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What are the hazards of vitamin A deficiency?

Update:24-09-2021

Vitamin A deficiency has clinical and functional characteristics. For primary vitamin A deficiency, everyone's tolerance is different, depending on a series of geographic and epidemiological factors. The clinical manifestations of vitamin A deficiency are mainly symptoms and signs of abnormal eye and vision and other epithelial function. 
Eye and visual performance
Dry eye is one of the typical clinical features of vitamin A deficiency. According to specific ocular manifestations, dry eye can be divided into several stages. XN stage is the earliest stage, and night blindness caused by dark adaptation function damage mainly occurs. Then there is the X1A stage, when the goblet cells secrete mucus reduced, resulting in dryness of the conjunctiva; the next is the X1B stage, when a blister-like Pitt's spot appears on the frontal surface of the conjunctiva. Stage X2 is the advanced stage of the disease, manifested as simple corneal dryness. When the cornea has softening or ulceration, or a liquefaction process of both, it is X3 stage. At this time, if the liquefied surface is less than 1/3 of the corneal area, it is X3A stage, and more than 1/3 is X3B stage. Eyeball damage caused by corneal softening is called dry eye fundus disease, also known as stage XF. 
Other manifestations of epithelial dysfunction
Thickening of hair follicles (keratinization of hair follicles) is a skin sign of vitamin A deficiency. The production of mucin in the mucosa is reduced, and the morphology, structure and function of the mucosa are abnormal, which can lead to pain and decreased mucosal barrier function, which can involve the throat, tonsils, bronchi, lungs and mucosa of the digestive tract. Vitamin A deficiency and marginal deficiency lead to increased risk of infectious diseases and mortality in children. 
Abnormal embryo growth and development
Vitamin A deficiency can impair embryo growth. Experimental animals that are severely deficient in vitamin A mostly undergo embryonic absorption, and the surviving embryos will also have abnormalities in the eyes, lungs, urinary tract, and cardiovascular system. When the human body lacks vitamin A, morphological abnormalities are rarely seen, but abnormal lung functions can be seen. 


Impaired immune function
Vitamin A deficiency can lead to a decrease in the number of blood lymphocytes, natural killer cells, and a weakened specific antibody response. When vitamin A intake is insufficient, the number of white blood cells can be observed to decrease, the weight of lymphatic organs is reduced, the function of T cells is impaired, and the resistance to immunogenic tumors is reduced. In experimental animals and human experiments, vitamin A deficiency often leads to abnormal humoral and cellular immune functions. 
Increased prevalence and mortality of infectious diseases
Vitamin A deficiency can lead to increased morbidity and mortality of infectious diseases in laboratory animals and humans, especially in developing countries. Children with mild to moderate vitamin A deficiency are at increased risk of respiratory infections and diarrhea; the mortality rate of children with mild dry eye is four times that of children without dry eye. Supplementing large doses of vitamin A to hospitalized children suffering from measles can significantly reduce the mortality rate of children and reduce the severity of complications. Research shows that vitamin A supplementation can reduce the severity of diarrhea and malaria in young children