Effects of Dietary Fiber and Statins on Serum Cholesterol Levels
Written by Belle Roberts
Cholesterol has a bad rap. Rather than being the villainous molecule the media makes it out to be, it’s actually essential to the structural integrity of our cells, a precursor for certain hormones, a vital component of vitamin D metabolism, and a precursor for bile acids. Our cholesterol content includes endogenous cholesterol, which the liver produces and recycles, and exogenous cholesterol that we intake through animal products.
When you intake fats, they’re converted and broken down into chylomicrons, which are taken into the blood by lacteals, vessels of the lymphatic system. These cholesterol-storing chylomicrons are then transported to the liver. Very-low-density lipoprotein, or VLDL cholesterol, is produced in the liver from endogenous triglycerides (the storage form of fats). VLDL circulates from the liver to the small intestine to interact with lipid absorption; 50% of this VLDL is returned to the liver, while the rest is converted to low-density lipoprotein (LDL) cholesterol, or “bad” cholesterol. 25% of that LDL enters non-hepatic tissues, which in excess can deposit in coronary walls. “Good” cholesterol, high-density lipoprotein (HDL) cholesterol, however, is transported from extrahepatic tissues to the liver. In short, cholesterol rotates through a cycle between the small intestine and liver, where it is stored and recycled into bile acids, which emulsify fats into smaller particles in order to be digested.
“Okay, where does fiber fit into this?” Shh, I’m getting there.
We hear a lot about “heart healthy” foods like Cheerios and rolled oats that help lower cholesterol levels, but the real reason these foods are helpful is their dietary fiber content. Fruits, veggies, nuts, and seeds are heart healthy too, they just don’t have a cartoon bee promoting them.
Dietary fiber is plant polysaccharides (complex carbohydrates) that are resistant to hydrolysis by digestive enzymes in humans. The power of fiber is that we cannot digest it. Soluble fiber, like that found in most fruits and vegetables, will undergo gel formation within the large intestine, increasing digestion and softening the consistency of a bowel movement. It can also bind directly to molecules or feed our colon microbiome (prebiotics are just soluble fiber pills). Insoluble fiber, found in grains, nuts, and seeds, increases fecal bulk and acts like a large intestine Zamboni, forcing undigested molecules out of the GI tract.
Fiber decreases serum cholesterol by decreasing bile acid recycling, cholesterol absorption, and cholesterol synthesis, as well as reducing insulin stimulation of cholesterol production.
Fiber binds to bile acids in the small intestine, pushing them out of the GI tract along with waste material; thus, forcing the liver to convert a stored cholesterol molecule into a new bile acid. Similarly, fiber can bind to cholesterol in the GI tract, forcing it out directly. Fiber also decreases cholesterol synthesis in two ways: generating propionic acid and slowing changes in blood glucose concentration. When the bacteria in our gut ferment dietary fiber, they convert it to lactate and short chain fatty acids. The short chain fatty acid propionate may inhibit cholesterol synthesis by entering the liver and inhibiting HMG-CoA reductase, the key regulatory enzyme in cholesterol formation. By controlling that enzyme’s activity, we are able to dramatically alter cholesterol synthesis. Insulin also stimulates HMG-CoA reductase. Gel formation of fiber in the gut decreases blood glucose response, which slows insulin activity; thereby, reducing stimulation of HMG-CoA reductase.
The dietary AI, or adequate intake, of fiber is 38g per day for men ages 19-50 and 25g per day for women ages 19-50. Not surprisingly, the average intake in the US is 17g per day and 13g per day for men and women, respectively.
It’s no wonder that cardiovascular disease accounts for 1 in every 4 deaths in the US.
The fat content of our blood is comprised of triglycerides and cholesterol. Healthy triglyceride levels hover under 150 mg/dL, while normal cholesterol levels lie under 200 mg/dL. High lipid levels increase risk of developing cardiovascular diseases, including atherosclerosis, heart attack, and stroke.
Atherosclerosis is especially prevalent in the western world and involves the accumulation of cholesterol in the arteries, the narrowing of the arteries, and the formation of abnormal surfaces in the arteries. There are general risk factors to developing atherosclerosis, like high blood pressure, cigarette smoking, and obesity; however biochemical risk factors include high LDL and low HDL cholesterol.
US corporate pharmacopeia has taken advantage of these risk factors by developing prescription drugs called statins, which inhibit HMG-CoA reductase. Its mechanism is targeted to the liver, but the disease-fighting effects rely on sufficient circulatory function; meaning prescription dosage must be increased to accommodate this. Statins rank number one among all prescribed drugs in the US, with Crestor selling nearly $5.6 billion worth of prescriptions inbetween July of 2013 and June of 2014. As of 2012, 28% of American adults over 40 reported taking a cholesterol-lowering medication in the past 30 days. Statins are incredibly effective at lowering serum cholesterol but require a lifelong commitment. Unlike fiber, which builds your intestinal resistance to cholesterol absorption and synthesis, statins work only when taking them as prescribed. Once you stop taking them, your cholesterol levels will shoot back up to unhealthy levels. However, statins and fiber can used symbiotically to maintain healthy lipid levels and gut function.
To sum up my ramblings for you, fiber elicits its effects via gel formation, binding to molecules, and fermentation by the gut microbiome. Statins lower endogenous cholesterol by inhibiting the key regulatory enzyme in cholesterol formation. Moreover, fiber and statins can be used together in disease prevention and management. Lipid levels can increase risk of diseases like atherosclerosis, diabetes mellitus, obesity, and gastrointestinal disorders like diverticulosis.
So stick it to the man -- eat your veggies.
Brown, A. J. (2007), CHOLESTEROL, STATINS AND CANCER. Clinical and Experimental Pharmacology and Physiology, 34: 135-141. doi:10.1111/j.1440-1681.2007.04565.x
Gropper, S., & Smith, J. (2013). Advanced nutrition and human metabolism (6th ed.). Belmont, CA: Wadsworth/Cengage Learning.