For Heart Health, Which Diets Really Work? A Conversation with Dr. Philip Greenland – Part I

Dr. Philip Greenland’s research has helped to shape cardiovascular care guidelines around the world. His work, which has been cited thousands of times, was among the first to reveal that women are more likely to die from heart attacks than men, and his studies illustrated that major risk factors almost always precede heart attacks. Dr. Kadish sat down with Dr. Greenland to discuss cardiovascular health.

Dr. Kadish: How important is diet to the prevention of cardiovascular disease or the treatment of it once somebody develops heart disease?

Dr. Greenland: The best I can answer that question is with published estimates of the proportion of the risk, both before the event and after the event, that people attribute to diet and weight control. I consider these related to one another, obviously. The estimates range as high as 50% of the risk in some way or another is related to diet. That might seem like a high number initially, until you start considering the influence of diet to obesity, insulin resistance, cholesterol levels and blood pressure. It’s not inconceivable that it could really be as high as 50%.

Dr. Kadish: Knowing that diet is so important for the prevention of heart disease, there have been a lot of papers published about many different diets. For the average person, that might be quite confusing.  Can you summarize your thinking about what kind of diet one ought to use, of the various ones that have been published? Low-fat diets, low-carbohydrate diets, diets with olive oil, etc.—How do you advise the average person who is interested in cardiovascular health?

Dr. Greenland: That’s a great question, and I think it’s a topic area that makes giving counsel to patients very confusing, because they read about this diet and that diet. Most of the literature on any of these particular diets is based on very, very short clinical trials, at best. Long-term trials are hard to do. But that said, there are some long-term trials, and those are pretty important and very convincing. In the PREDIMED trial that was done several years ago, they had to do a subsequent analysis, because they discovered there had been some misclassification and miscoding, but that’s a glitch. When they did the reanalysis, the study ended up showing quite substantial reductions in the combined end point of MI, stroke, and cardiovascular mortality. It was the Mediterranean diet, supplemented by olive oil or nuts.

I think people who are diet neutral—not advocates trying to support any one particular position—are generally recommending diets emphasizing vegetables, fruit, nuts, legumes and whole grains. The other study that was pretty convincing for secondary prevention was the Lyon Heart Study.

Dr. Kadish: Secondary prevention, meaning preventing further cardiac events after someone has suffered a heart attack?

Dr. Greenland: Correct. So the Lyon Heart Study was another example of a long-term study that was vegetarian-oriented and there’s pretty good consensus around it for heart disease prevention. I’m well aware that there are other studies that have been short-term studies, looking at things like lipid levels and blood pressure levels and so on. I actually don’t put a whole lot of weight, no pun intended, in those studies, because there are a lot of things that can influence blood pressure and lipid levels in the short term. What we’re trying to do with diet recommendations is give advice that people can actually stick with—that they will be able to follow long term. So some of the “oddball” diets that people advocate for—very-high-fat diet or all grapefruit diet—these are fine for short term, and they usually lead to weight loss, which usually leads to improvement in cardiovascular risk factors. But very few people can actually follow those kinds of diets long term, as opposed to a diet with an emphasis on vegetables, fruit, nuts and whole grains. 

Dr. Kadish: It’s interesting that you mentioned olive oil.  The Mediterranean diet plus olive oil, which had the best outcomes, that was a lot of olive oil.

Dr. Greenland: Yes, and it was actually due to supplementary olive oil. In my point of view, that’s a little bit out there, a little bit crazy. Part of the reason that I think that is because when you’re giving a dietary recommendation to people, you want to make sure that they’re not going to do something that’s actually going to cause them to gain weight. But if you’re eating your regular diet and supplementing it with a high-caloric additive, like drinking olive oil, that’s going to lead to trouble. Once you start gaining weight on any diet, this cascade starts to occur where you start to increase insulin resistance, you start to get higher blood pressure, and you start to get adverse changes in lipids. I think if you come back to a conservative approach to dietary recommendations, most of the recommendations out there do not emphasize adding supplemental fat with any type of a diet.

Dr. Kadish: Let’s talk about a few specific foods now. There was a recent study regarding whole milk versus skim milk, which of course seems to be the opposite of a low-fat diet.  Should we put a lot of stock in that study or do you think skim milk is really still a good part of a Mediterranean diet, which is also relatively low in fats?

Dr. Greenland: Another good question.  I assume you’re talking about the PURE Study?

Dr. Kadish: Yes.

Dr. Greenland: It’s an interesting finding.  I think what many people have noticed about that particular study is that it was done in lower socio-economic countries. It’s hard to extrapolate dietary advice from country to country because, as you know, the whole life style is completely different.  So, people have said about that study that the real concern of most of the people in those populations is under-nutrition, not over-nutrition. If you’re in a population where under-nutrition exists, and you’re able to have high-fat dairy, it might actually lead to a better outcome.  It’s one of the things that make evaluating these studies so difficult: you need to understand the whole context.

Continue reading Part II of this conversation with Dr. Philip Greenland.

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