Page Two of Low Carb Diet Study in the New England Journal of Medicine, 11/09/06
The Details
The researchers looked at the same data three different ways, looking for patterns associated with lower carb diets. First, they just looked at diets that tended to fit the profile of a low-carb diet, with less carbohydrate and more protein and fat. They scored each woman's diet on these factors.
Next, they generated scores looking at carbohydrate patterns combined with animal fat and protein consumption, instead of total fat and protein. Finally, they generated scores looking at carbohydrate patterns with vegetable fat and protein consumption. They were looking within each analysis for those who ate less carb and more of the types of protein and fat. Remember: All three analyses were done on data from the same women, not different groups.
The first two analyses showed no association between the low-carb pattern of eating and heart disease risk. The third one showed about a 30% reduction in risk for those who ate a pattern of less carbohydrate and more vegetable protein and fat. Here we come to the first point of misinterpretation. In doing the analysis, they looked at vegetable protein and fat totally separately from animal protein and fat. It's entirely possible that a person eating a lot of fat in their diet ate a lot of animal AND plant fat, not one instead of the other as many news articles are reporting.
When we look further at the third analysis (vegetable protein and fat, plus carb intake) we can find some interesting things:
1) Vegetable protein does not seem to be a factor. The 10% of women who ate the most vegetable protein only got 5.6% or more of their calories from vegetable protein. The lowest 10% got less than 4.2% of their calories from vegetable protein. This is not a big range, nor is it much vegetable protein. Even that top 10% of vegetable protein eaters still got 70% of their protein from animal sources. Not surprisingly, when vegetable protein was looked at as a separate factor, it was not significantly correlated with heart disease. Thus when news reports talk about getting protein from "vegetables instead of meat", they are reading more into the data than is actually there.
This does not mean that vegetable protein doesn't make a difference, by the way. It only means that this study does not show whether or not it makes a difference, because the women weren't eating much vegetable protein.
2) Vegetable fat consumption was related to reduced heart disease. The most interesting thing to me about this was pointed out to me by Regina Wilshire - that the women who were found to have an increased risk in this analysis (lowest consumption of vegetable fat with a higher carb consumption) were not eating the recommended daily intake of essential fatty acids. So possibly what was causing the decreased heart disease risk in the high vegetable fat/low-carb eaters was simply eating more essential fatty acids. The highest vegetable-fat eaters in this group also ate 4 times more nuts than those who ate the least vegetable fat. Nuts have repeatedly been associated with heart health.
3) Saturated Fat: Those with the most reduced risk in the study (most vegetable fat) got about a third of their fat calories from saturated fat. Interestingly, this is the percentage recommended for the Atkins Diet in Atkins for Life.
4) Carb consumption didn't vary as much in this part of the analysis. This is sort of a technical point, but it does lend credence to the idea that carbohydrate wasn't the factor in this section as much as essential fatty acids.
Why didn't the study show that low-carb eating reduces heart disease overall?
I think this is a reasonable question. Since many studies have shown that a low-carb diet lowers the risk factors for heart disease, and since this study showed that a low glycemic diet was associated with a half the incidence of heart attack as a high glycemic diet, and low-carb diets are low-glycemic by nature, why didn't the study show a decreased risk? We don't know, but I would guess that one factor could be that these data were gathered over a period of 18 years, from 1980 to 1998. Most of this period was before most people were aware of low-carb diets and before they had been studied much at all - most of the major books about it began coming out in the mid 1990's. Also, not that many people stick with any one "diet" over a long period of time. It's actually kind of surprising that as many of the women were actually eating a diet that was low in carbohydrate. 9% of the women were eating a diet where less than 30% of calories came from carbohydrate.References:
- Halton, Thomas, et al. "Low-Carbohydrate-Diet Score and the Risk of Coronary Heart Disease in Women." New England Journal of Medicine 355:19 (2006):1991-2002.
- Liu, Simin, et al. "A prospective study of dietary glycemic load, carbohydrate intake, and risk of coronary heart disease in US women." American Journal of Clinical Nutrition 71.6 (2000):1455-1461.
