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Which Diet Wins for Weight Loss – Low-Fat or Low-Carb?
Startling results from new study suggests the answer depends your partially gene-driven blood chemistry 08/10/2017 By Craig Weatherby

Weary of conflicting, ever-changing diet advice?

Nutrition-health studies are notoriously difficult to conduct, and prone to murky conclusions.

That’s because most such studies are epidemiological, and can only compare people’s self-reported diets to their health outcomes over periods of years or decades.

Unfortunately, participants’ self-reported claims about their regular diets are not very reliable.

And, although researchers try to adjust the results of epidemiological studies to account for the effects of various non-dietary factors, the accuracy of those adjustments is open to question.

Clinical studies can overcome many of those barriers — and an international team recently analyzed three large, randomized clinical trials, with surprising results.

The participants in those three trials had been assigned to eat diets containing specific proportions of carbs, protein, fiber, and fat.

Critically, the participants’ blood was tested, to determine the levels of sugar (glucose) and insulin — which allowed researchers to compare people's blood chemistry with their success at weight and blood-sugar control.

Before we examine the remarkable results of the new clinical-evidence review, let’s take a quick trip through the history of diet advice for weight and blood-sugar control.

The rise and fall of diets over the decades
Low-fat, high-carb diets held sway from the 1980s through the 1990s.

The dominance of low-fat diets persisted despite a lack of evidence supporting their superiority for weight control or overall health.

You can pin the over-persistent popularity of low-fat diets on two factors.

First, fat has nine calories per gram, versus only four per gram in carbohydrates and protein, which means that eating less fat also means you’re consuming fewer calories.

Second, heart disease was blamed on saturated fat until recently, and high-fat diets were considered generally worse for health versus high-carb diets.

As to the first reason for the popularity of low-fat diets, we now know that it’s not accurate to say “a calorie is a calorie”.

It’s become clear that calories from fats, carbs, and protein exert different metabolic effects, including distinct effects on appetite and fat-burning.

Although fat has twice the calories of carbs or protein, the differing metabolic effects of these different “macronutrients” matter when it comes to controlling your weight and blood sugar.

The first version of Dr. Atkins’ famous diet was remarkably high in fat, but Atkins eventually retreated to advising a low-carb, moderate-fat, high-protein diet.

High-fat diets have become overnight sensations, and the subject of best-selling books — such as ones by Jonny Bowden, Ph.D. and Stephen Masley, M.D. (see "Smart Fat": New Book Gets it Right) and Dr. Joseph Mercola, D.O. (see “Fat for Fuel” – Our Review).

These authors cite strong evidence that low-carb, high-fat diets put people in a mildly “ketonic” state that promotes fat-burning and may bring serious brain-protection and anti-cancer benefits.

We think they present persuasive cases that high-fat/low-carb diets offer the best choice — at least for people with healthy weights and blood sugar levels.

However, that describes a shrinking portion of the population, because rates of excess weight, obesity, prediabetes, diabetes rose sharply over the past few decades, and continue to climb (see "Positive implications for diabetes prevention", below).

We all want simple solutions like “eat a low-carb, high-fat diet”, but nature — including human nature — doesn’t always cooperate.

New study undermines one-size solutions
An international team led by scientists from Denmark’s University of Copenhagen recently published an evidence review may mark a revolution in diet advice.

The team also included scientists from the University of Colorado, Tufts University, and Spain’s research center for obesity and nutrition, and their findings were presented at the American Diabetes Association’s 77th Scientific Sessions (Hjorth MF et al. 2017).

Lead author Arne Astrup M.D., Ph.D., from the University of Copenhagen described the history leading to their study:
“When the obesity problem would not go away, it led to the fat-reduction era with the replacement of a lot of fat with fiber, but also a lot of starch and carbohydrates, and in some instances more sugars, which did not solve the problem. In fact, the obesity rate when up.”

As Dr. Astrup said, “This started a new trend, the Atkins Diet, and the major focus on restricting sugars and starchy foods, which like the low-fat diet worked for some, but not everyone.”

Clinical studies that have directly compared low-carb to low-fat diets found that each diet worked well for some, but not for others, “which, as Astrup said, “didn’t make sense.”

So, his international team searched three large clinical studies for “biomarkers” that might accurately predict weight loss success based on different diets and body types.

New findings rest on data from three large clinical studies
The Danish-led team analyzed data from three major clinical studies.

Each of the three clinical studies tested the effects of different types of diets on weight and blood-sugar control.

In short, the team concluded that each person’s best weight and blood sugar control diet depends on the status of two key “biomarkers” — their blood sugar (glucose) and/or fasting insulin levels.

And, in all three clinical studies, those biomarkers accurately predicted whether a particular diet — low-fat, low-carb, or high-fiber — would best help each individual shed pounds and keep them off.

After scrutinizing the three clinical studies, they concluded that success in weight control (and blood sugar control) depends upon these factors:

  • A person’s fasting blood sugar and insulin levels
  • The glycemic load, fiber content, and whole-grain content of a diet

And the accuracy of those diet-selection biomarkers was particularly high in people with prediabetes or diabetes, who need an accurate diet prescription more than most.

In summary, the diet that will work best for each person will depend on whether they have normal blood sugar, prediabetes, or full-blown diabetes.

In practical terms, fasting blood sugar and fasting insulin should be tested and used to select the recommended proportion of carbs, fat, protein, and fiber in each person’s diet.

Custom diets shed many more pounds
Dr. Astrup described core conclusion with dramatic implications:
“Remarkably, for many patients, use of these biomarkers can lead to a six- to seven-fold greater weight loss.”

And, as he said, “Going forward, we can educate patients when a diet they planned to follow would actually make them gain weight, and redirect them to a strategy that we know will work for them.”

These were some other conclusions:

  • For most people with prediabetes, a fiber-rich diet will be most effective.
  • Among people with prediabetes, the proportions of carbohydrate and fats should be customized to each person’s fasting insulin levels.
  • For people with type 2 (adult-onset) diabetes, a diet rich in unsaturated fats — such as those in extra virgin olive oil, nuts, and avocados — is the best way to achieve weight loss.

“Our research shows that weight loss strategies should be customized based on an individual's biomarkers, which is a big step forward in using personalized nutrition to help people achieve greater weight loss success,” said Professor Astrup.

Dr. Astrup expressed a key point: “These findings … allow us to provide those with prediabetes a custom strategy to help them lose weight, which can ultimately prevent or delay the development of type 2 diabetes.”

Positive implications for diabetes prevention
According to the American Diabetes Association, 29 million Americans had diabetes in 2012, and another 1.4 million are diagnosed annually.

And about 86 million Americans over the age of 20 had prediabetes in 2012, up from 79 million in 2010.

Some 30 percent of people who develop prediabetes will develop type 2 diabetes within three years, but 90% of people with the condition don’t know they’ve got it.

Getting more active and losing five to 10 percent of body weight can prevent or delay the development of type 2 diabetes among high-risk adults.

Together with the findings from the new evidence review — and the fact that weight loss can prevent type 2 diabetes — one-size-fits-all diet advice should be taken with a grain of salt.

Conversely, customized diets may hold unprecedented promise for putting a big dent in the nation’s epidemic of prediabetes and diabetes.

The challenges lie in education and execution.

Lay people and health professionals need detailed guidance on how to devise diets that provide customized proportions of fat, fiber, protein, and carbs in response to each person’s blood-sugar and insulin status.

One hopes that these findings will be translated into practical diet guidance that will quickly spread to physicians and regular people alike.

That said, such guidance will do little good unless most Americans get their blood sugar and fasting insulin tested — a serious challenge that’s well worth the funding it would require, in terms of public and private healthcare savings.

Food choices, not just fat-carb balance, matter
Needless to say, the proportions of macronutrients in a diet isn’t all that matters.

Clearly, it’s also critical to choose whole, unprocessed, healthful foods in each category.

So, it makes sense to avoid junky foods that offer little or no fiber, and hurt metabolisms, immune systems, and hearts.

For example, it would be unwise to choose omega-6-laden cooking oils such as corn, safflower, or soy — versus extra virgin olive oil, which has a far healthier fat profile and is rich in a highly beneficial, virtually unique family of antioxidants.

As to protein, the evidence favors fatty wild seafood like salmon, sardines, and tuna over meat and poultry, and poultry over red meats — mostly because of their differing fat profiles.

Wild fish provides the best fat profile, thanks to its unique abundance of omega-3 fatty acids in their most useful and healthful forms, called DHA and EPA.

It also makes sense to favor wild fish over farmed fish, because it has a much healthier fat profile (more omega-3s and fewer omega-6 fats), and — unlike most farmed salmon — doesn’t pollute the ocean floor or act as an incubator for viruses and parasites.

Poultry — especially pasture-raised chicken, which has a healthier fat profile versus standard, grain-fed chicken — is generally healthier than red meat.

That said, when eaten in moderation, grass-fed beef, grass-fed bison, and pasture-raised pork appear perfectly healthful.


  • Bullard KM, Saydah SH, Imperatore G, Cowie CC, Gregg EW, Geiss LS, Cheng YJ, Rolka DB, Williams DE, Caspersen CJ. Secular changes in U.S. Prediabetes prevalence defined by hemoglobin A1c and fasting plasma glucose: National Health and Nutrition Examination Surveys, 1999-2010. Diabetes Care. 2013 Aug;36(8):2286-93. doi: 10.2337/dc12-2563. Epub 2013 Apr 19.
  • Hjorth MF, Ritz C, Blaak EE, Saris WH, Langin D, Poulsen SK, Larsen TM, Sørensen TI, Zohar Y, Astrup A. Pretreatment fasting plasma glucose and insulin modify dietary weight loss success: results from 3 randomized clinical trials. Am J Clin Nutr. 2017 Aug;106(2):499-505. doi: 10.3945/ajcn.117.155200. Epub 2017 Jul 5.
  • Menke A, Casagrande S, Geiss L, Cowie CC. Prevalence of and Trends in Diabetes Among Adults in the United States, 1988-2012. JAMA. 2015 Sep 8;314(10):1021-9. doi: 10.1001/jama.2015.10029.
  • Tay J, Luscombe-Marsh ND, Thompson CH, Noakes M, Buckley JD, Wittert GA, Yancy WS Jr, Brinkworth GD. Comparison of low- and high-carbohydrate diets for type 2 diabetes management: a randomized trial. Am J Clin Nutr. 2015 Oct;102(4):780-90. doi: 10.3945/ajcn.115.112581. Epub 2015 Jul 29.
  • U.S. Centers for Disease Control and Prevention. Prediabetes. July 25, 2017

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