Nearly everyone today worries about being overweight. We're constantly inundated with messages from TV and the Internet about widespread obesity and the risks of being obese. Whether it's health issues or social acceptance, no one wants to be fat.

But if you look at it from an evolutionary aspect, the ability to store excess calories in the form of fat cells is a very life-serving ability. When a person consumes more calories than the body uses for muscle movement, internal temperature maintenance and cellular repair the remainder is stored in the chemical bonds of fat cells. Technically, it's stored in something called 'adipose tissue'.

Energy - which calories measure - isn't a substance, so it's not the same as fat. But that energy becomes available for use when those fat molecules break down into simpler products. That happens when a person increases movement or otherwise triggers a need for more energy.

Carbohydrates are one major source of that energy. Sugars (chiefly glucose) and starches are the two main forms and they participate in something called the Krebs Cycle. Also known in scientific circles as the tricarboxylic acid cycle, but don't bother trying to pronounce it. Sugars come in, get broken down into something called ATP, then into ADP releasing energy in the process.

When the body runs out of glucose to use in the cycle, it turns to stored body fat as a substitute. Breaking down those fat molecules is, in essence, what causes a person to decrease the percentage of body fat. Sometimes increased muscle mass results, so the final result isn't always a net weight loss.

But in biology, as in life, everything is best in moderation. When more calories are consumed than used over a long period of time, body fat increases to the point that the health risks can outweigh the benefits of a ready supply of energy. The result is an increase in the odds of heart difficulties, diabetes and other real medical problems. The social consequences are equally well known.

Knowing this, many will strive to maintain their weight and percentage of body fat within a certain range. That range differs from person to person (people have different body types), season to season (winter fat can actually be healthy) and according to their individual BMI (Body Mass Index).

So, in order to decide whether you are obese, only moderately over the preferred weight range, or just lack muscle tone, you need to consider those factors. Doing so requires knowing your specific body type, the ability to calculate BMI (very simple, actually) and recognizing that there is no exact, static, ideal weight for you.