|
| Mechanisms | References |
|
| Indirect mechanisms | |
| Alteration in body weight | |
| Nicotine suppresses appetite. | [17] |
| Low BMI or low body weight (1) decreases the effect of mechanical loading necessary to enhance osteogenesis; (2) is associated with less fatty tissue, thus the extraovarian conversion of androgen to estrogen is reduced in smokers; or (3) may be associated with lower leptin. | [5] |
| The effect of smoking on BMD and risk for fracture persists after controlling for low body weight and low BMI. | [4] |
| Alteration in parathyroid hormone- (PTH-) vitamin D axis | |
| Tobacco smoking reduces bone mass through its effect on vitamin D and calcium absorption. | [4] |
| Low 25-OH-D and 1, 25-OH2-D in smokers were reported in several studies. | [20–27] |
| Low vitamin D in smokers may due to (1) the induction of the liver enzyme that enhances hepatic metabolism of vitamin D metabolites or (2) suppression of PTH release. | [5, 16] |
| Suppression of PTH in smokers was not consistently reported. | [20–23, 25, 28] |
| Smoking impairs intestinal calcium absorption via changes in calciotropic hormone metabolism, remaining significantly lower in smokers despite adjustment for confounders. | [30–32] |
| Alteration of Adrenal Hormones | |
| Smoking increases cortisol level. | [33–35] |
| Smokers had higher levels of androstenedione and dehydroepiandrosterone. | [37, 38] |
| A high level of glucocorticoid in smokers alters bone metabolism and decreases bone mass either directly by changing the osteoblast and osteoclast activities or indirectly by altering the gastrointestinal absorption and renal reabsorption of calcium. | [39–42] |
| Alteration of Gonadal (Sex) Hormones | |
| Tobacco smoking enhances estrogen metabolism resulting in a lower level of estradiol. | [52, 53] |
| Regarding testosterone, some studies found levels of testosterone were similar in both smokers and nonsmokers, while other studies found levels of testosterone were higher in smokers. | [35, 60–62] |
| Women who smoke usually experience menopause two years earlier than women who do not smoke. | [54, 55] |
| Smoking may modify the production and metabolism of estrogen through (1) inhibition of aromatase enzyme (estrogen synthase) and suppression of the production of estrogen; (2) increase in the hepatic breakdown of estradiol to irreversible inactive metabolite; and (3) increase in the level of the serum SHBG that may reduce the level of free estradiol. | [53, 56–59] |
| Increased Oxidative Stress | |
| Tobacco smoking is associated with high levels of free radicals. | [63, 64] |
| High levels of free radicals may increase bone resorption and contribute to lower bone mass. | [65] |
| Smokers have significantly lower antioxidant enzyme levels and higher levels of oxidative stress products than nonsmokers. | [66] |
| Direct Effect on Bone Tissue | |
| Polycyclic aryl hydrocarbon compounds have deleterious effects on bone. | [4] |
| Nicotine has an inhibitory effect on osteogenesis and on angiogenesis that play key roles in bone metabolism. | [16] |
| Nicotine at low levels increases cell proliferation, while at higher levels it inhibits osteoblast production, resulting in cell death. | [70] |
| Nicotine had a dose-dependent inhibitory effect on osteoblast development and on vascular endothelial growth factor, necessary for angiogenesis. | [71] |
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