[Intro]   We here at SciShow want you to keep being curious -- about the world, the universe, even what’s going on in your own body.    And that is why we’ve teamed up with Google and YouTube to answer ten of the most popular science questions searched on the internet.   I'm Michael Aranda, and this is The World’s Most Asked Questions.     Today’s question: How many calories should I have in a day?   You’re probably asking because you want to know how many is too many.

After all, you know that if you take in more energy than you need, your body will store that excess energy as fat.   But before we can talk about how many calories you need, let’s start with what a calorie is.   The calories that you find on food labels aren’t the same as what scientists call calories.   In chemistry, a calorie is the amount of energy it takes to raise the temperature of one gram of water by one degree Celsius.   But the calories listed on your can of soda are actually kilocalories -- each one equal to 1,000 of the “small calories” that chemists use to measure energy.   So when your soda pop says it has 150 food Calories -- you’ll notice it’s written with a capital C -- that’s the same as 150,000 small calories -- with a little c.   So that’s enough energy to raise the temperature of a whole liter of water by 150 degrees.   Kinda puts things in perspective.   But how many calories you need depends on who you are, and how you live your life.   First, there’s your age to consider.   Despite the stereotype of the ravenous teenager, your calorie demands actually peak when you’re in your mid-20s.    That’s when your metabolism is higher than at any other point in your life, and because you keep growing into your 20s, once you're done, you have more lean muscle mass which requires more energy to maintain.   So, depending on your lifestyle and other factors, when you’re in your 20s, you may need from 2200 to 3000 calories a day!   Your sex is a factor, too. Men tend to have more total body mass, and more muscle mass, than women on average, so their caloric requirements can be slightly higher.   According to the US Institute of Medicine, the average calorie range for an adult woman is 1800 to 2400 calories a day; for men, it could be anywhere from 2000 to 3000.   But that’s obviously a pretty broad range.

And as delicious as it sounds, most of us don’t need to be eating 3000 calories a day.   And that’s because the most important factor, by far, that affects your calorie needs is your activity level.    For example, if you’re a woman in your 30s or 40s, and you live a rather sedentary lifestyle -- meaning you don’t set aside time for any exercise -- then probably don't need any more than 1800 calories on average.   But if you regularly take a nice brisk walk -- say between 2.5 and 5 kilometers a day -- then your caloric needs go up about 10%, to 2000 calories.   And if you regularly walk more than 5 kilometers -- or burn the equivalent amount of energy doing some other exercise, like running -- then you’re looking at another bump, up to 2,200 calories.   But when it comes to calorie intake, medical professionals will tell you that the real goal is to focus on your energy balance -- that’s the balance of calories you take in compared to the calories you burn through physical activity.   So, naturally, if you burn more calories than you take in, you’re going to have to use more of the energy that you have stored up as fat.    And, on the other hand, if you consume more than you use, you’ll just keep building up those “energy reserves” around your midsection.   Since no one burns the same exact number of calories that they eat every day, the key is to maintain energy balance over the long term.   So if you consistently ingest more energy than you use, you’ll be out of balance -- just as you would be if you keep burning more energy than you supply for your body.   But, if you ask me? You look fantastic.   Now, SciShow survey funtime! Survey takers who reported counting calories also reported getting hiccups less than other survey takers.

Interesting, but it's important to note that correlation does not equal causation.    Of all of the fascinating questions in the world, what question do you want answered most? Let us know on Facebook or Twitter or down in the comments below and we will answer the best questions in a new video at the end of the month. And don’t forget to use the hashtag #WMAQ and stay tuned for other videos this week.