人體的肝臟(以及小腸、皮膚)每天製造膽固醇，材料來自飲食攝入之脂肪(尤其是飽和 脂肪)，以幫助身體建構細胞壁、合成性激素與維生素D及消化脂肪的膽汁酸。當一個人血 中之總膽固醇和低密度脂蛋白膽固醇(low density lipoprotein cholesterol,LDL-C)濃度上升，罹 患冠狀動脈疾病(coronary artery disease,CAD)與中風(stroke)之風險隨著增加，兩者間因果關 係已獲確認且有充份實證文獻支持，透過低脂低膽固醇飲食與規律運動，甚至加上藥物治 療，可大幅降低血清中LDL-C水平，罹患冠心病(coronary heart disease,CHD)之風險亦跟隨 著減少。 臨床上亦可能遇到低膽固醇血症(hypocholesterolemia)，目前國際上並無一致定義，美 國心臟協會認為小於160 mg/dL，有些學者標準更低，即小於 130 mg/dL，多數以120-150 mg/dL為界定值。先天基因異常，可能出現血中膽固醇濃度偏低，通常會低於50 mg/dL，例 如低β脂蛋白血症(hypobetalipoproteinemia)、無β脂蛋白血症(abetalipoproteinemia)，後者大 部分出現在猶太人身上。後天方面，血脂異常者(dyslipidemia)服用降血脂藥物，亦可能導 致血中膽固醇濃度偏低；常見續發性低膽固醇血症，例如感染(急性或慢性)、貧血、營養不 良(malnutrition)、腸道吸收不良(malabsorption)、慢性發炎、甲狀腺機能亢進、慢性肝疾 病、惡性腫瘤、腎上腺功能不全、危重病(critical illness)等。另外身心障礙者因日常行動功 能降低，無論是在居家或長照機構接受照護，一段時間後有可能出現貧血、低白蛋白血症 (hypoalbuminemia)、低膽固醇血症等情況。 研究報告指出膽固醇濃度與死亡率間關係呈現U型曲線，即濃度在135-200 mg/dL間呈 現水平(死亡率最低)，當濃度小於135 mg/dL或大於200 mg/dL，則呈現上升趨勢。一些前瞻 性世代研究報告指出，血清總膽固醇濃度偏低與癌症之發生有關聯性。一般認為膽固醇對 於細胞結構和功能之恆定維持，扮演著重要角色，同時也作為一些生化路徑(如合成維生素 D與類固醇荷爾蒙)之前趨物(precursor)，學者亦發現在癌症確認前期，便可能出現總膽固醇 濃度下降，因癌細胞有可能會干擾身體內之新陳代謝與合成。

Cholesterol is synthesized mainly in the liver(also by intestine, skin), materiasl coming from daily intake fat, especially saturatic fat, cholesterol can help our body to construct cell wall, synthesizing sexual hormones, vitamin D, and bile acids. When serum total cholesterol and low-density lipoprotein cholesterol level rises, it increases the morbidity risk of stroke and coronary artery disease. By means of lifestyle modification, such as exercise, low fat and low cholesterol diet, we may lower the lipid level to some extent, hence decrease the CAD risks, if added lipid lowering agents such as statins, help to reduce more risks. So far, there is no exact definition of hypocholesterolemia, some authors suggested the cut-off value was 160 mg/dL, but others proposed as below 130 mg/dL, mostly proposed 120-150 mg/dL as cut-off value. Causes of hypocholsterolemia may be divided as primary or secondary. Primary hypocholesterolemia is the results of congenital genetic defect such as hypobetalipoproteinemia, abetalipoproteinemia, the serum cholesterol level may be lower than 50 mg/dL.As for secondary hypocholesterolemia, clinically commonly encountered, such as malnutrition,malabsorption, infection, chronic inflammation, anemia, cancer, adrenal insufficiency, hyperthyroidism, critically illness, chronic parenychymal disease, Gaucher disease, drug induced(statins),etc. Some study results pointed out that the relationship between cholesterol level and mortality was U-shaped, when cholesterol concentration within 135-200 mg/dL, mortality is lowest, while level below 135 mg/dL or above 200 mg/dL, mortality rate rises up. One pros-pective cohort study revealed low cholesterol level significantly assocated with cancer incidence. The cholesterol plays the role in homeostasis of cell structure and function, also as a precursor of some biochemical pathways(such as vitamin D and steroid hormones). Some authors also reported that period preceding of cancer confirmed, may represent with lower cholesterol level, because cancer cells may interrupt metabolism and synthesis in the body.