1. MYTH: Thinner people are healthy, heavier people are not.
2. MYTH: You need to drink eight glasses of water a day.
Nope. OK, yes, you should be drinking water throughout the day, and you'll definitely be healthier if you drink water instead of drinks with a lot of added sugars (like soda, juices, and other sweet drinks). But everyone's hydration needs are a bit different, and the specific number of eight glasses isn't based on any actual research. (FiveThirtyEight goes into detail about the history of this myth, and what research does exist, if you're interested.) General rule of thumb about staying hydrated: Drink enough water throughout the day so that your urine is pale yellow. The darker your pee, the more dehydrated you are.
3. MYTH: Natural is healthy, and chemicals are bad for you.
4. MYTH: Breakfast is the most important meal of the day.
5. MYTH: You can "spot target" certain body parts for weight loss — like blasting your belly fat by doing crunches.
Unfortunately that's not how it works. Working out specific muscle groups can make those muscles stronger, but doing a gajillion crunches isn't going to do anything to your belly fat. Ditto lunges and your thighs, and tricep dips and your upper arms. And so on. That's because muscle doesn't turn into fat: It grows and gets stronger underneath whatever fat you have. And you also can't do any exercises that target particular areas of fat.
This all means that if you are trying to lose weight in a particular spot, you'll need to reduce your total body-fat percentage through a mix of exercise and a modified diet.
6. MYTH: You shouldn't exercise if you're pregnant.
7. MYTH: Taking vitamins and supplements will make you healthier.
8. MYTH: Doing hours of cardio — such as running — is the best exercise to do to lose weight.
9. MYTH: Women who lift weights get bulky.
10. MYTH: You're eating for two when you're pregnant.
11. MYTH: If you have a cold, you should take vitamin C to make it go away.
12. MYTH: Detox diets or cleanses will purify your body of toxins.
13. MYTH: Working out will make you lose weight.
14. MYTH: Antibiotics will help a cold or the flu.
15. MYTH: Low-fat products are better for you than full-fat products.
A lot of low-fat products might have added sugars or carbohydrates to make the food taste better (since so much of the good taste comes from the fat). Also, more and more research is finding that fat (at least the kind that isn't trans-fat) isn't the enemy anymore. As long as you're not eating too much of it, a little fat in your diet can be good for you.
16. MYTH: You can't get pregnant if you're on your period.
Some commenters were asking for additional information. So we added a tip for how to tell if you're hydrated to the eight glasses of water point, and we included a few links to studies for the point that breakfast isn't the most important meal of the day.
You should aim to have pale yellow urine, not completely clear (which can be a sign of over-hydration).