Food plant fibers can be mainly divided into two categories: soluble and insoluble.
First, soluble plant fibers.
Soluble plant fibers are those that have water-absorbing properties, including cellulose, which is the main component of plant cell walls; and non-cellulosic polysaccharides, also known as hemicellulose, which can form water-containing gels. Soluble plant fibers are mainly found in vegetables, fruits, legumes, oat bran (sugar), corn skin, agar, pectin, and alginate.
Second, insoluble plant fibers.
Insoluble plant fibers mainly include lignin, which is the essential part of plants and belongs to the phenylpropane polymer.
Dietary plant fibers are mainly composed of non-utilizable carbohydrates (only lignin does not belong to carbohydrates) and provide very little energy. However, animal experiments and clinical dietary metabolism studies have confirmed that consumption of plant fibers has a regulatory effect on blood lipids, with soluble fibers being superior to insoluble fibers.
Research has found that when adding soluble fibers compared to adding insoluble fibers, the amount of substances such as lithocholic acid, bile acid, and cholesterol in the stool decreased by 83%, indicating that soluble fibers have a stronger lowering effect on serum total cholesterol (TC). This is related to the fact that more lithocholic acid, bile acid, and cholesterol are adsorbed by soluble fibers in the body. However, there was no significant difference in the effect of both fiber diets on serum high-density lipoprotein cholesterol (HDL-C).
In conclusion, even after lipid-lowering diet treatment, when serum TC has decreased, soluble plant fibers still have a significant lowering effect on serum TC.
The possible mechanism of dietary plant fibers reducing blood lipids is as follows:
- Soluble fibers expand when they come into contact with water, increasing the volume of feces and promoting peristalsis, which facilitates the excretion of cholesterol in feces.
- Soluble fibers bind with bile acids or other lipids, reducing the absorption of cholesterol.
- They decrease the synthesis of lipoproteins.
- They accelerate the clearance of LDL-C.
- Insoluble fibers are not easily digestible and absorbed, and they form insoluble complexes with lignin and cellulose in the intestine, which also affect cholesterol absorption and promote cholesterol excretion.
Although dietary plant fibers can reduce serum TC and LDC-C levels, excessive consumption can lead to the production of large amounts of methane in the intestinal tract, causing side effects such as increased stool volume and frequency, flatulence, and abdominal bloating.
One study on the relationship between dietary fiber and high blood lipid levels
One notable study on the relationship between dietary fiber and high blood lipid levels in Europe was conducted by the Department of Nutrition at the University of Oslo, Norway. This research aimed to investigate the effects of different types of dietary fiber on blood lipid profiles, particularly total cholesterol, LDL (low-density lipoprotein) cholesterol, and HDL (high-density lipoprotein) cholesterol, in individuals with hyperlipidemia or at risk of cardiovascular disease.
The study involved a randomized controlled trial where participants were assigned to consume diets varying in fiber type and content. The researchers focused on fibers such as soluble fiber (e.g., from oats, fruits) and insoluble fiber (e.g., from whole grains, vegetables), which have different physiological effects. Dietary interventions were carefully designed to maintain overall caloric intake while altering the fiber composition. Blood lipid levels and other relevant biomarkers were measured before and after the intervention period.
The findings indicated that increased intake of soluble fiber was associated with significant reductions in total and LDL cholesterol levels, which are known risk factors for cardiovascular disease. This effect was attributed to the ability of soluble fiber to bind bile acids in the gut, leading to their excretion and subsequent upregulation of cholesterol synthesis in the liver, thereby lowering blood cholesterol levels. Insoluble fiber, while not directly affecting cholesterol levels, contributed to improved gut health and overall dietary quality.
This research supports the inclusion of high-fiber diets, particularly those rich in soluble fiber, as a strategy for managing blood lipid levels and reducing the risk of cardiovascular diseases.
