Those first few strands of grays on your head are often not a welcome sight. They're a sign that we're getting older, whether they arrive prematurely in our 20s or spare us until we're in our late 30s. But in some instances, gray hair may indicate more than our biological age: It could signal one of these health issues.

1. Men may have increased risk of heart disease

According to a study published by the European Society of Cardiology, gray hair is linked with an increased risk of heart disease in men.

In the study, 545 adult men were divided into groups based on whether or not they had coronary artery disease and how much grey or white hair they had. The amount of grey hair on each man's head was graded on a scale of 1 to 5, with 1 being almost none and 5 being pure white. Scientists also collected data on the men regarding hypertension, diabetes, smoking habits and family medical history.

Researchers found that a hair score of 3 or more "was associated with increased risk of coronary artery disease independent of chronological age and established cardiovascular risk factors," according to the press release. Participants who already had coronary artery disease had "a statistically significant higher hair whitening score and higher coronary artery calcification" than those without it.

"Ageing is an unavoidable coronary risk factor and is associated with dermatological signs that could signal increased risk," said Dr. Irini Samuel, a cardiologist at Cairo University in Egypt. More research is needed, she said, adding: "If our findings are confirmed, standardisation of the scoring system for evaluation of hair greying could be used as a predictor for coronary artery disease."

2. You may have a vitamin deficiency

The '50-50-50' rule of thumb states that at 50 years old, 50% of the population has at least 50% grey hair. cunaplus/Shutterstock

Before we get into this one, let's back up. Each hair follicle contains pigment (called melanin — the same thing that colors our skin) that gives our hair its color. As we get older, we produce less pigment, resulting in gray hair. Dermatologists usually use the "50-50-50" rule of thumb which states that at 50 years old, 50% of the population has at least 50% grey hair.

However, some people go gray early. What's early? As WebMD reports: "Typically, white people start going gray in their mid-30s, Asians in their late 30s, and African-Americans in their mid-40s." So early is before those benchmarks, and its those early gray strands that may indicate a nutritional imbalance.

"Low vitamin B12 levels are notorious for causing loss of hair pigment," says Dr. Karthik Krishnamurthy, director of the Dermatology Center's Cosmetic Clinic at Montefiore Medical Center in the Bronx, New York, tells Good Housekeeping. And a study published in the International Journal of Trichology found low vitamin D3, serum calcium and serum ferritin levels in people who went gray prematurely.

Early gray hair also can be a sign of a problem with your pituitary or thyroid gland, according to WebMD.

3. Stress may play a role

Researchers say stress can trigger your hair to change colors. goodluz/Shutterstock

Maybe you've blamed your gray hair on your boss or your kids, but it does look like stress can trigger a change in your hair color, according to a Harvard study.

Researchers found that stress activates nerves that are part of the fight-or-flight response. That action can cause permanent damage to the pigment-regenerating stem cells in hair follicles.

For the study, published in the journal Nature, researchers experimented on mice, putting them under physical stress.

“When we started to study this, I expected that stress was bad for the body — but the detrimental impact of stress that we discovered was beyond what I imagined,” senior author Ya-Chieh Hsu, associate professor of stem cell and regenerative biology, said in a statement. “After just a few days, all of the pigment-regenerating stem cells were lost. Once they’re gone, you can’t regenerate pigments anymore. The damage is permanent.”

4. Your immune system may cause sudden graying

Your hair develops its color from melanocyte stem cells, which live inside the follicles. When your old hair falls out, the stem cells add melanocytes to new follicles — giving your hair its color. When the cells stop working, the follicles no longer develop pigment.

According to a study published in PLOS Biology on mice, when the immune system is attacked it can affect the MITF protein which helps melanocytes function. Researchers found that certain mice with genetic mutations for the MITF protein will have their immune systems overreact to fight viruses which causes melanocytes to no longer form — leading to sudden graying. While the study hasn't been confirmed with humans, researchers believe it sheds light on how a weakened immune system can cause sudden graying.

"We are hesitant to make too many inferences to humans without further experimentation,” Lead Author Melissa L. Harris told Reuters. "However, we would love to test whether the mechanism in this study could explain those anecdotal stories where people experience premature gray hair. Could the combination of a genetic predisposition and an everyday viral infection be just enough to negatively affect the melanocytes and melanocyte stem cells in humans, and cause early hair graying?”"

5. Your follicles may suffer from 'oxidative stress'

Hair follicles produce small amounts of hydrogen peroxide, a chemical that has been used for decades as an inexpensive way to lighten or bleach hair. However, if a hydrogen peroxide buildup occurs, your hair color may start to fade. As this study published in the Federation of American Societies for Experimental Biology journal reports:

In a new research report ... people who are going gray develop massive oxidative stress via accumulation of hydrogen peroxide in the hair follicle, which causes our hair to bleach itself from the inside out. Most importantly, the report shows that this massive accumulation of hydrogen peroxide can be remedied with a proprietary treatment ... described as a topical, UVB-activated compound called PC-KUS (a modified pseudocatalase). What's more, the study also shows that the same treatment works for the skin condition, vitiligo.

6. Gray is in your genes

A study found the exact gene responsible for giving us that salt-and-pepper look. Mangostar/Shutterstock

This we know: If your parents or grandparents went gray and did so early, then chances are you will, too. But in a relatively new twist, this study found the exact gene responsible for gray hair. The study looked at the genomes of more than 6,000 people from Latin America and identified 18 genes that influence hair traits, including IRF4, which previously was known for producing light hair in people of European origin but now is associated with gray hair.

“This is the first time a gene for graying has been identified in humans,” said lead author Dr. Kaustubh Adhikari, a researcher at University College London, in a press release. “As hair grays something happens that causes this gene to produce even lower levels of melanin. Now we can ask more specific functional questions,” Adhikari told Newsweek.

Then again, if you're sporting the salt-and-pepper look these days, it may mean simply that you've accumulated some life experience and earned your stripes — whether you choose to dye them or not.

7. You are (or were) a smoker

A study published in the Indian Dermatology Online Journal found that smokers are two and half times more likely to develop premature gray hair than people who do not smoke. And smoking can go one step further in damaging your tresses: Certain chemicals in smoke break down in hair cells, which leads to baldness, according to the New York Times.