Four Major Harms of Smoking to Blood Lipids: Analysis of the Mechanisms of Influence and Health Risks

Smoking has multiple effects on blood lipids, mainly manifested in the following four aspects:

● Increased serum total cholesterol levels: Studies have found that smokers have higher serum total cholesterol levels than non-smokers, which is speculated to be related to the concentration of carbon monoxide in the blood.

● Decreased serum high-density lipoprotein cholesterol (HDL-C): Many studies suggest a negative correlation between smoking and serum HDL-C levels. Regardless of gender, smokers have serum HDL-C levels that are 0.13–0.23 mmol/L lower than non-smokers. Smokers' serum HDL-C is negatively correlated with triglyceride levels. The low serum HDL-C levels in smokers cannot be explained by elevated triglycerides; it is currently believed to be related to carbon monoxide inhibiting the synthesis of low-density lipoprotein cholesterol in hepatocyte mitochondria.

● Increased serum triglycerides: Cigarettes contain large amounts of nicotine and carbon monoxide, which stimulate the sympathetic nervous system to release catecholamines, increasing serum free fatty acids. Free fatty acids are ultimately taken up by adipose tissue to form triglycerides. Catecholamines can further promote the release of lipids from adipose tissue, leading to elevated triglyceride levels.

●Promoting the oxidative modification of low-density lipoprotein (LDL)
Recent experimental studies have found that LDL exposed to smoke is easily oxidized to form oxidized LDL, possibly because carbon monoxide increases the sensitivity of LDL to oxidation. Oxidized LDL is a major direct cause of atherosclerosis.

Thyroid hormones promote both the synthesis of cholesterol in the liver and the excretion of cholesterol and its metabolites from bile. Therefore, thyroid hormones affect the production and degradation of serum cholesterol. In other words, when thyroid hormones are insufficient, although cholesterol synthesis decreases, its excretion rate is even lower, leading to an increase in total cholesterol concentration in the blood. Therefore, in patients with hypothyroidism, lipid synthesis, mobilization, and degradation are all reduced, with the latter being the most significant. The overall result is an increase in blood lipid concentration, but a significant increase in triglycerides is less common.

The increase in lipoproteins is mainly due to an increase in LDL levels. In hypothyroidism, the activity of low-density lipoprotein receptors on hepatocyte membranes decreases, which can impair the receptor-dependent degradation pathway of low-density lipoprotein in the body, thus causing elevated serum low-density lipoprotein cholesterol and apolipoprotein B levels. Other studies have shown that in hypothyroidism, the rate of low-density lipoprotein production in the body increases, which can also lead to elevated serum low-density lipoprotein cholesterol levels.