Tuesday, March 14, 2017

The Truth About Saturated Fat

The Controversy
When I was studying nutrition in the late 90’s and early 2000’s, it was widely accepted that saturated fat had a negative impact on serum cholesterol and would increase risk of heart disease when consumed in large amounts. In the past few years, there has been some question as to whether or not this is true.

There are several reasons why this doubt has crept into the minds of the public:
1) Several popular diets actively promote the consumption of saturated fats.

2) The idea that coconut oil is a healthy food has really taken off. Proponents of coconut oil argue that different saturated fatty acids will impact health in distinct ways. They argue that coconut oil is high in the type of saturated fats that are health promoting.

3) Several research studies have come out in the past few years suggesting that saturated fat does not increase risk of heart disease.

So is it true, are saturated fats harmless even when consumed in large amounts? Could some types of saturated fats even be good for you?

Saturated Fats
Let’s start from the top. Just what is a saturated fat? 

There are 4 basic types of fats; saturated fat, trans fat, polyunsaturated fats and monounsaturated fats. These different types of fats have very different impacts on our health.

Let’s focus on saturated fats. Saturated fats are fats that contain no double bonds. All of their carbons are saturated by hydrogens. They are found, for the most part, in animal products.

There are several different classes of saturated fatty acids, identified by the number of carbons in their chain length:

Lauric acid: has a 12 carbon chain and is found in coconut oil.
Myristic acid: has a 14 carbon chain and is found in palm and coconut oil.
Palmitic acid: has a 16 carbon chain and is found in palm oil, butter, and beef tallow.
Stearic acid: has an 18 carbon chain and is found in cocoa butter, chocolate and beef.

A study was published recently in The British Medical Journal that sheds a lot of light on this controversial topic.

The Study
The study in question was conducted by Zong et. al. and utilized both of Harvard University’s Nurses’ Health Cohort and Health Professional Follow-up Study (Reference 1). Over 73,000 women and over 42,000 men were followed for 20+ years. Subjects were categorized by saturated fat consumption and monitored for risk of heart disease. What makes this study different is that the researchers didn’t focus exclusively on saturated fat, but also on the individual subtypes of saturated fat most commonly consumed by these subjects. Results were presented for total saturated fat, as well as lauric acid, myristic acid, palmitic acid and stearic acid.

The results were convincing:
-For total saturated fat, there was a statistically significant 18% higher risk of coronary heart disease when comparing subjects consuming the highest amounts of saturated fat to those consuming the lowest amounts.

-When comparing subjects who consumed the most stearic acid to those who consumed the least, there was a statistically significant 18% higher risk of coronary heart disease.

-When comparing subjects who consumed the most palmitic acid to those who consumed the least, there was a statistically significant 18% higher risk of coronary heart disease.

-When comparing subjects who consumed the most myristic acid to those who consumed the least, there was a statistically significant 13% higher risk of coronary heart disease.

-When comparing subjects who consumed the most lauric acid to those who consumed the least, there was a borderline significant 7% higher risk of coronary heart disease.

-Replacing 1% of energy from saturated fat with 1% of energy from:
            Polyunsaturated fat- resulted in an 8% reduction in risk of heart disease.
            Whole grains- resulted in a 6% reduction in risk of heart disease.
            Plant protein- resulted in a 7% reduction in risk of heart disease.

Conclusions
This study tells us several things:
1) Saturated fat, in all of its forms, has the potential to increase risk of heart disease. The increases in risk were statistically significant for total saturated fat, stearic acid, myristic acid and palmitic acid. The only fatty acid that did not reach statistical significance was lauric acid. This relationship was very close to being significant as well (those consuming the most lauric acid had a 7% increased risk of heart disease and the p-value for trend was .05).

2) Replacing saturated fats with healthier foods (polyunsaturated fat, whole grains, plant protein) reduced risk of heart disease in these cohorts.

So, with such strong evidence of harm, why the controversy?
1) Much of the information floating around about saturated fat and coconut oil is not based on research science but on opinion.

2) The few studies that showed no increased risk in heart disease from saturated fat generally used statistical models that replaced saturated fat with carbohydrate. Since most of the carbohydrate that we consume in the U.S. is processed, it is not surprising that replacing one unhealthy food for another does not materially change risk of heart disease. Well designed studies like the one presented here use models that replace saturated fats with more healthful options, such a vegetable based oils, healthy protein and whole grains. These studies show consistent benefit to doing so.

Recommendations
My recommendations to my clients regarding saturated fats are not new. Saturated fat is still associated with an increased in risk of heart disease, regardless of the subtype. It is important to limit consumption to about 7% of calories. When you do substitute the sources of saturated fat in your diet, make sure it is not with refined carbohydrate, but with nuts, avocado, healthy vegetable oils high in poly- and monounsaturated fats, fruits, vegetables, whole grains and vegetable sources of protein.

References

1) Zong G, Yanping L, Wanders A, et al. Intake of individual saturated fatty acids and risk of coronary heart disease in U.S. men and women. British Medical Journal 2016 355:i5796.

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