Losing weight is hard to do.

Some of the USA TODAY readers who participated in the seventh annual Weight-Loss Challenge, especially those who are middle-aged or older, know it well. After losing 10 pounds or so, they hit a brick wall and their weight loss stalled.

Cindy Groover, 54, of Palm City, Fla., who cut calories and walked 3 miles a day to lose 15 pounds since late September, says: "When I was young, I could drop 10 pounds in two weeks by going on a low-carb diet. These days, it just doesn't happen fast."

Tom White, 66, of Waukegan, Ill., who has shed 17 pounds since late September by cutting back on his food intake and working out regularly, describes himself as "a slower loser."

And Jonieta Stone, 68, of Scottsdale, Ariz., lost 11 pounds recently and says losing weight is "taking much longer and is much harder" than ever.

All three of these readers volunteered to try out the USA TODAY eating and exercise program at the end of September. The program was designed to help dieters lose 10 pounds in 10 weeks, but it was more of a struggle for some than for others.

National obesity experts aren't surprised by the experiences of these three dieters.

Research shows that people usually drop about 5 percent to 10 percent of their starting weight in the first three to six months on a program. For many people, that's a loss of 10 to 20 pounds. After that, some people hit the wall and their weight plateaus.

Losing this much is the "sweet spot" for many people, but if they "want to go beyond that 10 percent loss, there is going to be some pain and suffering," says Tim Church, director of preventive medicine research at the Pennington Biomedical Research Center in Baton Rouge. "You are going to have to limit your calorie intake without starving yourself."

One reason it's difficult to drop more and keep it off is there's a cascade of biological responses designed to return dieters to pre-diet levels. A hunger hormone called ghrelin increases, and a fullness hormone called leptin decreases, research shows. In essence, your body defends its own weight, Church says.

And other factors are at work that may make weight loss more difficult for some people than others, such as genes, loss of muscle mass, lower overall levels of physical activity, deeply entrenched poor eating habits and changes in sex hormones, says Anne McTiernan, director of the Prevention Center at the Fred Hutchinson Cancer Research Center in Seattle. She has conducted several weight-loss studies with middle-aged people.

She says there could be some genetic variability in people's ability to hold on to or lose weight: "There are so many genes controlling this."

Changing levels of hormones, including estrogen in women, also affect weight as people age. Scientists at Pennington found women had lower metabolism after menopause than before.

The postmenopausal women in one study burned an average of 100 to 150 fewer calories a day just resting and doing their everyday activities, and they were less physically active for a total drop of about 200 calories a day after menopause, says lead researcher Jennifer Lovejoy, who now works for a health coaching company, Free & Clear in Seattle.

The lower metabolism appears to have to do with changing levels of estrogen and not changes in muscle mass, she says. And there is evidence that a lack of estrogen increases appetite and can cause specific cravings for certain foods, especially carbohydrates and fats. That means women need to be careful about consuming too many cookies, cakes, candy bars and chips, she says.

Lovejoy recommends that women in their early to mid-40s begin gradually increasing their physical activity and watching their dietary habits to help offset metabolic changes that can lead to weight gain with menopause.

Keeping track

McTiernan says a major obstacle for many people is their life-long unhealthy relationships with foods and beverages.

"If someone is eating a bag of Oreos every night, that's extremely hard to change," she says. "They need to replace that habit with something else that will make them feel good. That's a big challenge."

Dieters who have the easiest time slimming down are those who need to modify only one or two unhealthy habits, she says — for instance, people who need only to stop drinking high-calorie beverages such as juice, regular soda or alcoholic drinks.

One of her recent studies showed that middle-aged women lost an average of 21 pounds over several months if they did 45 minutes of aerobic exercise five days a week and limited their calories to 1,200 to 1,800 a day. Those who were the most successful wrote down what they ate, prepared food at home instead of eating out and made weight loss a priority.

Some women can consume only about 1,200 to 1,500 calories a day if they want to lose weight, McTiernan says. That's not much food, so many people get tired of dieting, she says.

And many may think they're limiting their calories to 1,500 a day, but they are actually eating more, Church says. "What people think they are doing and what they are actually doing are two different things."

It's important to track calories either with a daily diary or log, he says. Research shows that dieters who do that lose twice as much as those who don't.

After you lose weight and become smaller, you need fewer calories to maintain your smaller body, Church says. And that means you have to pump up the physical activity to get that negative caloric balance.

He recommends both calorie-burning aerobic exercise such as walking, jogging, biking and swimming, and strength training, which helps preserve lean muscle. As you age, you are slowly losing muscle mass and gaining fat mass, he says. Strength training helps tone your muscles and minimize the loss.

McTiernan agrees. Women who strength-train make their muscles more efficient at burning calories. It's especially important to preserve muscle when dieting, she says. If you diet without physical activity, you are going to lose muscle mass as well as fat mass.

And there's a big weight-loss advantage to physical activity. McTiernan's studies show that dieters who exercise in addition to cutting calories lose about 3 to 5 pounds more in the three to six months they are on a program than those who don't.

Moving is important

Many dieters who participated in the weight-loss challenge say their struggles may be a result of a general slowing down in their physical activity.

White, a retired information technology consultant, works out several days a week lifting weights and riding an exercise bike. He goes to Florida for several months a year, and there he walks on the beach, swims and bikes.

Still, he says he's not as physically active as he used to be. "When you are younger, your kids keep you going."

Stone, a retired college professor, says some older people have a lot of aches and pains, so they don't want to move much. "Among friends my age, I am the exception," she says. "I work out hard, am very strong, can still jog and ride bikes with my grandchildren, but there are days when I hurt and am tired."

Many 'competing interests'

Nan DiGangi, 45, of Fort Wayne, Ind., who has lost 13 pounds in the last few months, says it's harder than ever before to slim down because "I have more obstacles — dealing with family, job, grandkids — and you put yourself on the back burner."

Many middle-aged people are often really busy and have a hard time sticking to their eating and exercise programs, says Donna Ryan, president of the Obesity Society, a group of weight-loss researchers and professionals.

"It's the time in your life when you have the most competing interests. You may still have kids on the payroll. You probably reached the point in your career where you have a fair amount of responsibility. You are probably at or near the point of having to take care of your parents," she says.

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With all that on your plate, it may be more difficult to do the four things that are critical to success: create a calorie deficit, monitor food intake, track exercise expenditure and weigh yourself regularly, Ryan says. If you adhere to those basics, chances are you'll succeed.

People shouldn't get discouraged if they lose 10 pounds and then plateau, McTiernan says. Instead, they should focus on not gaining that back by staying physically active and sticking with their healthier habits.

"If you hit a plateau, then stay there for a while and try again in a couple of months."