When you're trying to shed more than a few pounds, chances are pretty high that both your brain and hormones are conspiring against you.
Researchers have long noted that when overweight people lose a significant amount of weight, their metabolism slows down and they have hormonal changes that increase appetite.
In effect, their bodies think they're starving and make them prone to overeat and put most or all of the lost pounds right back on.
But scientists have been uncertain about just how long the inclination to overeat lasts after weight loss.
Apparently, it lasts quite a while. An Australian research team tackled the issue in a study published by The New England Journal of Medicine last year. They studied 50 overweight or obese patients without diabetes and entered them in a weight-loss program that uses a very low-calorie diet intended to have them shed at least 10 percent of their weight.
Researchers measured nine hormones related to appetite as well as participants' self-reported appetite levels before they started the 10-week diet, at its end and then a year later.
Just 36 patients completed the diet and lost the minimum 10 percent of weight. But among this group, the metabolic rate and the hunger hormone levels were still higher than they were when the diets were started.
The researchers said it appears people who drop major weight will need some long-term strategies to counteract urges to eat.
A September 2011 report in the Journal of Clinical Investigation used brain-imaging scans to document how hungry brains come to crave high-fat foods.
Researchers at Yale University and the University of Southern California manipulated subjects' blood glucose levels and conducted brain scans while showing them three types of photos: of high- and low-calorie foods and of nonfood.
The scans showed that when glucose levels dropped, an area of the brain known to regulate emotions and impulses lost its ability to dampen desire for higher-calorie foods. The response was particularly pronounced in obese subjects when they were shown high-calorie foods.
"Our prefrontal cortex is a sucker for glucose," said Rajita Sinha, a neurobiologist at Yale and one of the study's senior authors.
Beyond brain and hormonal changes, researchers are also finding significant differences between obese people and slimmer people, and men and women, as to how fast they eat and what it means for total food intake.
Scientists at the University of Rhode Island described results from two studies during a meeting of the Obesity Society in Orlando, Fla., in November, 2011.
One lab study sought to confirm and quantify self-reported eating rates into actual consumption. Kathleen Melanson, an associate professor of nutrition, said self-reported "fast" eaters consumed about 3.1 ounces of food a minute; medium-speed eaters downed about 2.5 ounces and slow eaters ingested about 2 ounces.
The researchers also found "very strong gender differences" in eating rates, Melanson said. During a lab lunch, men consumed food at the rate of about 80 calories per minute while women consumed an average of 52 calories.
"The men who reported eating slowly ate at about the same rate as the women who reported eating quickly," Melanson said.
The second study found a close association between eating rate and body mass index, with subjects who had high BMI typically eating faster than those who had a low BMI.
Melanson, director of the Energy Balance Laboratory at the University of Rhode Island, speculates that fast eating may be related to having higher energy needs, since men and heavier people generally have higher energy needs.
She wants to do additional studies to see how specific slow-eating techniques influence appetite and weight loss, and whether teaching fast-paced eaters to eat slowly can help with weight management.
(Contact Lee Bowman at BowmanL@shns.com.)