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SEPTEMBER / OCTOBER 2004

AROUND HARVARD

This article originally appeared in the August 2004 Harvard Health Letter and is provided courtesy of Harvard Health Publications.

Low carb vs. low fat

Although people lose weight faster on low-carb diets, those taking the low-fat route catch up in a year. But that’s only part of the story.

If weight loss were a sprint, low-carb dieting would win, hands down. Four recent studies have shown that at the six-month mark, low-carb dieters lose, on average, about 9–13 more pounds than those on a low-fat plan. No one is completely certain why, although one of the most likely explanations is that the low-carb approach is better at suppressing appetite for a time, making it easier to achieve the ultimate goal of all diets, cutting calories. There isn’t strong evidence, though, for the theory proposed by the late Dr. Robert C. Atkins — that carbohydrate restriction forces the body into a mild state of ketosis when it “burns off” fat for energy because the diet isn’t supplying enough of it.

But two studies have now shown that for weight loss, the two diets end up in a statistical tie after a year. In the first one, published in 2003 in the New England Journal of Medicine (NEJM), low-carb dieters spurted ahead of their low-fat counterparts during the first six months, only to regain pounds in the next six. In the second study, published in early 2004 in the Annals of Internal Medicine, the low-carb group kept off the pounds lost during the first six months, but the low-fat group caught up with them by continuing to lose weight. In the weight-loss race, low carb seems to be the hare and low fat, the tortoise.

The plot thickens
But it’s more complicated than that familiar fable with its moral that “slow and steady wins the race.” Heaviness is a lifelong issue for most people who are overweight or obese. A year of slimming down isn’t all that meaningful. Longer studies are needed.

The numbers of pounds being lost are relatively modest. The people in the Annals study were heavy to begin with: The average starting weight was about 288 pounds. After a year, the low-carb group (including the dropouts) had lost an average of 11.2 pounds and the low-fat group, just 6.8 (the difference between them didn’t meet the test for statistical significance). The people in the NEJM study weren’t as heavy — they averaged about 215 pounds — but the results were pretty similar.

On the other hand, losing even just a few pounds may be a bigger victory than it seems. People in these studies may have been gaining weight month after month and that trend was reversed, notes Dr. Frederick Samaha at the Philadelphia Veterans Affairs Medical Center and one of the authors of the study in the Annals.

Low carb good for HDL, triglycerides
Low-carb dieting — which should be called low-carb, high-fat dieting — lets people eat greasy and buttery foods, although recent versions of the Atkins diet steer people toward healthier plant-based polyunsaturated and monounsaturated fats, as does the South Beach diet. The saturated fat found mainly in meat and dairy products tends to increase levels of “bad” LDL cholesterol. So one of the chief objections to low-carb dieting has been that it would ratchet up LDL levels. But both yearlong studies found that low-carb and low-fat diets had the same effect on LDL levels. And low-carb diets outdid low-fat diets with respect to other blood fats related to heart disease. Triglyceride levels fell further and the HDL results were better. In the Annals study, the low-carb diet was better for blood sugar control for people with diabetes.

The average is only that
Averages can be good summaries, but behind them there can be a lot of variation. Dr. Samaha says someone in the low-fat group in his study lost 79 pounds — and that person was among the 34% who dropped out of the study. At the other extreme, another person gained 31 pounds sticking with the low-fat diet program. There was a huge range among the low-carb dieters, too: from 65 pounds lost to 18 pounds gained.

Three take-home messages
First, low-carb dieting can’t live up to the hype (what could?) but it has some merit. You can improve upon it by sticking with polyunsaturated and monounsaturated fats as your fats and whole grains as your carbs. Second, diets have differing effects on cholesterol levels and metabolic factors. If you’re serious about losing weight, you should talk to your doctor about getting a cholesterol test. The results may help you choose the best diet for you. Third, for reasons of taste, upbringing, genetics, and other factors, the individual response to diets varies tremendously. Experiment. See what works for you. And by all means, get some exercise, too.