woman running and smiling on the beach

A swimmer runs with relish toward the Atlantic Ocean in Devon, England, in July 1926. (Photo: Fox Photos/Getty Images)

The human body is meant to move. We're all athletes at heart, fine-tuned by natural selection to run across savannas, leap over logs, hurl rocks and even throw punches, among other things.

We're also Earth's nerdiest animals, though, and we've spent the past few millennia inventing clever ways to reduce our physical workload. Our intelligence has helped us make life safer and stabler, but it has slowly removed us from the wilderness, too, where our bodies evolved to be awesome.

Inactivity is bad for our health, as science finally proved decades ago, and many people now exercise regularly in hopes of losing weight or living longer. Yet even though our brains are a big reason why we became so sedentary, inertia may harm them as much as any muscle. A growing body of research has begun to expose long-overlooked links between physical activity and mental health, suggesting exercise can significantly improve mood over time — maybe even as effectively as drugs.

"When you look at the treatment of anxiety problems and also depression, there's good evidence for the effectiveness of exercise," says Jasper Smits, a professor of clinical psychology at the University of Texas-Austin who studies exercise and mental health. "The number of trials for exercise is far fewer than for other types of interventions, so we're more cautious. But the trials and studies that have been done show promise. Particularly when you look at depression, the effects are really in the range you would expect to see with any other intervention, including pharmaceutical medications."

Considering how often drugs are now used to treat anxiety and depression, exercise could be a game changer for mental health care. It's cheap, natural and holistic, and it poses little risk of unwanted side effects when done wisely. More research will be needed to know the full range of its benefits, experts point out, and pharmaceuticals will remain vital for many who can't exercise or whose symptoms defy it. Still, what we do know is enough to make most people jump — and run, walk or swim — for joy.

surfers running toward water smiling

Surfers cavort along a beach in Sydney, Australia, on May Day 1931. (Photo: Fox Photos/Getty Images)

Running down a dream

Jogging can seem arbitrary today, but it's a big part of what makes us human. After our distant ancestors branched off from other apes to walk (and run) upright, some anthropologists think early humans caught large prey with persistence hunting, a strategy that involves patiently chasing animals over long distances until they collapse from exhaustion. Although humans have a long history as omnivores, eating meat could have provided the extra energy we needed to develop big brains.

"Running was mankind's first fine art, our original act of inspired creation," Chris McDougall, author of the 2009 best-selling book "Born to Run," told MNN in a 2010 interview. "Way before we were scratching pictures on caves or beating rhythms on hollow trees, we were perfecting the art of combining our breath and mind and muscles into fluid self-propulsion over wild terrain. Today, when we set off for a jog, we're tapping into that same primal tradition."

Humans may be born to run, but there are plenty of ways to get aerobic exercise. A 2013 study found that walking offers similar health benefits as running, and in some cases it might be better. Other popular options include swimming, cycling and fast-paced sports like basketball or soccer. Less intense activity can also encourage happiness — yoga and meditation have been linked to improved moods — but a 2010 study reported high-energy aerobic exercise reduced depression symptoms more than lower-energy workouts did. For now, Smits says, there's just more research on aerobic exercise.

"We only have evidence that's reasonably strong for aerobic exercise," he says. "So when it comes to recommending exercise, we feel more comfortable with aerobic. But that's not to say resistance training doesn't have an effect. Some studies have been done on that, and also we did a yoga intervention here, and have seen some really nice effects on stress measures. And we're not the only ones."

Spending time in nature is also good for the brain, so it would make sense that exercising outside is best. It might be, Smits says, but not for everyone. "A lot of people don't like to exercise outside and prefer the treadmill," he says. "Whereas a lot of others, myself included, prefer the outdoors. There are sights and sounds outdoors that might distract from your symptoms, things more in the realm of mindfulness that might be easier to experience in nature. It depends what works for each person."

For physical health alone, the U.S. government suggests people ages 18 to 64 get 150 minutes of moderate aerobic exercise per week, or 75 minutes if it's vigorous. For faster weight loss, the Mayo Clinic suggests up to 300 moderate minutes per week. But how much is needed to feel happier?

kid exercising

Resistance training is often used to build muscle, but it may also benefit weight loss and mental health. (Photo: Fox Photos/Getty Images)

Weighting game

While aerobic exercise has become broadly popular in recent decades, people are often compelled mainly by below-the-neck benefits. But as science continues to erase the imaginary line between mental and physical health, we may be in the early stages of a paradigm shift about physical activity.

"A lot of people are most motivated to exercise by losing weight," says Mary De Groot, a psychology professor at Indiana University who studies exercise and mood in people with diabetes. "That's a worthy goal, but as far as treating depression, the exercise that's required is usually lower intensity than what would be required for weight loss. So that's good news. You don't need to lose weight to feel better."

Much like medication, the effects of exercise vary from person to person. Yet while some people chase a "runner's high" and others are miserable climbing a flight of stairs, most of us do see mood benefits after exercise. But, also like medication, you can't expect lasting rewards if you don't commit.

"Regardless of how people feel during exercise, which is quite mixed, we know most people will leave an exercise session feeling much better than they did before they started," Smits says. "So there's an acute positive effect of exercise. And we try to use that to get people to adopt exercise as a habit."

Activity might offer quick relief from a bad mood, but the risk of injury looms large for inexperienced or rusty athletes. And since regular exercise is required for durable mental and physical benefits anyway, people are typically advised to start slowly and use short-term gains as long-term motivation.

"When you're stressed, that's exactly the time you should exercise," Smits says. "Then monitor your symptoms as you go, and use that as a way to motivate yourself and continue to exercise over time. The evidence shows that after three to four months of exercise, you see those effects I talked about earlier."

"The first rule of thumb is exercise needs to be safe and enjoyable," De Groot adds. "Many folks have not done regular physical activity for years. So we emphasize getting recommendations from a primary care physician on the type of exercise and how intensely to exercise."

happy women jumping over tennis net

Tennis can be aerobic or anaerobic exercise, depending how intensely it's played. (Photo: Agence France-Presse/Getty Images)

Pursuit of happiness

Even if exercise comes naturally to us, we still aren't sure how exactly it fights depression or anxiety. We know it wields powerful influence on moods, though, and research has shed some light on that.

For one thing, physical activity modulates brain chemicals known as neurotransmitters, which let neurons send signals. This includes dopamine, a neurotransmitter that regulates movement, emotional responses and feelings of pleasure, as well as serotonin, which affects mood, appetite, sleep and memory. It also includes norepinephrine, part of our fight-or-flight response that may help us deal with stress, and gamma-aminobutyric acid (GABA), which has been shown to have anti-anxiety effects.

Exercise can also increase levels of brain-derived neurotrophic factor (BDNF), a protein that promotes the growth and survival of neurons, thus helping fend off neurological diseases like Alzheimer's and Parkinson's. In fact, research on mice has found voluntary exercise — an important distinction in lab animals — can enhance neuron growth in brain regions involved with emotion, memory and learning.

Using an exercise wheel, for instance, can protect mouse brains from the effects of social stress. "If an animal were to engage in voluntary exercise and environmental enrichment, it buffered their stress response such that it seemed their mood didn't change when they were exposed to chronic social stressors," says Michael Lehmann, a staff scientist at the U.S. National Institutes of Health who conducted the research. "When we looked at the brain, we were able to map out a circuitry that seemed to indicate the animals experience much less fear during this stressful experience."

The exercised mice also showed more activity in an emotion-processing brain region called the infralimbic cortex, and removing it (sorry, mice) eliminated the stress-buffering effect — but only if it was removed before they exercised, not after. That suggests this region sends signals to other parts of the brain during exercise, Lehmann says, and highlights how complex of a process is at work.

The corresponding region in human brains has been linked to depression, hinting at the potential for similar stress-buffering effects of exercise. Beyond that, exercise may also help our brains learn to withstand stress just by exposing our bodies to small, controlled doses of it.

"For people with certain anxiety problems, particularly panic problems, it seems exercise may help them get more used to anxiety, the sensations that come with anxiety, this sort of physical distress," Smits says. "Exercise gives you an opportunity to get more used to that and start responding differently to it."

happy women jumping rope

London office workers take a break to jump rope on the roof in 1931. (Photo: Topical Press Agency/Getty Images)

Mind over matter

Mental health problems rarely occur in a vacuum, and they can become tangled up with physical ailments like an injury, a disability or a chronic condition. Exercise isn't an option for some people with limited mobility, in which case drugs or talk therapy might be life-saving. But for those who are able to exercise, physical activity has potential to strengthen the mind and the body at the same time.

"We used to think of mental health issues as being separate from physical health issues, but that's not the case," De Groot says. "Stress as an emotional experience has a major effect on the body, and the co-occurrence of diabetes and depression lies at the intersection of the mind-body relationship."

One in four people with diabetes will develop clinical depression at some point in their lives, De Groot says. Research has already shown that treating their depression with talk therapy and medication can help alleviate both conditions, and De Groot is now studying whether exercise amplifies this. The results are encouraging, she says, with 12 weeks of exercise spurring significant improvements in diabetes and depression that remained at three-month follow-up appointments. "So once people were depression-free," she says, "they stayed depression-free for three months afterward."

For people with anxiety problems, Smits has also found activity can help them overcome harmful habits like smoking, drinking or maladaptive eating that are often used as self-medication to manage stress. He has specifically studied smokers who suffer from anxiety, showing that exercise can improve their odds of quitting. "When they quit smoking they get really anxious and are quick to go back to smoking to address those symptoms," he says. "They have low distress tolerance." Vigorous exercise for 75 minutes a week seemed to give many the boost they needed, he says, resulting in an abstinence rate of 23 percent at a six-month follow-up compared with 10 percent for the control group.

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These are just a few of the ways humans might improve or even extend our lives by doing something that comes naturally to us. And while psychologists who study the mental effects of physical activity are cautious about making generalizations, many are bullish about exercise in general.

"I'm excited by these findings, but it's good to be conservative," Smits says. "I think there's a lot of promise here, mostly because of the evidence but also because of the theory behind it. There's a lot of reason to believe exercise really works."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.