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I'm a scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. For the past ten years, our laboratory has studied the health benefits of strength training. My particular research has shown that strengthening exercise can prevent the loss of muscle and bone that debilitates so many women later in life. Along the way, I've become increasingly excited by another discovery: that strength training is also a remarkably effective aid to weight loss.

You may have read my first book, Strong Women Stay Young, which explains the benefits of weight lifting. The book was based on my research, published in the Journal of the American Medical Association (JAMA), showing that strengthening exercise not only makes women stronger, but also builds bone, improves balance and flexibility, and increases energy. I had recruited forty women who were at risk for muscle and bone loss: they were postmenopausal, sedentary, and not taking hormones. Before they started, I warned the volunteers that they'd have to avoid changes that could confuse our findings: for the next year they could not lose weight or begin aerobic exercise.

The women were very conscientious. But they changed anyway—they couldn't help it:

Dorothy, who had been wearing size 16, was careful to maintain her weight. But after a few months of strength training, she noticed that her clothing was becoming loose. "I knew I needed a smaller size, so I bought 14s," she says. "Then I had to take them back because I needed 12s and sometimes 10s. My legs and hips became trimmer, and my arms got much more firm."

Verna was shapelier, thanks to strength training. "My weight was never out of control, but there was fat in the wrong places," she says. "My inner thigh flab trimmed up, my upper arms got firmer, and I lost my tummy."

Flora kept her promise not to start aerobics. But her new strength gave her so much extra energy that she took up ballroom dancing. Says Flora, "My girlfriend kept asking me to come with her, but I used to think the muscles in my calves would bother me. Now I can dance all night."

While this project was going on, my colleagues and I began a pilot study to see if strength training could prevent muscle and bone loss in another especially vulnerable group: women who were losing weight. Many women are shocked to learn that this is an at-risk category. They assume that when they lose weight, the only thing their body burns up is unwanted excess fat. Until recently that's what doctors and scientists believed too. But researchers have taken a closer look and discovered something disturbing: When women diet, at least 25 to 30 percent of the weight they shed isn't fat, but water, muscle, bone, and other lean tissue. This is true no matter how much protein and calcium their food plan includes. And the faster they lose weight, the larger the proportion that isn't fat.

I had been greatly encouraged by research from the University of Michigan showing that strength training could help women preserve muscle while they lost weight, and I wanted to explore this further. For our pilot study, my colleagues and I put ten overweight women on individually customized food plans designed for slow but steady weight loss. Half of them came to our laboratory twice a week and did strength training; the others just followed the diet.

Our diet-only volunteers lost an average of 13.0 pounds during the study. The women who strength trained lost about the same amount—13.2 pounds. But the scale didn't tell the whole story. A dramatic difference emerged when we looked at body composition. Women in the diet-only group had lost an average 2.8 pounds of lean tissue—mainly muscle—along with the fat. In contrast, the women who'd done strength training actually gained 1.4 pounds of lean tissue. So every ounce they lost was fat. Indeed, since the new muscle replaced fat, their total fat loss was 14.6 pounds. They lost 44 percent more fat than the diet-only group.

Our volunteers were thrilled to find an exercise at which they could excel. They became stronger, fitter, healthier, much more physically active—and were filled with self-confidence. Pat, who lost twenty-nine pounds:

Strength training helped the weight loss a lot. My metabolic rate went up, so I can eat more. I wear a smaller size than if I'd just lost weight and didn't build muscle. My doctor was very happy because my cholesterol and blood pressure went down. I have so much more energy—I feel invigorated. It gives you more self esteem, a more positive feeling about anything you want to do.

After Strong Women Stay Young was published, I received hundreds of letters, faxes, phone calls, and e-mail messages from readers all over the world. Many of these women found, to their great delight, that strength training had helped them lose weight. Inches, as well as pounds, had vanished. They looked better, felt healthier. Becoming strong had boosted their energy and vitality. Letter after letter spoke of new happiness and self-confidence.

Diana: I've only lost ten pounds, but people are saying, "You are losing so fast!"

Bobbie: I'm now 42, but I feel like I'm in my twenties! Two years ago I was ten pounds heavier, had constant back pain, and felt slightly depressed. I was walking four miles a day, and thought this was all I should be doing. Then I started weight training, and everything changed. I wish I had discovered this ten years ago. If feel so good! I ski, and I do in-line skating—they were easy to master once I was strong.

Along with the progress reports came questions:

Should I be doing aerobics too—and if so, how much?
What food should I eat?
Do I need to take a vitamin supplement if I'm losing weight?

I have the great privilege to work at Tufts University, along with dozens of other scientists involved in exciting new research on nutrition, exercise, and health. I discussed the readers' concerns with my colleagues, as well as with some of the many experts from other institutions who visit our laboratories to exchange information. I began to realize how much different specialists have learned about what works for healthy long-term weight loss. What was needed, I became convinced, was a comprehensive program that would pull together the essential pieces. I wanted to combine new findings about strength training with up-to-date information on nutrition and physical activity. My aim was to come up with a practical program that would not only help women lose weight permanently, but would also become the foundation for a long, healthy, vibrant life. That's why I decided to write Strong Women Stay Slim.

* * *

Over and over, I've seen strength training open the door for weight loss. Dianne, one of the women in the group that helped me refine the program for this book, isn't sure exactly how much she weighed when she started because her scale doesn't register above 300 pounds. "I figure it was about 340," she says. Aerobic exercise was out of the question for her—even five minutes of walking left her winded and made her legs ache unbearably. "I wanted to exercise, but how could I? I was a ball of jelly," says Dianne. "Then I read about strength training. Boing! A light went off in my head. Here was something that would bring me to a point where I could be more active."

Dianne became the star of our test group, rapidly graduating from 3-pound dumbbells, to 5-pound, then 8-, 10-, and even 15-pound dumbbells for some exercises. Other changes followed. "Strength training gave me an overall sense of wellness, and it snowballed," says Dianne. "I started to eat better within two weeks. I could see myself getting more involved in my own life, being better to myself. I became more active in little ways—instead of sending my daughter upstairs to get my earrings or my watch, I'd go myself. I frequently parked at the far end of parking lots."

By the end of ten weeks, Dianne had lost more than 35 pounds—and she glowed with vitality. The woman who could barely manage a five-minute walk when she started had joined a gym and was regularly walking thirty minutes on a treadmill. Says Dianne, "It's been a great uplift—I feel so much more positive." I'm thrilled with Dianne's progress so far and look forward to watching her continue.