Why Polyunsaturated Fats Lower Cholesterol and Saturated Fats Raise Cholesterol
Nutrition / Natural Medicine Update No. 62 (August 10, 2017)
with Dr. James Meschino
Research Topic: Why Polyunsaturated Fats Lower Cholesterol and Saturated Fats Raise Cholesterol
Source: The Journal of Nutrition. 2005
A looming question for many years has been, why do saturated fats and transfats raise bad cholesterol levels and polyunsaturated fats lower blood levels of the bad cholesterol? Well in a brilliant review of the research, published in The Journal of Nutrition in 2005, two researchers reviewed the data on this subject and provided the explanation the medical and health community has been seeking. So, let me try to hit the highlights for you:
The first thing to know is that when you ingest foods with saturated fat, transfats or polyunsaturated fat (including omega-3 fats), they are absorbed from the gut in small transport vehicles called chylomicrons. Chylomicrons enter the lymphatic system and eventually enter the bloodstream. Once in the bloodstream some these fats get taken up by fat cells and get stored in fat tissue, and can re-enter the bloodstream bound to the protein carrier albumin, and act as a source of energy for our muscles and other tissues between meals, during exercise and during fasting. The fats in the chylomicrons not picked up by fat cells and muscle cells inevitably are picked up by liver cells.
What Happens in The Liver
The presence of saturated fat, transfats and polyunsaturated fats in the liver prompts liver cells to turn on cholesterol production. Cholesterol is required to transport each of these types of fats back out into the bloodstream, this time incorporated into another type of shuttle vehicle, known as the VLDL (very low-density lipoprotein). Once secreted into the bloodstream by liver cells, the fats in the VLDL are further removed from the circulation by fat cells and muscle cells, primarily.
Fat Cells and Muscles
Fat cells store fat and muscle cells like to burn fat as a source of fuel during rest and light activity. Once the fat has been removed the VLDL is now transformed into LDL (or the bad cholesterol). With the fat removed, most of the LDL shuttle bus is filled with the cholesterol that was originally synthesized back in the liver.
LDL-Receptors Clear Bad Cholesterol from the Circulation
Now here is where the big distinction occurs that changes everything from the standpoint of heart disease, stroke, and other vascular diseases. The ingestion of polyunsaturated fats, including omega-3 fats and monounsaturated fats (olive oil, canola oil, avocados, nuts) also increase the number of LDL-receptors on liver cells. This enables the liver to clear the extra LDL’s from the bloodstream so that the bad cholesterol does not rise. And the receptors work better because these polyunsaturated fats improve what’s known as the fluidity of the cell membrane (outer skin of the cell which houses the LDL-receptors). The ingestion of saturated fat does not increase the synthesis of LDL-receptors and so LDL-cholesterol circulates through the bloodstream for 3-5 days and very easily becomes taken up by cells in the artery wall (called macrophages), which promotes the narrowing of arteries (atherosclerosis) and related vascular problems (heart attack, stroke etc.).
Other Adaptations in the Liver Induced by Polyunsaturated Fats
Polyunsaturated fats also decrease the conversion of carbohydrates into fat within the liver. The liver often converts carbohydrates into a cholesterol-raising saturated fat known as palmitic acid. But polyunsaturated fats slow this process down. As a result, the liver makes less cholesterol. Polyunsaturated fats also increase the conversion of cholesterol into bile acids in the liver. With food ingestion, the liver and gallbladder secrete these bile acids into the intestinal tract to help digest the fats eaten a meal. As bile acids cannot be converted back into cholesterol, polyunsaturated fats help to reduce the total cholesterol pool in the liver, and thus, less is secreted into the bloodstream.
Which Foods Contain Cholesterol-Raising Saturated Fats?
Finally, the saturated fats that raise blood cholesterol to the greatest degree are lauric, myristic and palmitic acid, found in beef, pork and high-fat dairy products to the greatest degree, although coconut and palm oil have a significant amount of lauric acid and other long-chain, cholesterol-raising saturated fats, as well. As for transfats, they not only raise blood cholesterol to the same extent as these saturated fats, but they also lower the good cholesterol (HDL), which makes them double trouble. HDL vacuums up some of the cholesterol in the artery wall, helping to reverse clogged arteries. So, higher HLD and lower LDL is the blood profile you are shooting for. In summary, to help lower bad cholesterol (LDL) and raise the good cholesterol (HDL) studies continue to support the following dietary and lifestyle strategies:
- Reduce or eliminate beef, pork, and high-fat dairy products (butter, any milk or yogurt above 1%, ice cream, whipped cream, cream, and cheese – unless the cheese is under 4% milk fat)
- Reduce or avoid coconut oil and palm oil
- Reduce or eliminate transfats, often found in pastries, creamy salad dressings, shortenings, and certain processed foods.
- Do your best to consume fish twice per week, olive oil, a handful of nuts each day, and maybe some avocado.
- Substitute chicken breast, turkey breast, and soy products for other meats, and know that soy products also reduce cholesterol by blocking cholesterol absorption in the gut and blocking the absorption of cholesterol building blocks.
- Beans and peas also help to lower cholesterol by dragging it out of the body via the fecal route. The same is true for 1 tablespoon per day of ground flaxseed, 1-3 teaspoons of psyllium husk fiber or the use of oat bran or an oat bran cereal. Apples and artichokes also help lower the bad cholesterol.
- Over consumption of refined sugars and starchy foods also promote the conversion of carbohydrate into palmitic acid (a cholesterol-raising saturated fat) in the liver. So, keep your carbohydrate intake in check, as well as your body weight.
- Endurance exercise helps to raise the good cholesterol (HDL), as does losing excess weight.
So, there you have it. Do your best to use diet and exercise to get your cholesterol into the ideal range: That means aiming for a:
Total cholesterol below 3.9 mmol/L (150 mg/dl)
And an LDL cholesterol below 2.0 mmol/L (76 mg/dl)
I’ve included the reference and link for the journal cited here in the text below
Fernandez ML and West KL. Mechanisms by which dietary fatty acids modulate plasma lipids. The Journal of Nutrition. 2005. 135:2075-2078. (The American Society for Nutritional Sciences) http://jn.nutrition.org/content/135/9/2075.full
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