Posted by Dave on April 14, 2010 | 6 Comments
Do a search for “high blood pressure” or “hypertension” and you’ll find that nearly every health website recommends the DASH diet to control blood pressure. It makes some sense: If sodium and saturated fat cause high blood pressure, then removing them from your diet should make it come back down.
But changing your eating habits is easier said than done. It’s easy to say you want to cut down on fat and sodium; it’s hard to resist a hot slice of Chicago-style pizza piled high with sausage and cheese. If it was easy to control what you eat, the U.S. wouldn’t look like this (source):
Yet several studies have indicated that the DASH diet can bring down blood pressure over the short term. A thornier question is whether it has significant health benefits over the long term. To find out, ideally we’d assign a large group of people to follow the diet for several decades, and compare them to people who don’t follow it. Realistically, such a study is unworkable. But a team led by Teresa Fung has done the next best thing. Starting in 1976, over 121,000 female nurses have been followed in a massive longitudinal health study, reporting on their health, eating, and exercise habits every two years.
Fung’s team looked at the data for over 88,000 of these nurses (the ones who didn’t already have a history of heart disease, stroke, or diabetes) through 2004 to see if what they said they ate had an impact on incidence of heart attacks or strokes. The difficulty is, the nurses weren’t explicitly following any particular diet. They just filled out a survey indicating how often they ate each of 55 different types of foods.
So the researchers developed a scoring system. Fruits, whole grains, vegetables, low-fat dairy, and nuts were “good” foods, and sodium, red meats, and sweetened drinks were “bad.” For each of these categories, the nurses were divided into quintiles. The top fifth in terms of number of servings consumed got 5 points, the next fifth got 4, and so on. Bad foods were scored in reverse, so being in the top fifth of red meat consumers gave you 1 point. The scores were added up for each woman, for a maximum of 40 points and minimum of 8. It’s not exactly rigorous, but it gives a sense of how close these women’s diets came to the DASH guidelines.
So did a higher DASH score lead to better health outcomes? Quite possibly. Once again, the women were divided into quintiles based on their DASH scores. Women in the highest quintiles — those closest to following the DASH guidelines — experienced significantly fewer heart attacks and strokes than women in lower quintiles. Even after adjusting for age, smoking, body mass index, energy intake, physical activity, and several other factors, the effect was still there. Following a reasonable approximation of a DASH diet correlated with fewer heart attacks and strokes. But there are some complications. First of all, this is just a correlation. Maybe people who are more prone to heart disease and stroke naturally have a harder time following the diet. And still it doesn’t work for everyone. Consider this graph:
This shows relative risk of getting heart disease for each DASH score quintile, but the nurses are further divided by BMI. A BMI above 25 is considered “overweight,” and as you can see, the effect of DASH score is not nearly as clear for women with a BMI over 25.
The graph assigns a risk of 1 to the women in the first quintile, with the lowest DASH scores. Then each successive quintile’s risk of getting heart disease is compared to the first quintile. For normal weight women, the trend is significant: the higher your DASH score, the lower your risk of heart disease. For overweight women, the trend isn’t significant. Although there’s no significant difference between the two groups, only the normal-weight group can claim a clear advantage for better adherence to DASH. The results were similar for strokes.
Another potential problem with this study is the fact that the entire study population is relatively active. Nurses are more like the conductors in the sedentary lifestyle study I discussed last week than the bus drivers. They’re on the move all day, usually on their feet. Would the DASH diet be as effective long-term for less active individuals? It’s hard to say. Although this study didn’t find a significant difference in the DASH diet effect based on relative activity levels, those activity levels are probably much higher than in the general population.
Still, when these results are combined with short-term controlled studies showing reduced blood pressure for people following the DASH diet, the evidence is quite impressive. It does seem quite likely that the DASH diet could be one way to reduce risk of heart disease and stroke.
That said, what’s less certain how easy it is to follow the DASH guidelines in the 21st century. Compare this graph from 1990 to the one above:
As recently as 1990, obesity rates in the US were much lower than they are now. During most of the Fung study, American nutrition habits were much better than they are today. If we started a similar study today, would we see similar results 30 years from now?
Fung, T., Chiuve, S., McCullough, M., Rexrode, K., Logroscino, G., & Hu, F. (2008). Adherence to a DASH-Style Diet and Risk of Coronary Heart Disease and Stroke in Women Archives of Internal Medicine, 168 (7), 713-720 DOI: 10.1001/archinte.168.7.713