If the risks of heart disease, diabetes and a lower quality of life aren't reasons enough for you to watch your weight, then consider what being fat may be doing to your brain.

People who are overweight or obese during their 30s, 40s and 50s have a greater risk of dementia and Alzheimer's disease as they age, compared to people whose weight is normal, according to a 20-year study of Swedes published in the Archives of Neurology.

Researchers used body mass index (BMI), an equation based on height and weight, in the study. A healthy BMI is generally considered to be below 25. The study found that the risk of dementia increased for individuals with a BMI over 25.

That's not good news for the estimated 127 million Americans who are overweight (with BMIs between 25 and 30) or the 69 million who are obese (with BMIs over 30). Indeed, obesity is on the rise all over the world, and public health officials warn that it poses a huge risk to health in the coming decades.

Although the study didn't pinpoint the specific ways that extra pounds cause harm to the brain, previous studies provide plenty of evidence to suggest how being overweight takes a toll on the brain.

• Hypertension. Much of the cognitive decline that we attribute to aging may be caused by high blood pressure, which commonly goes hand in hand with obesity. Over time, high blood pressure weakens blood vessels, causing damage to the fragile inner lining of arteries. This damage not only raises the risk of stroke or aneurysm, but eventually, if not corrected, it reduces attention, learning and memory, studies show.
• Diabetes. Every year 1.3 million Americans are diagnosed with Type 2, or adult-onset, diabetes. High rates of obesity are driving this trend. Women with Type 2 diabetes have a four-fold greater risk of cognitive decline than non-diabetic women, according to a recent study. Diabetes seems to affect men's mental acuity to a lesser extent, for unknown reasons. A diagnosis of diabetes does not seal one's cognitive fate. A University of California-San Francisco study found that diabetics who do not control their blood-sugar levels are at the highest risk for mental decline later in life.
• Poor Diet. High cholesterol is another risk factor for cognitive problems in old age, the Swedish study found. High cholesterol can be caused by a diet high in saturated fat, such as fatty meats and hydrogenated vegetable oils.

Previous studies have linked "good" fats—polyunsaturated fats found in olive and canola oils, fatty fish like salmon and nuts—to brain health. These fats help raise levels of "good" cholesterol (HDL, or high density lipoproteins), which helps the body get rid of "bad" cholesterol (LDL, or low density lipoproteins).

While the research about obesity paints a grim picture, the good news is that you don't have to drop 50 pounds to see a real benefit to your health.

Studies show losing just a few pounds can raise levels of HDL cholesterol. And losing just 10 percent of your weight if you are overweight can significantly lower the risk of hypertension and diabetes.

What's good for the body is especially good for the brain.