When should you begin checking your blood pressure? Is your sunscreen really giving you the protection you think it is? "20/20" teams up anti-aging guru Dr. Michael F. Roizen and heart surgeon Dr. Mehmet Oz, authors of "You: The Owner's Manual" to debunk 10 body myths - from why you gain weight to aging and memory.
Here are the body myths we debunked with our health experts and the truth behind them.
Memory Loss Is Inevitable
A lot of people believe that we use only 10 percent of our brain. Is that true?
"It's not true. In fact, you'd be in big trouble if you only used 10 percent. The reality is that we use all of our brain, but we don't use it all the time," said Dr. Oz.
"The brain was really hidden from us until we had new technologies that could allow us either to operate on the brain, or to see what the brain does when you're actually thinking or doing tasks."
Thanks to these advances, we can now see how challenging your mind can keep your brain young. Suzanne, who speaks four languages fluently - English, Swiss-German, French, and German - was recently diagnosed with a brain tumor in the language center of her brain. The problem is trying to remove the tumor without destroying her ability to speak her languages. Her treatment will give doctors at UCLA a rare opportunity to literally see how learning a language exercises the brain.
During surgery to remove the tumor, Suzanne, at one point, was awakened. She was shown pictures and asked to identify them in each of her languages.
An MRI lights up the part of the brain that Suzanne uses as she speaks. This helps the surgeon avoid the language area when removing the tumor. But the doctors discover something quite amazing - each of Suzanne's languages resides in a different part of her brain.
The surgery is successful - the tumor is removed and Suzanne's knowledge of four languages is saved. Her case busted a big brain myth - that language resides in just one tiny area of the brain. Because Suzanne learned her languages at different ages, her brain stored them in different places - showing that challenging your brain creates new neural pathways.
"Just like you challenge a muscle to grow it, well, the brain gets new connections," said Dr. Roizen.
And that busts another big brain myth - that you can't teach an old dog new tricks. Memory loss is not inevitable. Want to keep your brain young? Exercise it.
Try learning to play a musical instrument, doing crossword puzzles, learning a language - even playing computer games.
"The data now indicate that an hour of games for 40 weeks can make your brain equivalent, your brain's real age, 10 years younger," Roizen said.
And one final brain myth: Does the size of a person's brain have anything to do with their intellectual capacity?
"There is no correlation with the size of a human brain and its intellect. In fact, Einstein was sort of a famous example. His brain was about average size - no difference between his and almost any other brain," Oz said.
Slow Metabolism Makes You Fatter
We've all heard for years that people who are thin have won the biological lottery. They look like they can eat whatever they want without gaining a pound. We think they're lucky to have a high metabolism.
"It's generally false that your metabolism is the reason for your weight," said Oz. "And this has been looked at numerous times."
To figure this out, we turned to Dr. Jim Levine, an obesity researcher at the Mayo Clinic, who's studied the metabolism of lean and heavy people, such as lean Kathy Strickland and a heavier Dawn Campion.
"Dawn's numbers are actually higher because we find continuously is that people with weight problems who have obesity have a higher basal metabolism compared to people who are lean," Levine said.
Levine says most patients that come to his clinic have a higher metabolism.
"Your basal metabolism is the calories you burn to keep your body going, so if your body is bigger of course your basal metabolism is greater," Levine said. "If your body is smaller your basal metabolism is less."
Levine says the reasons behind weight gain are simple. People who weigh more move less.
"People with obesity have a tendency to be seated, a natural tendency to be seated for two-and-a-half hours per day more," Levine said.
Levine believes people can combat obesity by simply moving more. Even standing up at your desk can increase your metabolism by 40 percent.
"We're talking about shaking up everything we do because we have no choice in it," Levine said. "Obesity is collapsing corporations in America, but much more importantly, it's devastating our health and it's taking over our children. We have no choice but to do something. We have no choice but to do something big."
Men Can Drink More Alcohol Than Women
This is not just a sexist myth. Men are biologically better equipped to drink more than women.
"It's true not just because they're bigger but because they have an extra enzyme in the lining of their stomach and that enzyme, when it sees alcohol, begins to metabolize it immediately," said Oz. "So only about half the alcohol that they actually drink gets into the bloodstream."
Women, on the other hand, don't have as much of that enzyme in their stomachs. So more of the alcohol gets into the bloodstream and they get drunk quicker.
That gender difference is even more problematic, because while overall drinking trends are down over the past 20 years, many young women are drinking more than ever.
But the more a person drinks, the more their body builds up a tolerance for alcohol, according to Oz. So they actually begin to metabolize it faster.
"So women who are exposed to a lot of alcohol, do begin to catch up to men in their ability to cope with the drinks that they have," Oz said.
But before women get their hopes up that excessive drinking will level the playing field, remember that men still have the biological advantage.
Marijuana Gives You the Munchies
In movie after movie, smoking pot soon leads to binge eating.
"Without any question, marijuana gives you the munchies," Oz said. "And, specifically, you start to crave sweet foods, comfort foods."
The drug affects people's brains in a way that actually makes food more appealing.
"It's not just a mindset," Oz said. "It actually chemically alters the way you perceive food."
Ironically, this drug that gives people the munchies may actually hold the key to controlling people's hunger.
"If we know what drug causes you to get hungry and we can understand its mechanism, maybe we can block that mechanism so you'll never get hungry," Oz said.
At 100 hospitals across the United States, researchers have been studying a drug called rimonabant that blocks the same hunger receptors that chemicals like marijuana stimulate.
"By blocking that chemical we may be able to block your ability to crave foods," Oz said.
Results from seven years of testing the drug are promising.
Pat Robison, who took rimonabant for a year, said it reduced her appetite, and she lost 15 pounds. On average, people lost 20 to 25 pounds and took two to three inches off their waists.
Research on the drug is continuing.
A High SPF Will Fully Protect You from Skin Cancer
You know the drill - summertime means sun and sun means sunscreen. We're a changed nation these days, fully aware that those warm, enveloping rays can not only crease the skin and age us, they can potentially kill.
But a sunscreen with a high SPF will protect us, right?
Not necessarily. Doctors say the idea that a high SPF is all you need for full sun protection - is a myth.
But Oz notes that a sunscreen with an SPF of 30 will give you a lot of protection, but it's protection against only one type of sun ray - UVB rays.
UVB rays are the short ultraviolet rays that cause most sunburns. A sunscreen with an SPF of 30 will do a fine job against them.
But there's another villain that you rarely hear about - the longer UVA rays that penetrate more deeply beneath the skin.
They're responsible for a lot of your wrinkles, and both can cause cancer. Much of what you buy today claims to offer protection against both UVB and UVA. But it can be difficult to be certain.
Many products contribute to the confusion with unclear labeling. The problem is, there is no FDA-approved measure of how well a product screens out UVA rays. So what can you do to protect yourself against them?
Dr. Rachel Herschenfeld, a dermatologist in Wellesley, Mass., says you've got to read the fine print.
"There are certain ingredients that actually provide effective protection against UVA. The three most common include zinc oxide, titanium dioxide and something called Avobenzone, or parsol 1789," Herschenfeld said.
She recommends using products with the first two ingredients, especially for children.
But be careful if you're relying on Avobenzone. It degrades quickly in the sun, so keep putting it on, or look for a stabilizer like Helioplex, available now in just two products approved for sale in the United States: Neutrogena with Helioplex and Aveeno Continuous Protection Sunscreen. Outside the United States, look for products with the ingredient Mexoryl.
Too much science? Well, this part's easy.
Stay in the shade during peak sun hours. When you do venture out, apply sunscreen early - at least 20 minutes before you go outside. Be generous with it: use about four times what you think you need, and reapply it all day long , especially, if you go into the water.
"There is no way to produce a sunscreen that is truly waterproof. The best it can be is water resistant," said Herschenfeld.
It may sound like a lot of work for a little sunshine in your life, but keep in mind, even with all the sunscreen, the incidence of skin cancer is on the rise. Melanomas, many sun-related, have doubled over the last 25 years.
But that doesn't mean you're condemned to stay inside.
"I don't tell my patients to be hermits. I tell them to put on their hats and their sunscreen and their sunglasses and get out there," Herschenfeld said.
Nothing Will Age You Faster Than the Sun
You might think it's just the sun that causes premature wrinkles and ages your body, but a growing body of research shows your own anxiety levels can take years off your life.
"Stress is the greatest ager of all and it's the only thing we know that causes an increase in arterial aging, heart disease, strokes, memory loss & immune aging, infections and cancer," said Roizen.
So how does stress attack your body? That's exactly what Ronald Glaser and his wife, Janice Kiecolt-Glaser, have been studying for 25 years. Their research actually began as a marital disagreement. Janice, a psychiatrist, believed stress harms the body; Ronald, a molecular virologist, did not.
"There were a lot of skeptics, and I was one of them," he admitted.
And what better subjects to test for stress than med students taking exams?
Early on in their research, the Glasers exposed their subject to microbes via a vaccination.
"We found immune changes associated with them taking the exams, and quite frankly, I was absolutely blown away with that," Ronald Glaser said.
More than two decades later, the couple is studying whether the stress of fighting with your spouse would slow the body's healing. The researchers created blisters on the volunteers' forearms and then stressed the couples by making them talk about their pet peeves. All the while, the researchers analyzed their blood and tissues samples.
"We found that even a minor stress can really slow how fast you heal," said Janice Kiecolt-Glaser. She also noted that when couples argued, they took longer to heal.
Surprisingly, not all types of stress actually damage the human body. Ironically, it's those nagging tasks that you put off that'll drive you nuts and age you.
"For one nagging unfinished task, it puts on about eight years on your life. So if you're 30, it makes you 38," said Roizen.
According to Dr. Oz, the stress of a specific event - like blowing out a car tire on the road - is "not as damaging as one that's a nagging unfinished task."
Stresses of day-to-day life - like a demanding boss or pressing deadlines - aren't particularly damaging either.
But major life stressors, like losing a spouse or a job can have a significant aging impact.
"They really are hugely aging. The great news is there are easy things each of us can do to ameliorate or modify it," Roizen said.
Oz and Roizen say there are some simple ways to de-stress and reduce your risk of major illness - just by simply bending, breathing and laughing.
Roizen also said friendships offer great anti-aging, health-boosting benefits.
"One important element of staying connected to the world is to maintain friendships," he said. "And we actually have numbers on this. People who see six friends a month actually do better long term. So, if you're gonna put a number in the back of your mind, try to reach out to six folks that you care about every month."
Superfit = Superhealthy
For the majority of us, the more we exercise, the stronger and healthier we are. But excess physical activity is as destructive to your body as Mike Tyson's right hand.
Over-exercising can damage your muscles, your bones and your joints, or it can mask other problems, because you think you're invulnerable, even when you're not.
Professional athletes are not the only ones at risk. From local marathons to your neighborhood gym, chances are you know that person who's obsessed with pushing harder and working out longer than anyone else.
"You always want to be the best at what you do," said former track star and spokesperson for Bally's Fitness Nikki Kimbrough. "So sometimes people tend to over-train, and that's not healthy for us."
It's not just your joints that can be affected. Experts say exercising for more than two hours a day can cause a host of other health problems - chronic fatigue, problems sleeping, headaches, depression, gastrointestinal problems, difficulty healing, a reduced sex drive, or disrupted menstrual cycle.
"Maximum health doesn't require maximum fitness," said Dr. Michael Roizen. "You don't have to kill yourself to get the huge wonderful benefits of exercise."
Roizen recommends 30 minutes of walking a day, a half an hour of aerobic exercise 3 times a week, and lifting weights for 30 minutes once a week.
And most important, listen to your body. If you're hurting, slow down.
Cholesterol Is the Major Cause of Heart Disease
"It's widely believed but absolutely false," said Oz. "Cholesterol levels are important, but they're not the best predictor of whether you'll have a heart attack. It turns out that honor belongs to high blood pressure."
Blood pressure can tear holes in your arteries. The way the body repairs the hole is to fill it with cholesterol.
"If it's healthy cholesterol you get a nice smooth lining," said Oz. "But if you've got cholesterol that's sort of rotten, now the body has to heal that. The body heals cuts by putting scabs on them and that scab quickly builds on top of this and blocks off the artery."
And that blocked artery can cause a heart attack.
At what age should we start being concerned about our blood pressure?
Dr. Oz says you should start to check your blood pressure at age 12.
"Certainly by age 20 you ought to know your blood pressure," Oz said. "And you ought to check your blood pressure at least every five years after that."
The optimal blood pressure is 115 over 75. But fewer than half of all Americans have that. And if you get above 140 over 90, you're in the danger zone. You can monitor the numbers yourself at the corner drug store, for free. And if your pressure is too high, the good news is, hypertension can be reversed.
"If you're going to remember one number, if you're going to focus and fixate on one number in your entire health profile, it better be your blood pressure because that's where the money is," Oz said.
How to lower your blood pressure:
1. Eat plenty of fruits and vegetables, nine servings a day.
2. Exercise - even walk - at least 30 minutes a day.
3. If necessary, talk to your doctor about medication.
There's a Wrong Way to Breathe
It turns out there's actually a better way to take a deep breath than what you're probably doing.
"I would say less than one in 10 Americans knows what a deep breath is because we were all taught to believe that it's about straining our ribcage up," said Oz. "It's not. That doesn't even feel good."
The key to deep breathing is to use the stomach, not just the chest, according to Oz. Take in a breath and push out the stomach. That pulls down the diaphragm, a strong muscle, which pulls down the lungs, allowing air deep in the lungs. When you breathe out, pull in the stomach. That moves the diaphragm up, pushing the air out of the lungs.
Oz recommends taking four seconds to breathe in and four seconds to breathe out.
"That's a deep breath," Oz said. "That's one of the best ways to cope with stress. It's one of the best ways to deal with asthma, it's one of the best ways to get your lungs to truly fill with the air we know is of value."
On June 10, rowers Jordan Hanssen, Brad Vickers, Greg Spooner and Dylan LeValley set out on an incredible journey - the only American team in what could be a grueling three-month long race across the Atlantic Ocean in rowboats. There is no prize money - just the chance to be the first Americans to do it.
It's a challenge that's particularly important to Hanssen, whose father died of an asthma-related illness and who also has a mild form of asthma himself.
Hanssen was diagnosed with mild asthma as child. In a twist of fate, when he got to college, he discovered he had a real talent for competitive rowing - a sport that demands extraordinary breathing power.
"A rower's power comes from his lungs, and so does everyone's power," Hanssen said.
But, Hanssen said, "I think I'm pretty lucky to be in a position to row across the ocean. I think I was given the lungs that my father didn't have. "
Right now the Americans are in the lead, on their boat named the James Robert Hanssen - in honor of Jordan's father and the gift of a healthy breath.
[Click here to read more about the rowers' and their progress.]