Funny that we so rarely think of fat as our friend, when it does so much for us. Most of all, body fat keeps us warm, stores energy from food so we can use it later, and protects our organs.  

Body fat also houses the chemical reactions that manage growth and the immune system. Within our fat, hormones and proteins talk to one another to keep the body running.

It's amazing how many diseases either are caused by being overweight or are made worse by extra pounds. Think infertility, arthritis, sleep apnea, diabetes, stroke, heart disease, high blood pressure, and even some cancers.

BMI is a number based on your height and weight. It's a good way to tell how much body fat you have. A BMI of 25 to 29.9 means you're overweight. A BMI over 30 means you're obese. 

You can figure out your BMI here.

If you want to lose weight and keep it off, your best bet is a diet-and-exercise combo. It's much better than diet alone.

Don't be afraid of muscle. It's a good thing! Losing fat while keeping lean muscle mass through exercise keeps you fitter and stronger, so you look and feel better.

Can't tighten your belt? People with apple-shaped bodies, who carry a lot of weight in their middles, are at higher risk for type 2 diabetes, heart disease, stroke, dementia, and other diseases.

Measure your waist. For a man, it's best if it's under 40 inches. For a non-pregnant woman, it should be under 35 inches. 

To lose 1 pound of weight, you need to burn off 3,500 more calories than you take in. Cut just 500 calories per day to lose 1 pound in a week, or 1,000 calories per day to lose 2 pounds in a week.

Fatty acids are part of the makeup of hair. They help give hair -- and your skin -- their glowing looks.

About 17% of kids and teens ages 2 to 17 are now obese. That puts them at higher risk for diabetes, heart disease, and other health problems, just as it does adults who are obese.

More than 1 in 3 adults are obese.

High LDL ("bad") cholesterol can come from your family tree. People with a faulty gene that leads to high cholesterol tend to have heart attacks at a very early age.

Mostly, though, a bad diet, being overweight, and not exercising regularly cause high LDL cholesterol -- which affects about 1 in 3 adults. Most doctors urge the right diet, more exercise, and losing weight to lower LDL cholesterol. Sometimes they also prescribe medicine.

Friends tend to have the same kinds of habits about food and exercise -- like ordering a second dessert or taking up running. They also share similar ideas, like what a good body size is. 

Peer pressure can make an overweight person want to lose weight, too. The closer your ties, the stronger the effect.

Avoid a score that lands you in the group "underweight," "overweight," or "obese," because all these carry health risks.

Aim for any BMI in the normal range. But know that when people are at the lower end of the normal range, they seem less likely to develop some weight-related diseases.