It has been six weeks since I started my journey to lose weight and make lasting lifestyle changes. I'm still eating whatever I want, but the portions are smaller and I'm making healthier choices most of the time, such as more fruits, vegetables and whole grains. The weight is coming off, and clothes that I haven't worn for years are now beginning to fit.

Honestly, this adventure seems relatively easy so far. How can that be? I believe it is thanks to the most important addition I've made to everyday life: exercise.

This has happened to me before. The last time I lost a great deal of weight (75 pounds in 9 months), I was a nanny in Massachusetts. My only diet change was to cut back on sweets. It was exercise that made the difference. I not only did a lot of walking, aerobics and tennis, but I also was pushing two small children around the neighborhood in a double stroller and carrying them (or the laundry) up two to three flights of stairs three or four times a day. (I worked in a very big house.)

Just cutting back on calories without adding exercise makes the job of losing weight that much harder. Your body will begin to work against you. "From prehistoric man, we have a natural inclination to retain body fat," said John Dur, owner of the Henniker Fitness Center. "It is in our genes. As soon as the body senses it is being deprived, it reduces its metabolic rate."

Because the human body is programmed to protect itself from famine, the minute it detects a reduction in calories, it slows down the metabolism. The more we cut calories, the slower our metabolism gets. This is especially true for women since our bodies fight to hold onto a certain amount of fat for childbearing.

The only way to combat this natural phenomenon is with exercise. Get moving, and you will boost your metabolism and force your body to get the energy it needs by burning stored fat.

A balanced exercise plan consists of cardiovascular activity and strength training. The first is probably the most common. Also known as aerobic exercise, cardio includes activities such as walking, running and aerobic dance; anything that gets the heart pumping. Not only does this form of exercise burn calories, but it also promotes cardiovascular health by strengthening the heart and lungs.

The second, strength training, is often underrated in its importance to weight loss and overall fitness. "People thought all you had to do was walk on a treadmill and burn calories," said Lindsay Barford, manager of Henniker Fitness Center and a certified personal and athletic trainer. "Strength training increases the calories you burn throughout the day, even when you are just sitting at your computer."

How is this possible? Strength training develops lean muscle tissue. The body burns 40 to 45 calories a day for every pound of lean muscle tissue. Compare that with only two to four calories a day it burns for every pound of fat. Also,

studies indicate that strength training promotes strong bones. "At age 30, we start to lose bone density," noted Barford. "We can't stop it, but we can slow it down. And it is never too late to start."

Before beginning an exercise program, it is a good idea to check with your doctor, especially if you have any medical issues or it has been a long time since you've exercised. For strength training, the ideal option is to find a gym with accredited trainers who can design a specific plan for you.

The Henniker Fitness Center offers with each membership a free orientation lesson on the weight machines and a personalized exercise program geared to your fitness goals. "It is hard to get strength training outside a place like this unless you have the equipment at home," said Barford.

If the idea of going to a gym is intimidating, or if you are a beginner, there are a number of books and videos available to help you get started. I recommend Dr. Miriam Nelson's series of "Strong Women" books. Nelson, director of the Center for Physical Activity and Nutrition and associate professor of nutrition at Tufts University, has developed a weight-training program to do at home with hand and ankle weights.

I go to the gym at least three days a week. I do 30 minutes of cardio exercise on the treadmill and elliptical machine. Then I do about 30 minutes of strength training on the Nautilus machines. While the weather is still nice, I go for a 30-minute walk on non-gym days. So far, I average four to five days of exercise each week.

I can't say enough about how much better I feel. My knees hurt less, and going up and down stairs is not the problem it was six weeks ago. Also, I find myself better able to deal with stress, probably thanks to endorphins, the feel-good hormones released during exercise.

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