You've heard the old "apple a day" expression. The original has been traced back to 1866 and was: "Eat an apple on going to bed, and you'll keep the doctor from earning his bread." Science seems to be backing up this old idea up.

From brand new medical research to common sense advice, here are several reasons to eat a nutritious apple every day.

1. Apples contain an anti-aging compound. Some of how we age is uncontrollable — it's in our genes. But some of how fast we age — and how healthfully we do so — is under our control. Scientists are always looking for ways to slow down aging, no matter our genetic destiny. In a new study, they looked to see which flavonoids (naturally occurring compounds in fruits and vegetables) had the most antioxidant and anti-inflammatory properties.

They discovered that fisetin, which is found in apples, strawberries, cucumbers, persimmons and onions, had the strongest effects. "Administration of fisetin to wild-type mice late in life ... extended median and maximum lifespan," the researchers wrote. The tests also were run on human fat tissue to see if the results translated — and they did.

"These results suggest that we can extend the period of health, termed healthspan, even towards the end of life," professor Paul D. Robbins from the University of Minnesota, told Medical News Today.

Depending on your age, one apple has about 15 to 20 percent of your daily fiber needs. Depending on your age, one apple has about 15 to 20 percent of your daily fiber needs. (Photo: 1tomm/Shutterstock)

2. They're packed with soluble fiber. A high-fiber diet is important for digestive health and keeping cholesterol low. Fiber also has been shown to reduce the risk of conditions such as heart disease, diabetes, obesity and diverticulitis. One apple with the peel on has about 4.4 grams of soluble fiber, which depending on your age, is about 15 percent of the daily recommended intake.

3. They can lower your asthma risk. A study in Australia of 1,600 people and another study in the U.K. of 1,500 people both found that apples increased pulmonary health in general and reduced the risk of asthma. Researchers found that people who ate five apples a week saw the biggest boost in respiratory health.

4. Apples are excellent thirst-quenchers. Did you know apples are 86 percent water? If you're looking for ways to stay hydrated during the day, toss a sturdy apple into your bag for a refreshing treat.

Apple cider vinegar bath Got an itchy bug bite? Dab apple cider vinegar onto the bite to relieve irritation. (Photo: Sea Wave/Shutterstock)

5. Apple cider vinegar is all-around awesome. Throughout history, apple cider vinegar has been used as a natural remedy for everything from soothing sore throats to relieving acid reflux to getting rid of the hiccups.

6. Apples can help with losing weight. This one isn't rocket science. Apples are a fruit, they have no fat or artificial ingredients, and as previously mentioned, they're high in fiber and water. So it's common sense that eating apples instead of, say, a bag of chips or a candy bar is a smarter nutritional decision that will lead to less weight gain. And studies have shown that women who ate three apples each day were more likely to lose weight than women who didn't.

7. They have very few calories. Depending on the size of your apple, it probably has about 90 calories. Which is another point in support of the claim above about weight loss.

8. They have a plethora of vitamins and minerals. Apples have about 14 percent of your daily recommended intake of vitamin C and 5 percent of the recommended amount of vitamin B6. They have 2 percent of your daily vitamin A, calcium and iron needs. And with 195 milligrams of potassium, an apple has 6 percent of your daily potassium needs.

Apples with peanut butter is an easy snack and a yummy way to use up all those apples you picked at the orchard. Apples with peanut butter is an easy, kid-friendly snack and a yummy way to use up all those apples you picked at the orchard. (Photo: Brent Hofacker/Shutterstock)

9. They're delicious and versatile! Make apple sauce, apple butter or apple jelly. Cook up a pie or cobbler. Taste different-colored apples from red to green to yellow. Go savory and incorporate apples into chicken or pork recipes, or add chopped apples to stuffing. Or just eat the fruit as-is in its perfect, natural form.

No matter how you choose to eat an apple, leave the skin on — it's where most of the nutrients are.

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.

9 reasons to eat an apple every day
Apples are as good for you as you've always been told — and maybe better. Consider all these health benefits.