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Diet, disease, and detrimental fats

Moyan Brenn/Flickr

Just how much of a role does diet play in overall health? And what dietary advice is best to follow?

This week on “Take Care,” Dr. Walter Willett, chair of the Nutrition Department at the Harvard School of Public Health and the Fredrick John Stare Professor of Epidemiology and Nutrition, discusses what we've learned from nutrition studies over the years and how that information has helped shape dietary guidelines to improve human health.

It’s no secret that diet plays an important role in health. But just how big is that role? This is the question Willett asked himself when he joined the still on-going Nurses’ Health Study in 1977. Developing a questionnaire that could be administered to the study’s 121,000 participants, Willett began to collect dietary data that he hoped could help explain the relationship between diet and other health conditions.

“We’re tracking who develops a heart attack, who develops cancer, pretty much every major condition,” Willett said. “We can connect diet with disease incidents, control for smoking and activity, and a lot of other factors, and provide uniquely high quality data on long term consequences of diet.”

The study had a large impact on the way medical community viewed diet.

“Hypotheses that were widely accepted were overthrown, actors that we were not even thinking about turned out to be important,” Willett said. "We can have a huge impact on health and wellbeing with the right collection of dietary actors, the right dietary pattern.”

One of the biggest reveals of the study was on the impact of fat—or rather, lack thereof. At the time of the study, fat was believed to be a contributing factor in cancers including breast cancer, rectal cancer, and prostate cancer.

“That belief drove dietary recommendations to incriminate all types of fat and emphasize carbohydrates,” Willett said. “The food guide pyramid, for example, emphasized at the base up to up to 13 servings of carbohydrates in a day.”

Realizing this carbohydrate-laden fat-condemning diet wasn’t based on any solid evidence, Willett incorporated data collection regarding different types of fat into the study. He found that the amount of fat in the diet has little impact on the diseases in question, but that certain types of fats—specifically trans?fats—were connected to health risks.

The process of partial hydrogenation that trans-fats are subject to, twists essential molecules, changing their biological function. At the time of the study, trans-fats were found in many common food products, including margarine, vegetable shortening, and deep-frying fats.

“We did see, as we were concerned, higher trans-fat intake related to increased risk of heart attacks and diabetes,” Willett said. “As time has gone on, we’ve seen trans fats related to many other adverse consequences including cognitive loss and dementia.”

Since the study’s beginning, the amount of trans-fat in the food supply has been reduced by over 80 percent. Willett still emphasizes the importance of maintaining healthy fats in the diet, which he says are best obtained through vegetable oils, particularly olive oil.

“Liquid vegetable oils are very healthy,” Willett said. “We do need both omega-3 and omega-6 polyunsaturated fats, so if you’re not really consuming olive oil, having a combination of other types of fats is a good thing.”

Willett advises adopting a Mediterranean-style diet, one high in vegetables, fruits, and healthy oils and low in red meat. He recommends sticking to whole grains, though not in large quantities, and incorporating healthy fats.

“The great thing is, you can put those factors together in thousands of different ways that provide variety,” Willett said. “The wide range of flavors can make eating both very healthy and enjoyable at the same time.”