Which diet is best for weight loss? What two big trials found
Every year brings a new diet that claims to beat the others. Researchers have tested that claim many times, by putting people on different diets at random and weighing them a year later. Two of the best-known trials give a clear answer.
Atkins, Zone, Weight Watchers and Ornish
In 2005, a team at Tufts-New England Medical Center in Boston published a one-year trial of four popular diets. They put 160 adults into four groups of 40. All were overweight or obese, and all had high blood pressure, high cholesterol or high blood sugar. The four diets:
- Atkins, which cuts carbohydrate
- Zone, which balances carbohydrate, fat and protein
- Weight Watchers, which cuts calories
- Ornish, which cuts fat
After two months of full effort, people chose for themselves how closely to follow their diet. Many dropped out. Only 50% to 65% of each group completed the year.
Average weight loss after one year, counting people who dropped out as losing nothing:
| Diet | Average loss at one year | Completed the year |
|---|---|---|
| Atkins | 2.1 kg | 53% |
| Zone | 3.2 kg | 65% |
| Weight Watchers | 3.0 kg | 65% |
| Ornish | 3.3 kg | 50% |
The diets came out close together. The study’s key finding was this: the amount of weight people lost went with how closely they said they followed their diet, and not with which diet they were on.
Healthy low-fat against healthy low-carb
In 2018, a team at Stanford published DIETFITS, a larger trial. It put 609 adults aged 18 to 50 with a BMI between 28 and 40 on one of two diets for a year:
- A healthy low-fat diet
- A healthy low-carb diet
Both groups had 22 small-group sessions with health educators. Both were taught to cut fat or carbohydrate as far as they could keep it up for the long term, and to eat good quality food.
After 12 months, the low-fat group had lost 5.3 kg on average and the low-carb group 6.0 kg. The difference of 0.7 kg was too small to be sure it was real.
The researchers also tested whether a person’s genes, or how much insulin they released after sugar, could predict which diet would suit them. Neither did.
What this means for you
- Pick a plan you can keep for a year, not one that looks fastest for a month. In the Boston trial, keeping to the plan predicted weight loss and the diet type did not.
- All of these diets work, when they work, the same way. They make you eat fewer calories than you burn. Our article on the calorie deficit explains how big a gap to aim for.
- Food quality still matters for your health. The DIETFITS diets were the healthy versions of low-fat and low-carb, and both stressed the quality of the food.
- Judge your plan by your trend weight over a month, not by the scale after one week. Low-carb diets in particular can drop water fast in the first days.
If you like bread and rice, a low-carb plan will be hard to keep. If you like meat and cheese, a low-fat plan will be hard to keep. The trials suggest that the plan you can live with is the plan that works. Whichever you pick, Downtrend shows your weekly rate and the date your current pace reaches your goal, so within a few weeks you can see whether the plan is working for you.
Sources
- Dansinger ML, Gleason JA, Griffith JL, Selker HP, Schaefer EJ. Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial. JAMA, 2005.
- Gardner CD and others. Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial. JAMA, 2018.
- Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. American Journal of Clinical Nutrition, 1992.
This article is general information, not medical advice. Talk to a doctor before a big change to what you eat or how you train, and especially if you are pregnant, under 18 or have a health condition.