Ever wonder why certain people are blessed with thick, healthy locks and others just can’t get their flimsy strands to grow? A whole slew of factors contribute to the state of your mane.

To figure out why someone is losing hair, docs look into four main factors, says Lauren Eckert Ploch, M.D., dermatologist at Georgia Dermatology and Skin Cancer Center. And those same factors also play a role in hair growth, she says.

Read on to learn what determines whether a person’s hair grows at sloth or Rapunzel-level pace.

Genetics

By and large, how fast your hair grows has a lot to do with genes. “Hair cycles through three stages: the active, resting, and degenerative phases,” says Joshua Zeichner, M.D., director of cosmetic and clinical research in dermatology at Mount Sinai Hospital in New York City.

The number of hair follicles we have doesn’t change, but those follicles are constantly producing new hairs throughout our lifetimes, explains a paper published in the Journal of Cell Science. In the growth (or ‘anagen’) phase, your follicles produce a completely new hair, from root to tip, which begins to grow beneath the skin surface and is then pushed out. Then, the follicle shifts to the regression (or ‘catagen’) phase, in which the bottom of the hair shaft seals off and the hair follicle recedes. In the rest (or ‘telogen’) phase, the hair follicle is completely dormant, until it shifts back to the growth phase, shedding the old hair and beginning the process all over again.

Your anagen phase determines your mane’s ability to grow, Zeichner says. The longer your anagen phase lasts, the more time your hair spends growing and longer it becomes. The length of your anagen phase is determined by—you guessed it—your genes.

A study published in PLOS Genetics identified a gene called LHX2, which regulates stem cell populations and contributes to our generation or organs and tissues, as a major player in hair formation.

Hence why dermatologists usually ask patients complaining of thin hair if stringy strands run in the family. You’re more likely to deal with thinning hair if someone else in the family did, says Zeichner. That same connection often exists for quick-growing locks, too, he says.

Related: Show your mane some love with these reliable hair-care products.

Your Diet

“Poor nutrition doesn’t provide the building blocks for healthy hair growth and leaves the scalp lacking in nutrients it needs for optimal functioning,” explains Zeichner.

So what do you need to fill up on? A healthy and diverse diet rich in protein (meats and legumes), omega-3 fatty acids (olive oil and healthy fats like avocados), vitamin D (oily fish or supplements), vitamin C (citrus fruits and bell peppers), iron (green, leafy vegetables), and zinc (oysters, beef, or sesame seeds) is your best bet for promoting healthy hair, says Ploch. “Copper, vitamin A, and vitamin E—plentiful in fresh fruits and vegetables—are also important for scalp and hair health,” she says.

While biotin, or vitamin B7, is often touted as essential for making the hair grow faster, it’s theoretically more likely to make hair stronger versus making it grow faster, Ploch notes. “Biotin deficiency leads to hair loss and brittle hair, so biotin does play a role in growing strong, healthy hair,” she says. “However, whether it truly impacts how fast our hair grows is not fully scientifically proven.”

Protein intake has a lot to do with hair growth, too, says Kavita Mariwalla, M.D., a dermatologist based in West Islip, NY. Hair is mostly made up of protein, which means fueling up on the nutrient is important for hair structure and overall health. You can use this calculator from the USDA to determine your daily minimum protein requirements.

Related: Help boost your protein intake with a supplement.

An Underlying Medical Issue

“Several systems in the body contribute to the process [of hair growth],” says Ploch. For instance, your thyroid hormones can play a big role. “Low thyroid function is often associated with thinning hair,” Zeichner says. “Thyroid hormones regulate a healthy metabolism, so thinning hair may occur when its cells are not adequately stimulated for proper functioning.”

Autoimmune diseases like lupus can also contribute to deteriorating hair health. In such conditions, your immune system—which typically attacks microorganisms that threaten your health—attacks your own healthy cells. The resulting inflammation can be associated with everything from thinning hair, joint aches, rashes, belly pains, and sensitivity to the sun, Zeichner says.

Even stress and a lack of sleep can play a role in how well your hair grows. Case in point: “After a particularly traumatic event like an illness, hospitalization, or childbirth, hair can fall out in what’s called telogen effluvium,” says Mariwalla. As previously mentioned, ‘telogen’ refers the resting stage of the hair growth cycle, and this type of hair loss occurs when stress prematurely shifts hairs into the resting stage, causing them to fall out, Mariwalla says. These hairs do eventually grow back after stress levels dial down.

The State Of Your Scalp

Suffering from a skin condition on your scalp? Issues like severe dandruff or psoriasis may be to blame for your lack of luscious locks, says Zeichner. Dandruff is best treated with antidandruff shampoos, which reduce yeast levels on the skin, he says. If you have psoriasis, touch base with your dermatologist about your best course of action.