What is the most likely consequence for a human of not spending at least 20 minutes a day with a portion of one's skin exposed to the sun?
digestion could slow down due to lack of warmth to speed up the reactions of enzymes.
bones could become brittle due to lack of vitamin d, which is needed for calcium absorption.
muscles could become weak due to lack of energy, which is needed to make the cells contract.
skin could lose its protective oily coating due to lack of adequate excretion from sweat glands.

Bones could become brittle due to lack of vitamin D, which is needed for calcium absorption.

Explanation:

The human who is not exposing to the sun for at least 20 minutes on daily basis may suffer from vitamin D deficiency as a result of which his bones will become brittle and fragile. Vitamin D is responsible for making our bones strong because it facilitates absorption of calcium which keeps our bones  healthy. Exposure to sunlight for at least 20 minutes in a day converts a steroid named 7 dehydrocholesterol in our body into vitamin D naturally. So, if a person will not expose to sunlight for at least 20 minutes in a day then he will suffer from vitamin D deficiency for sure.

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