Dan writes:

We're told that eating foods like oatmeal, which contains soluble fiber, can help lower cholesterol. My understanding is that this works because the fiber binds to the cholesterol in your food, preventing its absorption.

But we're also told that the amount of cholesterol in your food doesn't make much difference because, if you get more in your diet, your body just produces less.

If the soluble fiber is reducing absorption of dietary cholesterol, but dietary cholesterol doesn’t matter, then why would that reduce your serum cholesterol?

This is a great question but one that requires a bit of a deep dive to answer. But if this is not the place for deep dives into nutrition nerd-dom, I don’t know where is!

How dietary cholesterol affects blood cholesterol

About 80% of the cholesterol that’s circulating through your body right now was manufactured in your body, not extracted from your food. Most of that de novo cholesterol production happens in the liver, but small amounts are also manufactured in the small intestine.

And Dan’s absolutely right: The liver will ramp its production of cholesterol up or down in response to your dietary intake. Take in more cholesterol through food and the liver will make less, and vice versa. (Cholesterol production in the small intestine is not affected by that feedback loop, however.)

To some extent, the amount of cholesterol that you absorb from food is genetically determined. Some people are “hyper-absorbers” and for them, the amount of cholesterol in their food may have a bigger impact on their blood cholesterol levels.

But for most people, dietary cholesterol intake doesn’t have a significant impact on total blood (serum) cholesterol levels. More to the point, cholesterol intake does not appear to impact the risk of heart disease. This is why we are no longer advised to limit our consumption of dietary cholesterol (unless, of course, you’re one of those hyperabsorbers).

If you’re looking for a dietary culprit for high blood cholesterol levels, look to saturated fat and refined carbohydrates, not cholesterol. Eggs and shrimp, which are both high in cholesterol, are virtually carb-free and relatively low in saturated fat.

How soluble fiber affects blood cholesterol

According to the National Lipid Association:

“Soluble fiber can bind cholesterol in the intestine and remove it from the body. Eating 5 to 10 grams of soluble fiber a day can help lower total and LDL-cholesterol by 5 to 11 points, and sometimes more.”

You might imagine that when you eat...

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