My legs—and lungs—are burning as I pedal. The cycling interval’s only 10 seconds long, but it sure feels like the clock is moving in slow motion. Coach Mauricio Andrade stands in front of me, offering support that’s motivating but firm. There’s not a chance he’ll let me slow down or slack off.

When he finally calls time and I ease up my cadence, I glance around to the view of snow-capped peaks. That, and the thin air, have momentarily transported me to a place like Leadville, Colorado or Cusco, Peru, at an altitude of about 10,000 feet.

But once I’m mercifully finished with my two rounds of 10 high-intensity intervals, I’ll step outside, inhale deeply, and get back in my car to drive to my apartment on the North Side of Chicago. That mountain view? It’s a wall-sized decal.

I’m working out this morning in the altitude chamber at Well-Fit Performance, a training hub for many of the city’s triathletes and other endurance athletes. In addition to endless pools, strength- and functional-training equipment, and a full complement of treadmills and bike trainers, Well-Fit has now installed one of the few altitude chambers in the country, and the first in the region. 

The facility’s expensive compressors essentially suck the oxygen out of the air, simulating some of what I’d experience if I hiked to Machu Picchu or ran the Leadville 100-miler. There are two other women near me, doing their own workouts on top-of-the-line Woodway treadmills; when I catch my breath enough to chat with them afterward, I learn they’re training for a trek in Kathmandu.

If I visit the room regularly—twice a week for four to eight weeks—I just might see my race times come down and my fitness level reach new heights, Well-Fit’s owner and head coach Sharone Aharon tells me. “There’s such enormous benefit to training at altitude, at high intensity,” he says. “If I say one sentence about it, you train less and you gain more.”

RELATED: 10 Simple Ways to Actually Enjoy Running

Why athletes train at altitude

For decades, elite endurance athletes have headed to the mountains for altitude training. Because there’s less oxygen in the air to begin with—and less atmospheric pressure pushing it into athletes’ veins—their bodies respond by boosting the production of red blood cells. The effect is temporary, so they have to time it right. But when they then head back down to sea level for competition, these adaptations deliver hard-working muscles an augmented supply of oxygen to power each contraction.

The problem is that sweating in thinner air isn’t just harder for us mere mortals, it’s also more challenging for the likes of marathon champions like Shalane Flanagan. You just can’t pedal as hard or run as fast at higher elevation. So athletes have to find other ways to push their bodies to the limit, says Andrew Subudhi, PhD, professor and chair of the department of biology at the University of Colorado Colorado Springs, who’s studied the effects extensively…

Post Malone isn't the only person who loves face tattoos. Celebrities like Lena Dunham, Minka Kelly, and even Mandy Moore have jumped on the face-tat bandwagon with the recent trend of microblading (to make your eyebrows appear fuller). And now there's a new beauty tat fad called dark circle camouflage—aka tattooing the dark circles under your eyes to make the skin lighter.

Professional tattoo artist Rodolpho Torres has gained over 2 million Instagram followers in part for his "eye camouflage" work of covering up dark circles via tattoo. He also uses this tattooing method to "camouflage" stretch marks on the legs and chest. (Side note: We love our tiger stripes and so does Padma Lakshmi.)

RELATED: A Celebrity Makeup Artist’s Steps for Covering Up Dark Under Eye Circles

While Torres has more than 10 years of tattooing experience, derms say you shouldn't trust anyone with such fragile skin if they aren't a doctor. "No nonmedical personnel should be touching that area of your eyes—especially with a sharp instrument," says Lance Brown, M.D., a leading dermatologist in New York City and the Hamptons. "Under the eye, you need to be very cautious—you can cause an infection around the eyelid, or a sty or cyst could grow around the hair follicles," says Dr. Brown.

RELATED: My Perfectly Microbladed Eyebrows Are Ruining My Life Right Now

It is common for tattoo scarring to occur if the artist is inexperienced or presses too deeply with the needle. Apply these potential mishaps to the skin beneath your eyes and it's a recipe for serious concern. Scarring on the lower eyelids, in particular, can create a contraction in the skin that pulls the lower eyelid down, causing ectropion, a condition where the lid pulls or sags away from the eye. "Ectropion can lead to tear duct issues, cysts, and more," says Dr. Brown.

For the record, traditional tattoos are largely safe (and can even boost your health according to the American Journal of Human Biology) but it's probably not worth taking the risk when it comes to the sensitive skin under the eyes—especially considering the new report from the FDA that they've seen an alarming rise of infections and adverse reactions to tattoos as a result of moldy ink. (One woman recently experienced a life-threatening infection after her microblading appointment went south.)

RELATED: I Tried Kim Kardashian’s Trick for Making Dark Under Eye Circles Disappear

RELATED: Hyperrealistic Nipple Tattoos Are Changing the Game for Breast Cancer Survivors

If vanity wins out over your health concerns, consider this: While tattooing your circles may save you from having to pack on the concealer (I mean, we can't deny that the before-and-afters look pretty impressive) since it doesn't address the underlying cause of dark circles, it's likely just a temporary band-aid solution…

The low-carb, high-fat ketogenic diet continues to be the trendiest weight-loss plan around. Maybe that's because following keto is relatively simple: by cutting way back on your carbohydrate intake while eating lots of fat and a moderate amount of protein, your body goes into a state of ketosis, burning fat instead of carbs for energy. 

Stories of keto diet weight-loss success are all over social media. Yet the plan isn't without some side effects, especially when it comes to the way it can affect your workout. Here are four must-knows if you’re an athlete or regular exerciser who’s gone keto—or is thinking of trying it. 

RELATED: Your Ultimate Keto Diet Grocery List

You might feel weak during the first few weeks

Transitioning from burning carbs to burning fat takes about two weeks, so at the diet’s start, your system is not in ketosis yet. “That means that your body is still using stored glucose for fuel rather than ketones," says Jim White RDN, ACSM exercise physiologist, and owner of Jim White Fitness and Nutrition Studios in Virginia. "But because your carbohydrate intake is restricted, the amount of stored glucose (or energy) is limited, which can cause you to feel weak and fatigued.”

Some people experience symptoms such as brain fog, headaches, chills, and a sore throat during this transition. These symptoms are collectively referred to as “the keto flu," explains Dave Asprey, founder and CEO of performance nutrition company Bulletproof. It's not really the flu, of course. But symptoms like these will make you feel like a flu sufferer and “will negatively affect your ability to tackle a workout,” says Asprey.

The "keto poops" could interrupt your workout

Yep, this is a real thing. It's also known as keto diarrhea, and it can strike in the middle of a gym session. Keto poops are a common side effect of the high-fat diet. “This is likely due to the gallbladder—the organ that produces bile to help break down all the fat in the keto diet—feeling overwhelmed or overworked,” says Josh Axe, a natural medicine physician and author of the upcoming book Keto Diet.

Feeling an urgent need to go number two during yoga or CrossFit isn't too much of an issue; you can just hightail it to the gym restroom or locker room and then resume your workout. But when you’re on an outdoor run, well, that could be a problem. If you’re a casual runner, just pick a shorter route or one with known bathroom stops. And consider adjusting your meal schedule, so any poop problems happen before or after you lace up your running shoes.

RELATED: Keto vs. Atkins: Which Is the Better Low-Carb Diet?

You'll need to hydrate a lot more often

Dehydration is another common side side effect of keto. “Ketosis causes you to lose electrolytes because it increases urine output," says Axe…

You already know that vitamin D is good for your bones, your brain, and your heart. Now, new research suggests that it may also give your workout routine a boost. According to a study published in the European Journal of Preventive Cardiology, people with higher levels of vitamin D tend to be more physically fit.

Specifically, the study looked at cardiorespiratory fitness, a measure of how efficiently the heart and lungs supply oxygen to the muscles during exercise. People with higher cardiorespiratory fitness can exercise longer and harder, and they also tend to live longer and healthier lives.

For the study, researchers compared the vitamin D levels and cardiorespiratory fitness levels—measured by a treadmill test—of nearly 2,000 U.S. adults ages 20 to 49 who took part in a nationwide study from 2001 to 2004.

They found that people in the top quartile of vitamin D had cardiorespiratory fitness levels that were 4.3 times higher than those in the bottom quartile. Each 10-point increase in vitamin D was associated with a 0.78-point increase in VO2 max, the measurement for cardiorespiratory fitness.

RELATED: 12 Ways to Get Your Daily Vitamin D

Even after adjusting for participants’ age, sex, race, body mass index, and health history, fitness levels for those with the highest vitamin D levels were still 2.9 times higher than those with the lowest. The link held true for both men and women, and for all of the age groups and ethnicities in the study. It was also true regardless of whether participants were smokers or had hypertension or diabetes.

The study was observational, so it could not show a cause-and-effect relationship. But the association was “strong, incremental, and consistent across groups,” said lead author Amr Marawan, MD, assistant professor of internal medicine at Virginia Commonwealth University, in a news release from the European Society of Cardiology (ESC).

“This suggests that there is a robust connection and provides further impetus for having adequate vitamin D levels,” Dr. Marawan said, “which is particularly challenging in cold, cloudy places where people are less exposed to the sun.”

Vitamin D is known as the sunshine vitamin because the human body makes vitamin D in response to sun exposure. People can also get it from supplements or from fortified foods. (The study did not take into account how much vitamin D participants got from sun, supplements, or food.)

The study notes that vitamin D could potentially affect cardiorespiratory fitness in several ways. For starters, the nutrient has been shown to boost the production of muscle protein and aid in calcium and phosphorus transport on a cellular level. It may also affect the body’s makeup of fast-twitch muscle fibers, “suggesting that vitamin D may improve aerobic fitness,” the authors wrote.

RELATED: 27 Health Problems Linked to Low Vitamin D

This isn’t the first study to suggest a link between vitamin D and athletic performance: Previous research has noted that vitamin D-deficient ballet dancers jump higher and have fewer injuries—and pro athletes have better sprint times—when they take supplements…

What would you do if you were out for a Saturday morning run, training hard for your next race, and all of a sudden a car drives by and throws a McDonald’s cup at you? What about if a group of young boys comes up behind you and smacks your butt, giggling as they run away?

Believe it or not, both of these examples of supreme body shaming have happened to competitive runner Julie Creffield, author of The Fat Girls' Guide to Marathon Running ($9; amazon.com)

Creffield, who’s from the United Kingdom, considers herself a plus-size runner. But anyone who thinks her weight holds her back is seriously mistaken. She’s been running for about 15 years and has completed marathons, ultramarathons, and triathlons around the world. Now, she’s taking on what she calls her “bucket list” race: Sunday’s New York City Marathon.

RELATED: Everything You Need to Know to Train for a Half Marathon

For Creffield, 15 years of running has also meant 15 years of body shaming, exclusion, and misguided stereotypes. “People automatically think you run to lose weight, and then when you don’t lose weight, they’re like ‘Why are you still fat?’” she tells Health. “There’s this assumption that we only exercise for slimness, and for me that’s not the reason.”

Running is like therapy for Creffield. She used to struggle with depression, she says, and she credits exercise with pulling her out of it.

Another misconception Creffield can’t seem to escape: people thinking she’s a beginner. “They give you unsolicited advice about how to improve, and they say things like, ‘Once you’ve been doing it for a while, it’ll be easier.’ And I’m like, ‘Well, I’ve been running for 15 years.’”

RELATED: 6 Food Rules for Marathon Success

After years of putting up with body-shaming comments and actions, Creffield says she realized she couldn't be the only runner who had to deal with this kind of abuse. So in 2010, she started her blog, The Fat Girls' Guide to Running. In 2013, she turned it into a business, Too Fat to Run?

The name was inspired by one of Creffield’s most notable body-shaming experiences, when she went to the doctor with a pulled back muscle. As soon as she started complaining of pain, the doctor suggested she exercise more. “I was like, ‘Well actually, I’m training for a marathon,’ and he said, ‘Oh no, you’re too fat to run a marathon.’” Cue her determination to prove him wrong.

She calls her business a “virtual running club,” or an online resource runners can go to find carefully tailored workouts and training programs. It's also a platform for connecting with other women in the program. 

Creffield says she’s always felt excluded from the running community…

Hotly debated fitness topics are in no short supply: yoga vs. pilates, cardio vs. strength training, and the treadmill vs. outdoor running. But no dispute is as polarizing as the one surrounding morning vs. evening workouts.

Of course, the absolute best time to work out is whenever gym time meshes with your schedule so actually show up on the regular. If you can only squeeze in a jog or yoga flow late at night, it's smarter to do that then skip it altogether, exercise physiologists and trainers say.

But starting the day with a heart-pumping sweat session does come with indisputable health benefits. And now that daylight saving time has ended, mornings are brighter, so getting out of bed and into your workout leggings isn't as hard as you might think. Let us make the case with these five science-backed reasons to start setting your alarm a little bit earlier.

RELATED: How to Motivate Yourself to Go to the Gym on Cold, Dark Days

You’ll make better wellness choices all day

“When you start the day working on your health, you'll strive to maintain that healthiness," explains Zack Daley, head coach at Tone House in New York City.

Think about it: When you're proud of yourself for killing it at your 8 a.m. class, you won't want to ruin that healthy high by gorging on breakfast donuts, right? Instead, you'll likely try to keep that awesome feeling going by skipping the pastries, taking the stairs to get to your office, and indulging in a grain bowl at lunch rather than the taco Tuesday special. All of these little moves add up to a healthier you.

You’ll sleep like Sleeping Beauty

According to the  National Sleep Foundation (NSF), people who hit the treadmill at 7 a.m. sleep longer, experience deeper sleep cycles, and spend 75% more time in the most reparative stages of slumber than those who exercise at later times in the day. THe NSF also notes that those who sweat at night tend to have more trouble catching shuteye, possibly because working out raises your body temperature, which is a known sleep saboteur.

Anecdotally, Daley believes this to be true: “I find that when I work out early, I am able to get to bed easier at night. But when I work out later at night, my adrenaline is still going from my late night workout.”

RELATED: 20 Habits That Make You Miserable Every Winter

You might lower your blood pressure 

People with high blood pressure, aka hypertension, often need medications to control this dangerous condition. But lifestyle changes—such as morning exercise—may help. In one study published in the Journal of Vascular Health and Risk Management, researchers had participants exercise at three different times of day: 7 a.m., 11 a.m., and 7 p.m.

Those who worked out early in the morning reduced their post-workout blood pressure by 10%…