Non-alcoholic fatty liver disease (NAFLD) has become the most common form of chronic liver pathology, with a disease prevalence of up to 30% in Western countries, already surpassing viral hepatitis and alcoholic fatty liver disease. It has also become the number one liver disease in China. NAFLD includes simple steatosis to becoming cirrhosis and can progress to liver failure and hepatocellular carcinoma. Although the formation of NAFLD is associated with the pathogenesis of obesity, insulin resistance and metabolic syndrome. Currently, there is a “multiple-hit hypothesis” for the pathogenesis of NAFLD. In recent years, new evidence for the pathogenesis of NAFLD has been suggested to be related to vitamin D deficiency. Evidence of Vitamin D Deficiency in NAFLD In 2013, a study from Turkey found (ClinMed(Lond).2013Dec;13(6):576-9) that a total of 613 non-obese volunteers participated in the study, and there were 275 cases of NAFLD, and the results found that patients with fatty liver disease had a significantly lower concentration of serum 25(OH)D compared with the level of healthy people; in addition, the more severe the fatty liver disease, the more vitamin D deficiency was found in patients with fatty liver disease. the more severe the fatty liver disease, the more significant the reduction in vitamin D levels; multifactorial analysis of low serum vitamin D levels as an independent risk factor for NAFLD: another meta-analysis in the same year (Aliment Pharmacol Ther 2013 Aug;38(3):246-54), which included 17 cross-sectional case-control studies evaluating the vitamin D and NAFLD Relationship. Patients with NAFLD had a 0.36 ng/ml decrease in 25(OH)D levels and were 1.26 times more likely to be vitamin D deficient compared to controls. Research evidence of vitamin D deficiency in NAFLD in China Scholars at the Ninth Hospital of Shanghai Jiao Tong University in China also conducted a large-sample study (Br JNutr. 2016Apr;115(8):1352-9.), which was a cross-sectional study of the health and metabolic status of adults in 16 districts in East China. A total of 5,066 subjects were enrolled, and 2,193 (43.3%) were diagnosed with NAFLD; as a result, 84.56% of the subjects exhibited vitamin D deficiency. Subjects with high vitamin D levels had a lower prevalence of NAFLD, especially male subjects. In the highest quartile of vitamin D levels, the prevalence of NAFLD was 40.8%, whereas the lowest quartile of vitamin D levels showed a prevalence of 62.2%, and there was no change in vitamin D levels in women. Decreased vitamin D levels were associated with a 1.54-fold increased risk of NAFLD. This study suggests that vitamin D levels are significantly associated with NAFLD and that vitamin D is an independent factor in the prevalence of NAFLD, especially in men in eastern China. Vitamin D supplementation therapy may be a new target for controlling NAFLD. Synthesis and sources of vitamin D Vitamin D is a fat-soluble vitamin among the vitamins required by the body, and several types of vitamin D are known, the most important forms being vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Exogenous vitamin D is derived primarily from ergocalciferol in food, which is absorbed in the small intestine along with fats with the assistance of bile. Secondly, vitamin D is mainly derived from 7-dehydrocholesterol in the skin through photosynthesis to form vitamin D3 pro-vitamin, which is also converted to vitamin D3 in the body by temperature.Vitamin D from both pathways is first transported to the liver by vitamin D-binding protein (DBP), which is catalyzed by the enzyme cholecalciferol- 25-hydroxylase catalyzes the formation of the 25-hydroxyvitamin, from which vitamin D3 is preserved in the liver; it is then transported out of the liver when the body needs vitamin D and further hydroxylated in the kidneys under the control of thyroid hormones to 1,25-dihydroxyvitamin, which is an active form of the hormone. Role of Vitamin D The general public’s understanding of the role of vitamin D is limited to being involved in the regulation of blood calcium and bone homeostasis, and it has been used to prevent and treat rickets in children, osteomalacia and arthralgia in adults. People suffering from osteoporosis can effectively improve the absorption of calcium ions by adding the right vitamin D and magnesium. However, in recent years, the extra-skeletal role of vitamin D has received increasing attention. Vitamin D has been reported to control, in a direct or indirect form, nearly 200 genes in the body that regulate angiogenesis, apoptosis, growth, proliferation and differentiation. In the cardiovascular system, chronic obstructive pulmonary infections and other diseases play a role; even the study that vitamin D is a neurosteroid hormone, the development of the nervous system, neurotransmitters, neuroimmune regulation, and protection of damaged nerve cells can play an important role, and this role is lifelong; in addition, there are also reports that vitamin D in the prevention and treatment of tumors has its unique role, such as Liver cancer, leukemia, stomach cancer, breast cancer, lung cancer, colorectal cancer, etc. Vitamin D plays a key role in regulating cell reproduction, which shows the importance of vitamin D to health. This article is more to discuss the relationship between vitamin D and NAFLD, the specific mechanism is very complex and belongs to the cutting-edge basic research, which is not yet conclusive, but the fact that NAFLD and vitamin D deficiency is a growing evidence. Causes of Vitamin D Deficiency in NAFLD Patients The main factor in human vitamin D deficiency is inadequate exposure to ultraviolet light from sunlight, which varies. There are factors such as changes in modern human lifestyles, with more and more people favoring indoor work and activities and a lack of outdoor sunlight exposure, and there are behavioral factors such as deliberate avoidance of sunlight, the use of umbrellas and sunscreen cosmetics. In obese patients, the decrease in serum vitamin D levels may also be related to the excessive retention of 25(0H) D in adipose tissue. In nonobese NAFLD patients, low vitamin D levels have been associated with inadequate exposure to the sun’s ultraviolet rays or inadequate food intake. The Institute of Medicine has recently given criteria for categorizing vitamin D deficiency: 25(OH)D <10 ng/ml is considered vitamin D deficient, 10 ng/ml ≤25(OH)D <3012 ng/m1 is considered insufficient, and 25(OH)D ≥30 ng/ml is considered normal.Vitamin D deficiency is prevalent in patients with chronic liver disease, and at least one-third of them have severe Vitamin D deficiency is prevalent in patients with chronic liver disease, and at least one-third of patients have severe vitamin D deficiency (<12 ng/ml). Recommendations for supplementation of vitamin D deficiency in patients with NAFLD Studies have concluded that vitamin D deficiency in the body is closely related to the onset and progression of NAFLD.Vitamin D deficiency is prevalent in patients with NAFLD, and therefore supplementation with vitamin D or phototherapy should obviously have a positive impact on the recovery and improvement of liver disease. However, there are few prospective studies evaluating vitamin D supplementation in NAFLD patients with limited clinical data, so further clinical studies and large-sample clinical observations are needed to better provide a theoretical basis for clinical treatment. As oral vitamin D has the advantages of simplicity, economy, and no obvious adverse effects, but for those who lack of outdoor sun exposure; behavioral factors that deliberately avoid sun exposure, the use of umbrellas as well as sunscreen cosmetics and other behaviors are not conducive to health.