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Does the DASH diet have to be low-fat?
One of the challenges in studying diet is that we're studying, well, people. Asking people just to recall what they eat is prone to error (although there are ways to ensure more accurate data), and as anyone who has tried to lose weight on a strict diet will tell you, it's tough to stick to a restrictive diet for the long haul.

More poorly designed diet research: low-fat vs. low-carb diets
The media has been all over today's study from the Annals of Internal Medicine (doi:10.7326/M14-0180), for the most part acting as if this were the last word on healthy eating. Proponents of the Atkins Diet are understandably celebrating, as the results of the study as published are that a low-carbohydrate diet increases weight loss and improves cholesterol scores more than a low-fat diet.

Yes, fad diets work, but....
One of the concerns with diets that drastically restrict the amount of fat, protein, or carbohydrates is the lack of evidence regarding their long-term effects on one's health, especially cholesterol scores.


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Not all fats

a bottle of olive oil with an olive branch bearing green olives

For a long time low fat diets were all the rage. They were supposed to help you lose weight, improve your cholesterol scores, help you avoid heart disease, and and in extreme cases, make you "heart attack proof." Fortunately, in the last few years there's been a wealth of well-designed research showing that not all fats are bad for you: indeed, some are definitely good for you. Trans-fats? Definitely bad. Omega-3 fatty acids? Definitely good.

A team at Harvard's Medical School, School of Public Health, and the Brigham and Women's Hospital in Boston has looked at the associations between intake of various types of dietary fats and one's risk of death, not just from all causes but also from more specific causes like heart disease, cancer, neurogenerative disorders like ALS or Alzheimer's, and respiratory diseases (JAMA Int Med 2016;176(8):1134-1145).

For their analysis, they utilized data from two large-scale, long-term studies, the Nurses' Health Study (NHS) and the Health Professionals Follow-up Study (HPFS). The data used from the NHS included dietary questionnaires from the over 83,000 women participating responded to every 2 years starting in 1980 and continuing through 2012. The data from the HPFS included similar dietary questionnaires, also administered every 2 years, from over 42,000 men starting in 1986 and continuing through 2012. The authors analyzed the participants' dietary records and determined the individuals' intake of total fat, trans-fats, saturated fat, polyunsaturated fat, and monounsaturated fat. They also broke out the poly- and mono-unsaturated fats into subtypes, including omega-3 and omega-6 fatty acids as well as specifying the source (from fish and shellfish or otherwise) of those omega-3 and -6 fatty acids.

The fat intake of those who died from various causes was then compared to those who did not. Those with the highest of five levels of total fat intake were 13% more likely to die of any cause - but those with the highest quintile of polyunsaturated and monounsaturated fats were 19% and 11% less likely, respectively. Similarly, higher saturated fat intake meant a greater risk of death from cancer (about 7% higher), while higher total fat intake substantially increase the risk of death from respiratory diseases (a whopping 56%). Greater levels of polyunsaturated and monounsaturated fats, on the other hand, meant lower risk of death from neurodegenerative and respiratory disorders.

The researchers went a step further and used mathematical models to find out if the outcomes might change if the participants had replaced just 5% of the calories they consumed from saturated fats with the same number of calories from polyunsaturated or monounsaturated fats. Switching from a saturated fat to a polyunsaturated fat means a 27% drop in risk of death from all causes as well as a lower risk of death from heart disease, cancer, and neurodegenerative disorders, while switching to monounsatured fats cut the risk of death from neurodegenerative disorders like ALS or Alzheimer's by 29%.

What this means for you

This is a huge study involving many thousands of people, multiple measures of dietary intake (not just one at the start of the study) and lasting at least 20 years, making its conclusions very strong indeed. Further, the authors were careful to excluding anyone who had been already diagnosed with heart disease, cancer, or type 1 or type 2 diabetes before the start of the study. That said, this is an observational study, so it does not show causality - just association. Still, it's consistent with research we've seen elsewhere. Fats are not bad for you - just choose the right types of fats. Here's my article on dietary fats in general; follow it with these articles on saturated fats and unsaturated fats.

First posted: September 14, 2016