From the day children first step out onto the playground, friendships are a key part of life. According to Gallup polling data from 2004, 98 percent of Americans report having at least one close friend (the average number of friends is nine).

But friendship may be in trouble. Americans reported an average of 10 friends apiece in 1990, according to Gallup data, and a slew of sociology studies find that Americans have become more socially isolated over the decades. For example, a 2006 study on the number of friends people felt they could discuss important matters with found that the number fell from an average of 2.94 in 1985, to 2.08 in 2004.

Any amount of increase in our social isolation would be bad news, because friendship isn't just about fun, fellowship and emotional health. Having friends can improve physical health, too.

"One's social life matters above and beyond what we already know about the 'quick fixes'" of diet and exercise on health, said Yang Claire Yang, a sociologist at the University of North Carolina, Chapel Hill, who studies the physiological effects of social ties.

Researchers who study friendship have uncovered many of its health benefits. Here's how friendship can be good for you.

1. Friends may extend your life

People who have strong social relationships are less likely to die prematurely than people who are isolated. In fact, according to a 2010 review of research, the effect of social ties on life span is twice as strong as that of exercising, and equivalent to that of quitting smoking.

In the review, researchers examined 148 previous studies on social links and mortality, which together included more than 300,000 participants. These studies found that measures of the strength of people's social relationships, from their number of friends to their integration into the community, were all linked to decreased mortality.

Researchers think that friendships and health are linked through the body's processing of stress, Yang said. In the short term, stress is a good thing. If you're being chased by a lion, you want your body to respond with heightened alertness, a pounding heart and a flood of get-up-and-go hormones like norepinephrine. Likewise, if you've got a virus, you want your immune system to kick into gear and attack the intruder with specialized cells and inflammation.

But the chronic stress that can come with isolation can switch on these processes for long periods of time, causing physical wear and tear on the body, Yang said.

2. Your pals make you generally healthier

Yang and her colleagues studied this health effect by comparing the biological stats of people who reported being isolation with those who reported having lots of friends across their life span. Using four large studies of hundreds to thousands of people each, ages 12 to 91, the researchers compared biomarkers such as blood pressure, body mass index, waist circumference and levels of the inflammation marker C-reactive protein.

They found that these measures of health were worse in people who also had weaker social ties, reporting their work in January 2015 in the journal Proceedings of the National Academy of Sciences. For example, among the people in the study who were in old age, a lack of social connections more than doubled the risk of high blood pressure (raising it by 124 percent). For comparison, having diabetes raised the risk of high blood pressure by much less (70 percent).

Traditionally, it's been tough to determine if friendships and other social links are the cause of poor health, or if poor health causes isolation. The advantage of Yang and her colleagues' research is that they had data that spanned years, Yang said.

"We were able to see the change in biomarkers over time as a result of their earlier exposure to social connections, how many friends [they] have, how often [they're] talking to other people," Yang said. That method bolsters the argument that social ties do cause health effects.

3. Friendships might help keep your mind sharp

Having friends who make you feel like you belong may be a key for better physical health. A 2012 study found that older people's dementia risk increased with their feelings of loneliness.

The study followed more than 2,000 residents of the Netherlands ages 65 or older over three years. None of the participants had dementia at the beginning of the research, but 13.4 percent of those who said they felt lonely at the start of the study developed dementia during the study period, compared with 5.7 percent who didn't feel lonely. [10 Ways to Keep Your Mind Sharp ]

"The fact that 'feeling lonely' rather than 'being alone' was associated with dementia onset suggests that it is not the objective situation, but rather the perceived absence of social attachments that increases the risk of cognitive decline," the researchers wrote in their findings, published in the Journal of Neurology, Neurosurgery and Psychiatry. (However, the study found an association, and couldn't determine whether the loneliness was a cause of the dementia.)

4. Friends influence us (for better or worse)

Obesity is contagious, screamed headlines, after a 2007 study that found that when one person packed on extra pounds, his or her friends were more likely to become obese, too. But there was an overlooked bright side to the research, which appeared in the New England Journal of Medicine. Thinness spread like social wildfire, too.

The researchers pulled data from a large health study, the Framingham Heart Study. It followed people over time, allowing researchers to draw causal inferences. If one person became obese over the course of the study, they found that friends of that person were 57 percent more likely to become obese, too. [8 Reasons Our Waistlines Are Expanding]

But the converse was also true, study researcher James Fowler, a professor of global public health at the University of California, San Diego, noted in a statement. People also take cues from their friends who exercise or eat well to lose weight, as a separate 2011 study confirmed.

"When we help one person lose weight, we’re not just helping one person, we’re helping many," Fowler said. “And that needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier."

5. Your BFFs can help you through tough stuff

We all need somebody to lean on, as the song goes. And research on cancer patients finds that when the going gets tough, friends can help.

A major study published in the journal The Lancet in 1989 found that women with breast cancer who were randomly assigned to attend support groups with other cancer patients reported better quality of life and lived longer, compared with women in a control group who were not assigned to such support groups.

Since then, other studies have debated whether social support groups improve survival time, with some research suggesting they do and other studies finding no effect. However, there is widespread agreement that support groups improve quality of life in cancer patients. And while most of this research has been done on women, a 2014 study found that men with prostate cancer can benefit from support groups, too.

6. Friends can help you cope with rejection

Not all social relationships can go smoothly, unfortunately. But when they don't, friends can help you pick up the pieces. [The Science of Breakups: 7 Facts About Splitsville]

A 2011 study on fourth-graders found that having friends helped kids cope with the stress of being picked on or rejected by other classmates. The researchers measured cortisol, a stress hormone, in their study participants' saliva and found that being excluded by their peers raised the kids' cortisol levels, probably indicating chronic stress. (Getting picked on didn't raise cortisol levels, the researchers reported in the journal Child Development, suggesting that getting left out may hurt more than getting attention in a negative way.)

But the cortisol increase that came with being excluded was less pronounced in kids who had more friendships or closer friendships, compared with those who had few or low-quality friendships.

"Together, the results demonstrate that although friends cannot completely eliminate the stress of exclusion at school, they do reduce it," study researcher Marianne Riksen-Walraven, professor of developmental psychology at Radboud University Nijmegen in the Netherlands, said in a statement.

7. Friendships can last a lifetime

In an era when people move around a lot for school and jobs, maintaining friendships can be difficult — and the occasional Facebook update doesn't always satisfy. However, research finds that distance doesn't have to dampen a friendship.

In one study, researchers followed college friends beginning in 1983, asking them about their friendship and sense of closeness. They found that physical distance didn't necessarily track with the emotional closeness of a friendship over decades. Phones and email still kept friends in touch two decades later, the researchers found, especially those who had been friends longer in college and those who had similar interests when they became friends.

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