words, a low-fat diet resulted in more fat loss than a low-carb diet.19 While this is important research that sheds light on how these types of diets work, it’s important to remember we live in the real world. Not a heavily controlled environment.
Having said that, the take-home point is that while it may not be as simple as “a calorie is a calorie,” calories count. Both low-fat and low-carb diets can yield meaningful weight loss…provided you can stick to it. So, if a certain style of eating improves appetite control, reduces caloric intake, and you’re more likely to stick to it, then it may be advantageous. And although it was only briefly mentioned, it’s important to emphasize the significance of optimizing protein intake, which can positively influence both aspects of the energy balance equation—reduced caloric intake and increased metabolic rate.20
Which Is Better for YOU? It Depends
Here’s a fascinating and insightful quote from the authors of the A TO Z weight-loss study mentioned above: “Clinical trials comparing a spectrum of low- to high-carbohydrate diets demonstrate clinically meaningful weight loss on any reduced-calorie diet regardless of macronutrient composition, provided there is adequate adherence to the dietary regimen.”21 Hey, that’s what I just said!
In other words, both reduced-calorie low-fat and low-carb diets can work. The one that is going to work “best” for you is the one that you can stick to. I know that’s not earth-shattering. But it is important. And more important than the debate of low-carb vs low-fat.
While we typically look at psychological, emotional, behavioral, and environmental factors that can influence compliance, these authors set out to identify whether there were any physiological variables that might affect dietary adherence by influencing factors like hunger and metabolism. Interestingly, they found insulin resistant (IR) participants were less likely to stick to and lose weight on a low-fat diet compared to insulin-sensitive folks assigned to the same diet.
In other words, IR folks may do comparatively better on a low-carb diet. In the A TO Z study, for instance, IR folks lost nearly 3 TIMES more weight on the low-carb diet than on the low-fat diet.21,22 This isn’t the first time an observation of this nature has been noticed. And it suggests we may be able to personalize nutrition based on physiological variables.23
What’s cool is that there’s even more research emerging that’s helping paint this picture. For example, the Personalized Nutrition Project, led by Professors Eran Segal and Eran Elinav of the Weizmann Institute of Science, is aimed at constructing truly personalized diets. Along these lines, in a breakthrough study published in the journal Cell, professors Segal, Elinav, and colleagues found glycemic responses were highly variable across individuals to the same foods.
In other words, what may be “healthy” for one person may not be ideal for another. What’s particularly interesting is they found these variations could be traced back, at least in part, to the participants’ gut microbiome (the community of bacteria that makes up the gut).24
Low-carb vs Low-fat: What’s the REAL Answer?
If you ask Dr. Traci Mann, PhD, Professor of Social and Health Psychology at the University of Minnesota, the answer is NEITHER. In her book, Secrets from the Eating Lab, Dr. Mann spills the beans on the diet industry. She shares compelling science demonstrating that, by and large, diets rarely lead to significant and sustainable weight loss. Even more, she exposes gaping limitations in the existing research, suggesting that already grim statistics are most likely even worse than most people are led to believe.
For example, Dr. Mann and her colleagues have published a series of review studies looking at the long-term outcomes of calorie-restricted diets to assess whether “dieting” is truly an effective strategy for weight management. Their findings are astonishing, if not demoralizing.
For starters, they found that up to two-thirds of dieters regain more weight than they lose while dieting. And in the event weight loss is maintained, it’s a miniscule amount—about 2 pounds, on average. On top of that, these statistics likely underestimate the ineffectiveness of dieting because of several methodological problems, which bias the studies toward showing successful weight-loss maintenance. Considering that non-dieters gain about 1 pound during a similar time period, the most positive conclusion is: “Dieting [marginally] slows the slight weight gain that occurs with age among the average nondieter.”25,26
Dr. Mann isn’t the only one to draw the conclusion that, by and large, the traditional diet approach doesn’t work. Multiple studies comparing low fat to low carb (and everything in between) have reported overall low-adherence rates. Not surprisingly, folks who stick to their diets fair better. But, you cannot ignore the fact that compliance rates are historically poor—maybe 25% or so after a year’s time.13,15,22
So, what REALLY works?
At the risk of sounding cliché, the best long-term approach is one that’s lifestyle-based. This involves personalized nutrition, a sustainable physical activity program, and behavior modification. That’s not to say you can’t use a popular style of diet as a compass per se. But a long-term approach may be better than worrying about low-carb vs low-fat.
In fact, if you look at some of the most popular “diets” that stick, they’ve evolved from simply being a way of eating to becoming a “way of life.” For example, when people talk about “being” Paleo or Keto, they’re not just talking about how they eat. They’re talking about a lifestyle they’ve embraced.
Another prime example is the Mediterranean-style diet. It again falls into the context of a much broader lifestyle and set of practices. For instance, the Mediterranean lifestyle is known to involve regular physical activity, sharing meals with others, enjoying life, being outdoors, properly managing stress, and incorporating plenty of purposeful rest and sleep.
For what it’s worth, a Mediterranean-style diet doesn’t really classify as low fat or low carb. Although diet composition can vary significantly across regions and time of year, about 37% of calories come from fat and 43% of calories come from carbohydrate, according to a review study published in the journal Nutrients.27 Ironically, Ancel Keys was one of the first to coin the Mediterranean diet terminology in his book How to Eat Well and Stay Well the Mediterranean Way.28
And although Paleo-esque diets have no specific macronutrient targets, studies estimate that carbohydrate intake is roughly 35 – 40% (ranging as high as 65%), while total fat intake may be as high as 35% (or as low as 20%).7,29
On one hand, this suggests that in the battle of low-carb vs low-fat, neither may be the “best” way to go. In fact, when you look at these types of lifestyle-based eating practices, you may also draw the conclusion that (based on seasonality and food availability) certain times of year may be high-carb, some may be low carb, and others may be moderate fat and carb.
Perhaps most importantly, it highlights the significance of asking what (i.e., food quality) over how much, as these lifestyle-based approaches can be summed up quite simply with phrases such as “Eat REAL Food” and “Don’t eat ultra-processed junk.”