Despite widespread knowledge of the health risks, there continues to be a worldwide epidemic of obese and overweight people. Fortunately, daily calorie consumption is gradually declining in the United States, and the rates of obesity have stopped rising.

But most people in the U.S. and throughout the developed world still live with excess body fat. According to the World Health Organization, more than 1 billion people are overweight and 300 million are obese, increasing their risk for depression, accelerated cognitive decline, and dementia.

More than half of all obese people have a condition called “metabolic syndrome,” which is characterized by central obesity (too much fat around the belly), increased sugar and triglycerides in the blood, elevated blood pressure, and low levels of “good” HDL cholesterol.

The fat cells that comprise central obesity are pro-inflammatory, meaning that they elevate inflammation in the brain. Research has shown that brain inflammation is associated with reduced brain volume and a greater risk of accelerated cognitive decline.

Body mass index (BMI) is a ratio of height to weight that estimates whether a person is overweight or obese. French investigators showed that volunteers with elevated BMIs had more trouble learning words and substituting numbers for symbols than those with normal body weights.

Heavier individuals also have a harder time controlling appetite. Studies indicate that the brain’s frontal lobe — which regulates appetite — stops functioning normally as a person’s body weight increases, leading to difficulty resisting high-calorie foods.

Elevated BMI also increases insulin resistance, a condition in which sugar has difficulty entering cells from the bloodstream.

Insulin resistance raises the risk for diabetes and heart disease, both of which can compromise brain health.

Research has also linked obesity to depression. The link between mood and body weight appears to involve elevated inflammation in the brain, as well as psychological factors. For instance, many obese people suffer challenges with body image, social isolation, and poor self-esteem.

Overweight and obese people also exercise less, and research indicates that regular physical exercise benefits both mood and memory.

The good news is that when people do lose weight, they experience improvement in both their mood and their cognitive ability