While many people confuse hair density (the number of hair strands located in a given area of the scalp) with thick hair, thickness is actually just the width of a single strand of hair. Thick hair is generally clearly visible as a single strand, coarser to the touch, and has an average thickness of at least .08 mm. Thin hair, on the other hand, can be hard to spot as individual strands, is soft and smooth, and is usually no thicker than .05mm. 

Though one hair type is not necessarily considered "healthier" than the other, many people prefer thicker hair as it can better protect one's head and skin from sun damage, usually appears fuller with more volume, and can hide the top of one's scalp, including bald patches. 

What causes thick hair?

While multiple factors can affect the thickness of one's hair, it's primarily "determined by genetic factors," says Michelle Henry, MD, a board certified and Harvard trained dermatologist. More specifically, she says that genetics play a part in the number and size of hair follicles on the scalp, "which consequently influences hair thickness."

That means that with the exception of specific illnesses or conditions, the thickness of your parents' hair "is likely similar to the thickness of hair you have," says Angela Lamb, MD, a dermatologist at Mount Sinai in New York City. 

A few other external or individual factors have also been shown to marginally impact hair thickness. These include "hormonal fluctuations, age and overall health," explains Henry.

Which foods contribute to thicker hair?

Some people think that diet can make thin hair thick, but the experts say such misconceptions are likely rooted in misunderstandings about nutritional deficiencies and healthy hair. "While it is important to eat a well-balanced diet, there is no evidence that dietary changes can help grow thicker hair," explains Shari Lipner, MD, an associate professor of clinical dermatology at the Weill Cornell Medical Center. 

Instead, deficiencies in vitamins and minerals such as "biotin, zinc, iron and vitamin D can lead to hair loss and thinning," she says. For this reason, it's important to include certain foods in one's diet to help maintain healthy hair. The best foods that are known to help include "whole grains, bright vegetables, nuts and legumes," says Lamb.

How to get thicker hair

Beyond dietary choices, there are other tips and techniques known to thicken, strengthen or protect one's hair. "Minimize the use of excessive heat styling and harsh chemical treatments that could damage hair follicles," advises Henry. She recommends selecting gentle hair care products that are suitable for your hair type and to avoid over washing "as it can strip away natural oils."

"One of the best ways to strengthen hair that has become thin from either poor health or over styling is to use a protein-based conditioner," adds Lamb. Leave-in thickening and volumizing products including dry shampoo, mousses, serums, coconut oil, gels and sprays may also be helpful. 

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In some cases, medication may also be suggested which can be taken orally or applied topically. "Board-certified dermatologists can make recommendations and prescribe medication to grow hair and increase hair density," offers Lipner. "These medications include topical and oral minoxidil, oral spironolactone, and oral finasteride."