When we think of healthy hair, we don’t typically think of healthy hair follicles, but that’s where it starts.

On average, our scalps have about 100,000 hair follicles. Each follicle is made up of highly active, living cells that form a mini pouch that helps hold hair strands in place and, with blood vessels attached, create a nourishing environment for hair to grow.

Nutrients flow through the blood vessels and supply the hair follicle with the nutrition it needs to fuel growth. There are four distinct phases of the hair growth cycle:

1. Growth (anagen). As the name implies, hair strands grow during this phase—the most active phase of the cycle. New hair is formed during this time, and, as the hair follicle is nourished, hair growth can occur. During normal hair growth, about 90 percent of hair follicles are in this phase, which can last two to seven years.

2. Regression (catagen). After allowing lots of growth, the hair follicle goes through a transitional period where the base of the hair follicle begins to break down and detach from its nourishing blood vessels. This begins to weaken the connection between the hair strand and the hair follicle. One percent of hair follicles are in this stage at any given time, and it lasts about one to two weeks.

3. Rest (telogen). As time goes by, the base of the hair follicle that began breaking down in the regression phase becomes completely inactive, further weakening the connection between the hair strand and the hair follicle. At any point in the cycle, 9 percent of hair follicles are in this stage, which can last five to six weeks.

4. Shedding (exogen). Eventually, the hair strand completely detaches from the hair follicle and falls out.

Thousands of hair follicles simultaneously undergoing their own hair growth cycle demands lots of cellular activity and energy, which requires nutrients, especially B vitamins. Here are the key nutrients that support healthy hair:

1. B vitamins—thiamin, riboflavin, niacin, pantothenic acid, vitamin B6, biotin, folate, and vitamin B12—play key roles in cell health and activity. They support energy pathways by breaking down macronutrients to produce the energy needed for our cells to function well, like properly growing and dividing during the hair growth cycle.

2. Iron contributes to energy metabolism in hair follicle cells, too, and may be important for normal hair growth.

3. Like every other cell in the body, hair follicle cells need antioxidants to protect them from oxidative stress. Vitamins A, C, and E work together to protect these cells from the damaging effects of free radicals so they can function normally.

4. Zinc and selenium support the maintenance of normal hair by protecting the body against oxidative stress via the antioxidant enzyme superoxide dismutase (SOD).

And emerging research suggests that vitamin D may play a significant role in supporting healthy hair by regulating hair follicles throughout the hair growth cycle, especially during the growth phase.