A good daily multivitamin containing zinc, vitamin B, folate, iron, and calcium is a reasonable choice, although there is no good evidence that vitamins have any meaningful benefit in alopecia. Newer studies suggest that vitamin D may be somewhat helpful and worth considering. Specific vitamin and mineral deficiencies like iron or vitamin B12 may be diagnosed by blood tests and treated.
Generally, hair problems, especially hair thinning and hair loss, occur around the anagen phase or the resting phase. As we age, the length of the anagen phase also decreases as the hair follicles receive less and less nourishment from the body. The result is hair that is weaker and thinner after every cycle. In some cases, the hair enters the resting phase too early (or the catagen phase is too short) and this is when excessive shedding also happens.
Collagen is the most abundant protein in your body found in your skin, hair, bones, and tendons. Our body produces less and less of it as we age. I’ve long read about the benefits, including improved skin and nails and even pain reduction, of replenishing our depleting collagen stores with a form easily assimilated by the human body including hydrolyzed collagen and gelatin. I became particularly interested in collagen for hair loss when I read about a study published in Science in 2016. It all started with investigating the hair follicle stem cells of mice where researchers discovered that age-related DNA damage triggers the destruction of a protein called Collagen 17A1. The hair follicles of older people then convert themselves into skin cells, and over time baldness ensues. Think of the image of each hair follicle on your head disappearing leaving behind bare skin one at a time and on and on. My favorite brand is Vital Proteins Collagen Peptides grass-fed and pasture-raised.

Telogen effluvium - occurs mainly due to the body’s reaction to stress, brought about by, illnesses such as cancer, mental and emotional disturbances, medications such as blood thinners, hormonal imbalances, stress during childbirth and so on. In this condition hair thinning occurs on the scalp. The hair usually regrows after the stress period is over. 
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