The diet-heart hypothesis states three things:
- Dietary saturated fat increases blood cholesterol
- Elevated blood cholesterol increases the risk of having a heart attack
- Therefore, dietary saturated fat increases the risk of having a heart attack
To evaluate the second contention, investigators have examined the relationship between blood cholesterol and heart attack risk. Many studies including MRFIT have shown that the two are related (1):
The relationship becomes much more complex when you consider lipoprotein subtypes, density and oxidation level, among other factors, but at the very least there is an association between habitual blood cholesterol level and heart attack risk. This is what you would want to see if your hypothesis states that high blood cholesterol causes heart attacks.
Now let’s turn to the first contention, the hypothesis that dietary saturated fat increases serum cholesterol. This idea is so deeply ingrained in the scientific literature that many authors don’t even bother providing references for it anymore. When references are provided, they nearly always point to the same type of study: short-term controlled diet trials, in which volunteers are fed different fats for 2-13 weeks and their blood cholesterol measured (2)*. These are the studies on which the diet-heart hypothesis was built.
But now we have a problem. Nearly every high-quality (prospective) observational study ever conducted found that saturated fat intake is not associated with heart attack risk (3). So if saturated fat increases blood cholesterol, and higher blood cholesterol is associated with an increased risk of having a heart attack, then why don’t people who eat more saturated fat have more heart attacks**?
I’ll begin to answer that question with another question: why do researchers almost never cite observational studies to support the idea that dietary saturated fat increases blood cholesterol? Surely if the hypothesis is correct, then people who habitually eat a lot of saturated fat should have high cholesterol, right? One reason may be that in most instances, when researchers have looked for a relationship between saturated fat intake and blood cholesterol, they haven’t found one. Those findings have essentially been ignored, but let’s have a look…
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