**6.3 Smoking**

The amount and duration of smoking may influence how smoking affects bones. Smoking may affect 25 hydroxylases in the liver, which lowers serum 25-hydroxyvitamin D levels, altering the hepatic metabolism of vitamin D. This may explain why smoking and vitamin D deficiency appear to be related. Also, there is proof that smoking can affects the gastrointestinal absorption of calcium. Although it is debatable whether smoking affects estradiol levels, certain research studies have shown that smoking affects the production and metabolism of estrogen. Smoking accelerates the hepatic metabolism of estradiol and nicotine may diminish the synthesis of estrogen. Also, smokers have greater serum levels of the sex hormone-binding protein, which lowers free estradiol levels. All IBD patients should be encouraged to quit smoking, though, as it lowers the risk of secondary consequences such as heart disease, lung cancer, and changes in bone health [48–50].
