Low-Fat vs Low-Carb vs Mediterranean: Which Diet Works Best for Weight Loss?

People have argued about low-fat and low-carb diets for decades. Every few years a new book says one of them is the answer and the other one is a trap. Researchers have now run large trials that put the diets head to head for a year or longer, and the results are more useful than the arguments.

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The Israeli trial that started the debate

In 2008 the New England Journal of Medicine published a two-year trial of 322 people with obesity. Most were men, and their average age was 52. Each person was assigned to one of three diets. The low-fat diet limited calories. The Mediterranean diet also limited calories, with olive oil and nuts for fat. The low-carb diet didn’t limit calories at all, only carbohydrates.

Almost 85 percent of the people were still on their diet at two years, which is high for this kind of study. Average weight loss was 2.9 kilograms (about 6.4 pounds) on the low-fat diet, 4.4 kilograms (about 9.7 pounds) on the Mediterranean diet and 4.7 kilograms (about 10.4 pounds) on low-carb. The low-carb group had the best improvement in cholesterol ratio. Among the 36 people with diabetes, blood sugar and insulin improved more on the Mediterranean diet than on the low-fat diet.

That looked like a win for low-carb and Mediterranean. The differences were a few pounds over two years, though, and the authors said personal preference should help decide which diet a person uses.

The trial that tried to settle it

A few months later the same journal published a bigger answer. Researchers at Harvard and Louisiana State randomly assigned 811 overweight adults to four diets. The diets mixed low or high fat with average or high protein, so the carbohydrate share ranged from 35 to 65 percent. All four were built from healthy foods and cut calories.

At six months, people on every diet had lost about 6 kilograms, or 13 pounds, around 7 percent of their starting weight. Then weight started coming back after the first year. At two years the groups were almost the same: 3.3 kilograms lost on both the low-fat and high-fat diets, and 2.9 against 3.4 for the highest and lowest carb diets. None of those differences were meaningful.

What did make a difference was showing up. People who went to more of the group counseling sessions lost more weight, about 0.2 kilograms for every session they attended. Hunger and satisfaction were the same across diets. The authors concluded that reduced-calorie diets work no matter which nutrients they emphasize.

A newer look: healthy low-fat vs healthy low-carb

Stanford’s DIETFITS trial, published in JAMA in 2018, tested the question again with 609 adults aged 18 to 50. Both groups were coached on diet quality: plenty of vegetables, and as little added sugar, refined flour and processed food as possible. One group cut fat and the other cut carbs.

After a year the low-fat group had lost 5.3 kilograms (about 11.7 pounds) and the low-carb group 6.0 kilograms (about 13.2 pounds). That gap was small enough to be chance. The researchers also tested whether genes or how much insulin a person made could predict which diet would work better for them. Neither did.

What happens across all the named diets

A 2014 review in JAMA pooled 48 randomized trials with more than 7,000 people who tried named diets like Atkins, the Zone, Weight Watchers and Ornish. Compared with no diet, both low-carb and low-fat diets took off about 7 kilograms, or roughly 16 pounds, at 12 months. The gaps between individual named diets were small. Atkins beat the Zone by less than 4 pounds at six months.

The authors put it plainly: the best diet is the one a person will stick with.

Where the Mediterranean diet stands out

Weight isn’t the only reason to pick a diet. In PREDIMED, a Spanish trial of 7,447 older adults at high risk of heart disease, people were assigned to a Mediterranean diet with extra olive oil, a Mediterranean diet with extra nuts, or advice to cut fat. After about five years, both Mediterranean groups had roughly 30 percent fewer heart attacks, strokes and heart-related deaths than the low-fat group.

The original 2013 report was withdrawn because some people at a few sites weren’t properly randomized. The researchers reanalyzed the data and published it again in 2018, and the result held up.

So what should you do?

Pick the diet you can live with. If you love bread and fruit, a low-fat plan built on whole grains and vegetables can work. If cutting carbs keeps you from snacking, low-carb can work. If you want the pattern with the strongest heart evidence, the Mediterranean diet has it.

Whatever you choose, the trials point to a few things that matter more than the label:

• Eat fewer calories than you burn. Every diet that worked in these studies did that, even the low-carb one that didn’t count calories.
• Choose whole foods over processed ones. DIETFITS got good results from both diets by focusing on food quality.
• Get support. Group sessions made a real difference in the Harvard trial.
• Plan for the second year. Most people regained some weight after 12 months.

Portion control is a big part of that. The 21 Day Fix containers are one way to handle it without counting every calorie.

Exercise helps too, especially with keeping the weight off. In the 2014 review, exercise mattered more at 12 months than at six.

Most people in these trials regained some weight after the first year, and a log makes it easier to see why. Fitness Rocks has a free printable daily food and exercise log (PDF) with room for what you eat and drink and your workout. Two weeks of filling it in is usually enough to spot patterns.

What the research can’t tell you

Most of these trials lasted one to two years, and in most of them people had started to regain weight by the end. The Israeli trial was mostly men. Averages also hide a wide range: in every group some people lost a lot of weight and some gained. A diet that works for your friend may not work for you, and that’s normal.

If you have diabetes, kidney disease or take medication, talk to your doctor before changing how you eat. Cutting carbs fast while on insulin or some other diabetes drugs can push blood sugar too low.

Where to get help

A registered dietitian can build an eating plan around your tastes, your budget and your health, which matters more than the name of the diet. If you take medicine for diabetes or blood pressure, check with your doctor before you cut carbs or calories, because your medicine may need to change as your eating does.

• Academy of Nutrition and Dietetics: Find a Nutrition Expert, to find a registered dietitian near you, in person or by video.
• NIDDK: weight management, plain-language guides from the National Institute of Diabetes and Digestive and Kidney Diseases on healthy eating, activity and weight.
• NIDDK: healthy living with diabetes, how food, activity and medicine work together if you have diabetes.

Where this page came from

Fitness Rocks started as a podcast by the doctor I bought the site from. In 2008 he compared low-fat, low-carb and Mediterranean diets for weight loss, and in 2009 he came back with a post called the final answer on low-fat vs low-carb. Those episodes are gone, so this page covers the same fight with the trials that have come out since.