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Low-Fat Vs. Low-Carb Diets: The Rematch

A new study was just published in the Annals of Internal Medicine testing a low-carb diet against a conventional low-fat one.

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Researchers took 307 participants and randomly divided them into two groups. One group went on a low-carb diet, right out of the Atkins playbook. For three months these folks consumed no more than 20 grams of carbs a day from fibrous, low-glycemic vegetables (exactly what the first and rather strict “induction” phase of the Atkins four-stage approach calls for).

For each week after the initial three months, this group then added back in 5 grams per day of carbs and were told to keep doing this until their weight stabilized. In other words, the first week (after the initial three months) they consumed 25 grams of carbs, the second week 30 grams, and so forth. (More on that in a moment.)

Other than carbs, this first group had no restrictions and could eat all the protein and fat they wanted. In fact they were encouraged to, and told not to try a “low-fat” version of the diet because it would stall weight loss. In other words, standard, textbook Atkins.

The second group went on a standard low-fat diet of between 1,200-1,500 calories a day (for women), and 1,500-1,800 calories a day (for men). The only restriction was to keep fat to 30 percent or less of calories (standard advice). They were told to stick to this plan for the entire two years.

The researchers were interested in weight loss, which in research terminology was what was called the “primary outcome.” In other words, weight loss was what they were primarily interested in measuring. (More on that in a moment as well.) The results? There was no difference in weight loss between the two groups. But don’t think for a minute that’s all there is to this story.

You may recall the traditional rap on low-carb diets: they may work in the short term for weight loss, but they are ultimately dangerous. So the researchers had a “secondary outcome,” meaning there was something else besides weight loss that they wanted to look at: risk factors for heart disease-risk factors reputed to worsen when you follow the Atkins or any other very low-carb diet.

Six months into the study, however, the low-carb group had a significantly greater reduction in diastolic blood pressure, a significantly greater reduction in triglycerides, and significantly greater reduction in a particularly bad form of “bad” cholesterol called VLDL (very low-density lipoproteins). In other words, markers for heart disease risk actually improved. A lot.

And-hold on to your hats-at all time points throughout the 2 years, including at the finish line, the low-carb group had a significant increase in HDL (“good”) cholesterol: approximately 23 percent. There’s not a drug on earth that’s been able to do that.

There are a couple of other things to know about this study. First, the amount of weight lost by both groups was not enormous-a little more than 20 lb in the first year (average for both groups), with both groups regaining some during the second year so that the total weight loss at the end of 2 years (average for both groups) was about 15 lb. This isn’t surprising. Virtually every study I’ve ever seen has found that people start reverting to their old habits to some extent, which is why they gain some weight back. The more they slip back into their old ways, the more they gain back. Those who were able to stick with their program gained back the least amount of weight, or even continued to lose weight-the averages don’t tell us that.

Second, you may have noticed that those improved cardiovascular risk factors showed up for the low-carb group (only) after six months, but that after that, there was no difference between the groups in those risk factors-both had improvements. (Except of course, for the very important improvement in HDL cholesterol, which was seen only in the low-carb group and was sustained throughout the two years.)

This is not surprising either, and I would consider it likely evidence that the low-carb group started drifting more towards “conventional” eating after six months, thus wiping out the differences between the low-carb and the low-fat group. In fact, this is the part that’s worth a second look. You may recall that the low-carb group was instructed to add back carbs until their weight “stabilized.” The low-fat group was given no such parallel instructions. In other words, they weren’t told “add calories (or fat) until your weight stabilizes.” So what we’re seeing here is essentially a test between two groups-one of which was told to diet until their weight stabilized, the other of which was told no such thing.

Still, the weight loss was the same for both groups, which makes me wonder what would have happened if the low-carb group had actually stayed on a low-carb diet throughout the study. Tom Naughton, a journalist who has written widely about low-carb diets and who made a movie about the subject, Fat Head, put it this way:

“If you’re consuming 20 grams of carbohydrates per day at three months and then starting adding an extra 5 grams per day each week, at six months into the study, you’d be consuming 80 grams of carbohydrates per day. At 12 months, you’d be up to 210 grams per day. Ask anyone who’s lost weight and kept it off by restricting carbohydrates what happens if the carb count starts drifting up. The answer: first you stop losing, then start gaining again. Most of us find the magic number is somewhere below 100. So at some point in the first year, the low-carb dieters most likely hit their limit and stopped losing weight. From then on, it was merely a maintenance diet.”

Be that as it may, both groups lost the same amount of weight, and both groups regained a little (also about the same). But before you buy into the inevitable headlines about how these diets produced identical results, remember that while the weight loss might have been identical, the outcomes were not.

Averages often conceal real differences. For example, within the low-carb group, there were some folks who really stuck to the program, and I’m willing to bet that when the raw data are released, you’ll see a number of individuals who not only maintained their weight (or kept losing) but also maintained the significant gains in cardiovascular risk factors that were seen after six months. If you had your choice between two diets-both of which produced weight loss, but one of which did it with less hunger and better cardiovascular outcomes, which would you choose?

Don’t buy for a minute that this study showed no difference between low-fat and low-carb. It showed no difference between groups in the weight loss department, but a significant improvement in cardio risk factors for the low-carb group, especially at the beginning when folks were actually sticking to the program. And I suspect those improvements-so obvious after six months on the low-carb regimen-would have continued throughout the two years had the low-carbers really kept their carbs low. (I agree with Naughton that the “magic number” is somewhere around 100 grams a day.)

Finally, all the participants received counseling and support for “behavioral change.” The researchers believe that lifestyle support may be the single most important thing in successful weight loss, even more than which particular diet was followed. On that issue, they just might be right.