Harvard study: ‘Case against carbs, for fats grows stronger’


By Marika Sboros

It’s not the definitive word for the best weight-loss diets but it’s close. And it doesn’t venture into the contentious plant- versus animal-food divide. A major new Harvard study shows that replacing carbohydrates with fats speeds up metabolism.

Dr David Ludwig

It overcomes one of the biggest hurdles in conventional weight-loss diets: the “plateau”. That is the metabolism slowdown that prevents weight loss on conventional diets. It’s why so many lose some weight but stay hungry and find it increasingly harder to lose more.

The Harvard study in the BMJ is well-designed and randomised. It is also one of the largest feeding studies ever conducted, say the authors, led by Dr David Ludwig, Harvard professor of paediatrics.

Add recent US research by Virta Health on low-carb, high-fat (LCHF) diets to reverse type 2 diabetes and experts say it’s a recipe for speedy, safe, sustainable weight loss.

Always HungryLudwig is also a nutrition professor at Harvard’s TH Chan School of Public Health and co-director of the New Balance Foundation Obesity Prevention Centre at Boston Children’s Hospital. And author of Always Hungry? Conquer cravings, retrain your fat cells and lose weight permanently. 

US physician Michael Eades calls it “the most comprehensive (book) on low-carb dieting published to date’.

He says that the study shows that the case against carbohydrates is indeed growing. (That goes some way to balancing a recent Harvard study suggesting that low-carb diets shorten life-spans.)

Call for revision of dietary guidelines

Experts have hailed the Harvard study as ground-breaking and “profound”. They say that it supports what many have long suspected. LCHF diets really are superior to conventional low-fat, high-carb diets for weight loss.

It is more proof (were more needed) that official low-fat, high-carb dietary guidelines need radical revision.

Dr Tim Spector

UK professor of genetic epidemiology at King’s College, London, Tim Spector, says that the study “kills the myth that all calories are equal”.

It shows that “carbs and fats have different effects on our metabolic rate”, says Spector, a specialist in genetics, epigenetics, microbiome and diet.

Thus, the Harvard study further shakes the foundation of conventional low-fat weight-loss diets. That is the CICO (calories-in, calories-out) model of obesity.

The model holds that “a calorie is a calorie”. And that all you have to do to lose weight is eat less and move more.

Yet many experts continue to knock LCHF diets. They also say that health risks of high-fat diets (including heart attack and stroke) outweigh any benefits.

The REAL significance

Ludwig explains the significance of his team’s research in an editorial in the Los Angeles Times. The headline: The Case Against Carbohydrates Gets Stronger.

“People have a hard time believing that weight control isn’t just a matter of calories eaten and calories burned,” Ludwig writes. “But it’s not just how much but also what you eat that significantly affects your metabolism.”

In turn, this affects how much weight you will gain or lose, he says.

The CICO view offers “no compelling biological explanation for the obesity epidemic”. That’s apart from opinions that “it’s complicated” and there are “many factors are involved”. And ultimately, “that we eat too much”, Ludwig says.

Click here to read: Fung’s single best weight loss tip

But if the type of calories consumed affects the number of calories burned, this trend “starts to make more sense”, he says.

It points to an alternate hypothesis about obesity, the carbohydrate-insulin model, which his team studied, Ludwig says. The model holds that overeating is not the underlying cause of long-term weight gain. Instead, the cause is the “biological process of gaining weight that causes us to overeat”.

Why we get hungry

In the editorial, Ludwig explains the carbohydrate-insulin obesity model: “When we eat processed carbohydrates (particularly refined grains, potato products and sugars), our bodies produce more insulin. Too much insulin, one of the most powerful hormones, forces our fat cells into calorie-storage overdrive.”

These rapidly growing fat cells then hoard too many calories, leaving too few for the rest of the body. This makes us hungry and if we persist in eating less, our metabolism slows down.

In the Harvard study, Ludwig again fingers the processed carbohydrates. These flooded the food supply during the low-fat diet era of the last 40 years. They have “pushed the body weight set-point up across the population”.

Varying carb to fat ratio

In a BMJ release, the researchers clarify their aim:  To “better understand the role of dietary composition on energy expenditure”. Thus, they compared the effects of diets varying in carb-to-fat ratio on energy expenditure over a 20-week period.

The trial involved 234 overweight adults aged 18 to 65 with a body mass index (BMI) of 25 or higher who took part in an initial weight-loss diet for about 10 weeks. Of these, 164 achieved the target weight loss of around 10% of body weight. The researchers then randomly assigned participants to follow either a high (60%), moderate (40%) or low (20%) carbohydrate diet for 20 weeks.

Click here to read: Think you can outrun bad diet? Fat chance! 

After adjusting for potentially influential factors, they found that over the 20-weeks period, participants on the low-carb diet showed greater total energy expenditure compared with those on the high-carb diet. They burned 209 to 278 kilocalories a day more than those on the high-carb diet.

That’s about 50 to 70 kilocalories a day increase for every 10% decrease in the contribution of carbohydrate to total energy intake, the authors say.

In those with the highest insulin secretion at the start of the study, the difference in total energy expenditure between low- and high-carb diets was even greater: up to 478 kilocalories a day. That is consistent with the carbohydrate-insulin model.

Best weight-loss strategy

If this effect persisted, “it would translate into an estimated 10 kg weight loss after three years, assuming no change in calorie intake”, write the authors.

Hormones involved in energy balance (ghrelin and leptin) also showed “potentially advantageous” changes in participants on low-carb. That was compared to participants assigned to the high-carb diet.

Thus, Ludwig says the Harvard study suggests a better strategy for weight loss: “Not counting calories but focusing on reducing carbs.”

The study does not prove the validity of the carbohydrate-insulin model. However, it “credibly (makes) the case that all calories are not alike to the body”, Ludwig says. It makes the case that novel effects of specific foods “might make a big difference in our ability to lose weight — and keep it off”.

Dr Dariush Mozaffarian

On Twitter, Dr Dariush Mozaffarian calls the Harvard study “one more nail in the coffin of our global calorie-counting, low-fat fetish”. Mozaffarian is dean of the Friedman School of Nutrition Science and Policy at Tufts University and was not involved in the research.

‘Yo-yo’ effect

In a New York Times article by Anahad O’Connor, Mozaffarian describes the study as “profound. “(It) confirms that, remarkably, diets higher in starch and sugar change the body’s burn rate after weight loss, lowering metabolism.”

The observed metabolic difference was “large, more than enough to explain the yo-yo effect” that people trying to lose weight often experience, he says.

Mozaffariain makes a rallying call for revision of official dietary guidelines: “It’s time to shift guidelines, government policy and industry priorities away from calories and low-fat and toward better diet quality.”

The authors have declared study limitations: They cannot rule out the possibility that some observed effects may be due to other unmeasured factors. Nevertheless, this large trial shows that dietary composition “seems to affect energy expenditure independently of body weight”.

Of course, it’s true that one study can’t answer all questions about diet and obesity. And Ludwig says that his team’s research will need to be replicated.

Correct terminology

The main funder of the $12-million trial was the US-based Nutrition Science Initiative (NUSI), a non-profit research group.  Science and health journalist and author Gary Taubes, a well-known advocate for low-carb diets, is co-founder of NUSI. Taubes is also author of The Case Against Sugar, Why We Get Fat and Good Calories, Bad Calories.

Taubes says that it’s “problematic” to call the effects of replacing carbs with fats a “metabolic advantage”. That terminology “emerges from and has been used in the context of energy balance thinking”, he says.

The issue is not just whether carb-restricted diets have some sort of “metabolic advantage” over carb-rich diets. More important is what causes fat accumulation in the first place.

“The implication has always been that a diet providing a metabolic advantage is one that revs up metabolism,” says Taubes.

Vital message

“Ergo, you expend more energy and that’s why you lose more fat. Expending more energy is the cause; getting leaner is the effect. ”

However, Ludwig’s study demonstrates the opposite, he says. “Getting leaner (mobilising fat from the fat cells and oxidising them) is the cause and increase in energy expenditure is the effect.”

That’s a “vitally important message” to communicate, Taubes says.

The bigger picture, he says is that carb-rich diets, among other effects, stimulate insulin secretion. They also inhibit lipolysis (fat-burning, in laypeople’s language) and trigger excess fat accumulation.

As well, fat accumulation is “always primary a hormonal/enzymatic phenomenon  – just as other growth phenomena invariably are”.

In the New York Times article, Dr Kevin Hall, US scientist and obesity expert at the National Institute of Diabetes and Digestive and Kidney Diseases, calls Ludwig’s study “ambitious and very well run”.

However, Hall says that the researchers used methods that “raise questions about the results”. He cites the use of  “doubly labeled water” to track metabolism. This is not shown to be reliable in people on low-carb diets, Hall says. It “may have exaggerated the amount of calories the subjects burned”.

Ludwig disagrees and tells O’Connor: “We used a gold standard method that has been validated across a wide range of experimental conditions and universally adopted in the field.”



  1. For me the Carnivore diet (low carb) has been the best diet I have found so far. It healed my years of digestive issues and I no longer feel starved which I always did on plant based diets. The biggest lie of nutrition is that bulk fills you up and you eat less. I found that the brain and body wants nutrient dense food (Fats and Proteins) and does not care about bulk. Now I feel better on Carnivore diet, my digestion is excellent, my weight never goes up, and I never feel hungry. I don’t care about the cholesterol (never checked it). The fact is that Nutrition science is the most poorly conducted among all sciences, you cant trust it. I would gladly take the “feeling great” even if it cuts short 10-15 years of my life (which poorly conducted research claims will happen).

  2. I fortunately found Atkins back when he first wrote the book in ’72. It is the sole reason for my usually very slim weight. Every time I went off it there was a fast 30 lb (or more) gain. So I am generally always on it. I have always loved fatty steaks etc. so it really works for me.
    The interesting thing is that from the beginning my Drs have resisted it, even though all my heart measures went to the much improved zone when on the diet. (yes cholesterol goes DOWN on it) Still to this day Drs in general will NOT open their minds to it. It baffles me. I have told people about it for all these years and only about 5 years ago did people start to be more open. So it will take more time to re-educate and inform people and the new young Drs. But one day it will be understood by all, I believe.

    Back in the 50’s when I was young, my mother always used to say that if we wanted to lose weight just cut out all bread and potatoes. And that always worked. So it had always been understood that the the starches were the culprits, but then the AMA and seemingly all Drs everywhere suddenly adopted the food pyramid. Proteins and fats were to be avoided and all grains and fruits (sugars all) were to be enjoyed at will. (but who can eat bread alone, so on went the fats and deep frying which made everything even worse) So Americans became obese. That is only a slight simplification. It is hard to find full fat dairy products now. Nearly everything is low or non fat, even cream, which means that all that remains are the carbs. (sugar.)

    I love this article because it explains the affect of carbs so well.. I will share it with all my resistant facebook friends. Again.

    • My mother too! In fact when she was young she remembered they called dieting “Banting” the first time around.

      Another thing I’ve noticed, the young and middle aged who are generally fat (which used not to be the case) don’t seem to be able to go far without carrying snacks and “energy drinks”. Meanwhile the fit healthy old folks stick to their meals and generally don’t eat in the street. I suspect (partly from personal experience) it’s the carbs that make you hungry for more carbs a couple of hours later. Meat and vegetables don’t do this.

      It’s taken my doctor about a decade to realise perhaps I’m not killing myself with my diet after all. I think it’s starting to sink in, only slowly. Many medical professionals, especially dieticians, are too young to know there was ever a time when there were no “epidemics” of obesity, diabetes and other metabolic diseases, but some of them are catching on at last.

  3. I took a 30 day keto challenge a year and a half ago, and…I’ve never looked back. Though I so often read about keto fat loss, weight loss and so on…this diet changed my life profoundly in a different way. I spent years wracked with pain, every joint swollen, and my belly terribly bloated and causing me terrible pain with Crohn’s disease. All of that, all the pain, all the swelling, all that time lost by needing to lay down all the time…all of that is long gone.
    After 30 days keto, my son told me I should stick with it, he’d never known me to be energetic, ready to get going, to play, to move around by choice. It literally changed my life, and I had little to no weight to lose, and I’m roughly the same shape and size as ever, but…I am pain free for the first time in decades! This needs to be talked about. If my life ends up shorter, I wouldn’t even care, because what I can do in a day is at least twice as much as before I became ketogenic. This diet can heal people. People like me, that have been told by many doctors, that healing isn’t possible. Have hope. With a little will power to start out, the real you may just be sinking and strangled under carbs. Set yourselves free and start to feel amazing!
    Thanks for the well-researched article. I hope to see more research on pain reduction and inflammation from the keto diet. I was on many medications, for acid reflux, and for Crohn’s disease….now I’m on no medications at all, and I’m better than ever!
    Best of luck to you all. Keep at it, it’s so worth the wee bit of work to get started, after the first two weeks, it’s easy.
    Many blessings, all.

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