Not necessarily.

Truth is, obesity genes are responsible for just 5 percent of all weight problems.

So what causes the rest?

Harvard Medical School's Dr. David Ludwig has the answer. His groundbreaking research uncovers the real cause of obesity for most Americans.

And it isn't dietary fat!

That's right. Studies by Dr. Ludwig and others show that dietary fat doesn't really determine body fat.

For example, a large study called the Women's Health Initiative found that 50,000 women on low-fat diets didn't significantly lose weight. And another study found no dramatic differences in weight loss between people who followed either low-fat, low-carb, or very- low-carb diets.

So why aren't these diets working?

Well, for one thing, we are looking for answers in the wrong place.

For more than 15 years, I have sought answers about weight problems by testing almost all my patients with a simple, inexpensive test that can help determine not just the causes of obesity, but also the risk of diabetes, heart disease, cancer, Alzheimer's, and premature aging.

Yet it is a test most healthcare providers don't perform, don't know how to interpret, and often think is useless.

But thanks to Dr. Ludwig's research, this important test is being recognized.

Two of his recent studies show that the main determinant of changes in body weight and waist circumference (belly fat) is the way your body responds to any type of sugar, carbohydrate, or glucose load.

The best way to determine how your body responds to sugar, carbs, and glucose is to test your insulin level with a glucose tolerance test. This is done by measuring your insulin after you drink a sugary beverage with 75 grams of glucose.

This test has helped me learn more about my patients than any other test -- and it helps me create individualized nutritional approaches for them.

Let's take a closer look at those studies.

In one, Dr. Ludwig and his colleagues performed a glucose tolerance test on 276 people and looked at insulin concentrations 30 minutes after they consumed a sugary drink. This helped the researchers determine whether they were high- or low insulin secretors.

During the course of the 6-year study, they found that those who were the highest insulin secretors had the biggest change in weight and belly fat compared to the low insulin secretors. And people who were high insulin secretors and ate low-fat diets did even worse.

This makes a lot of sense, because insulin both stimulates hunger and makes you store belly fat. When you eat a high-carb meal, your insulin spikes and your blood sugar sinks, making you hungry.

Then you crave more carbs and more sugar -- and eat more the whole day.

And that's not all.

Dr. Ludwig also found that people who ate a low glycemic load diet -- which lowers blood sugar and keeps insulin levels low -- had much higher levels of HDL "good" cholesterol and much lower levels of triglycerides.

So the best way to treat cholesterol may not mean eating a low-fat diet, but eating a low glycemic load diet.

To learn more about Dr. Ludwig's exciting work, take a look at his research on National Library of Medicine's database, PubMed (http://www.ncbi.nlm.nih.gov/sites/entrez).

I also urge you to read his book, "Ending the Food Fight," a unique guide for addressing our skyrocketing childhood obesity epidemic.

Finally, please ask your physician for a glucose tolerance test. He or she should measure your insulin and blood sugar at 30 minutes, one hour, and two hours to get the best picture of your insulin profile.

If you are a high insulin secretor and your insulin goes over 30 at these intervals, you should eat a low glycemic load, whole-foods, unprocessed diet.

The answer to obesity and weight treatment is in personalizing our approach. Learning how you respond to sugar and carbs is an important step on that road.

References:

Ebbeling CB, Leidig MM, Feldman HA, Lovesky MM, Ludwig DS. Effects of a low-glycemic load vs low-fat diet in obese young adults: a randomized trial. JAMA. 2007 May 16;297(19):2092-102

Chaput JP, Tremblay A, Rimm EB, Bouchard C, Ludwig DS. A novel interaction between dietary composition and insulin secretion: effects on weight gain in the Quebec Family Study. Am J Clin Nutr. 2008 Feb;87(2):303-9