We all have that friend. The one who can eat double cheeseburgers and greasy French fries for breakfast, lunch, and dinner and never gain weight. Perhaps you are that friend. Some people are just born thin and healthy, right? Wrong.

Lean does not always equate to healthy, says Krishna Sudhir, MD, a cardiologist and division vice president in Abbott's vascular business. Skinny people could still face health risks if they make poor lifestyle decisions. Below, Sudhir clears up common misconceptions about body type and offers advice on how you can stay fit at any size.

The Skinny Myth

In a society obsessed with body size, the focus is often on obesity and the health dangers associated with being overweight, Sudhir says.

"Clearly, being overweight and obesity carries a lot of risk factors," he notes. "But the converse is not necessarily true. We can't just assume because someone is thin, they're healthy."

A common misconception is that thin people can eat whatever they want as long as they don't gain weight. Not true, says Sudhir. All those fatty foods and high-calorie snacks add up over time. The same goes for neglecting to exercise.

"It's logical, right? I'm already thin, why should I work out at the gym?" one often hears. "I'm reasonably thin, so why should I avoid high-fat foods?"

The truth is that eating junk and majoring in couch potato-ism can raise your cholesterol, impair your glucose tolerance, and increase your blood pressure over time. People who are genetically susceptible have a higher chance of developing these conditions regardless of body size, Sudhir says. Cardiovascular disease can be the unfortunate result later in life. 

Another myth is that exercise is only for the overweight, Sudhir adds. Sure, losing weight is a goal of many gym-goers, but working out has benefits beyond shedding body fat. 

"Exercise does a lot of other good things for the body like controlling risk factors," Sudhir says. "It's good for [strengthening] the musculo-skeletal system, it reduces our long-term risks of a variety of diseases, and it has beneficial effects on the brain.”

All in the Family 

Heavy or thin, your family history has a significant impact on your future health.

There are a number of contributors to conditions such as heart disease, diabetes, and stroke, but the number one risk factor is family history, Sudhir says.

Don't let a thin figure fool you. Frame size may not lower your risk for certain diseases if they run in your family.

"[For example], If your mother had a heart attack when she was 65, and your dad had a stroke when he was 52, and you're getting to your mid-40's, it's time to see a doctor," Sudhir says. "These genes tend to manifest in families. A strong family history is often a predictor of the increased risk."

A health screening can help pinpoint health conditions that mark your family line, he says. People with illnesses in their family background benefit from getting on the right health track as early as possible.

"It doesn't mean doom and gloom," Sudhir says. "What it means is you need to be aware of your risk. "One generation later, medical technology and science have advanced. There may a lot more than we can do about these diseases than what our parents knew."

Staying Fit at any Size

So now that we know the skinny on the "skinny myth," the question is: What makes a body healthy?

Eating right is key, says Sudhir. Avoid fatty, high-sugar, and processed foods.  Opt instead for fish and chicken, fruits and vegetables, whole grains like brown rice, quinoa, and nuts, such as walnuts or almonds.

Exercise at least three to four times a week or more, if possible, he adds. It's beneficial to mix cardio with strength training for a balanced workout. Cardio, such as jogging, swimming, running, dancing or even just walking, helps promote heart health and can keep blood pressure and cholesterol under control, Sudhir says. 

Weight training increases your upper body strength and keeps muscles firm, an important determinant of how well we cope as we get older. 

"Remember that all exercise counts," Sudhir says.  "Whenever possible, take the stairs rather than the elevator. Park the car a little further from the door at work so you get a little more of a walk. Take the dog for a walk in the evening."

Sleep is important to your overall health. A lack of rest has the potential to increase your risk for cardiovascular disease and other health conditions. So make sure you get adequate shut-eye each night.

Make time for an annual exam with your doctor, even if you feel fine. Some ailments, such as cancer or high blood pressure can be silent. Regular physicals can help your doctor monitor your health and potentially identify a lurking condition, getting you on the road to treatment, Sudhir says. Annual exams become even more vital as we approach the age of 40.

And lastly, invest in happiness. In today's world, we tend to underemphasize the importance of happiness, but contentment is a prime weapon against disease, Sudhir stresses.  

"Happy people seem to do better health-wise," he says. "Happiness and laughter decrease stress and less stress means less blood pressure, better glucose levels, better cholesterol levels, and so on."

Want to take these tips to go? Download this infographic.