Zinc is an essential dietary mineral that has numerous roles in the body, most notably as a catalytic and structural element in hundreds of metalloproteins. Meat, egg, and legume products are common sources. Oysters are particularly good sources of zinc.
Any benefits of oral zinc supplements or increased dietary intake will depend on the zinc status of the individual. Some research suggests that when zinc levels are low, insulin sensitivity and testosterone can decrease, and supplementation can bring levels closer to normal.
Zinc lozenges, in high doses, seem to reduce the duration of common colds, though it's unclear if they reduce the risk of getting colds in the first place or the severity of symptoms. Their effects are largely limited to throat and nasal symptoms, and high doses tend to come with some mild but notable side-effects like nausea and metallic taste in the mouth. Zinc lozenges have not been studied for COVID-19 and extrapolation from colds to COVID-19 is invalid. For more information, see this page.
Zinc is lost through sweat, making supplementation very important for athletes that don’t get a lot of zinc through food. Insulin resistance can also decrease zinc levels.
Large doses of zinc can produce nausea and other forms of gastrointestinal upset, particularly when taken on an empty stomach. Large doses of zinc can also cause a copper deficiency and lead to overdose. Many supplements use far more than the RDA, and a combination of supplements, fortified foods, and high zinc foods may lead to exceeding the tolerable upper intake level for adults, which is 40 mg per day. High dose zinc lozenges can lead to nausea and a metallic taste in the mouth, and should only be taken for a short time, if at all.
Zinc deficiency in childhood can impair growth. In adults, zinc deficiency can result in hypogonadism in men, as well as mental lethargy, depression, and skin abnormalities. Low levels of zinc due to diabetes can further worsen insulin resistance. It is estimated that inadequate zinc intake affects around 10% of persons in the US, but global insufficiency rates are over 50%.