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The question

I am a 37-year-old woman. I exercise regularly and eat well, but have always been almost too thin. I would love to try a new cardiovascular activity, but I don't want to lose weight. Is that possible?

The answer

Genetics, physical activity and diet are the three main variables that control both body weight and body composition.

Since genetically you seem to have a fast metabolism, you will have to be extra conscious of replacing any calories you expend while exercising with nutritious food. No activity is off limits, as long as you are diligent about replacing the calories you burn while exercising.

For example, if you go for a 40-minute run before work, it's not enough to eat a slightly larger than usual breakfast. You need to eat approximately 500 calories more a day than you would normally. In addition to your regular three meals, you could eat a mini-meal before your run, another one mid-morning and have a slightly larger than usual lunch.

This isn't an excuse to eat junk food, however. Replace the calories you burn while exercising with healthy nutritious food like whole grains, fruits, vegetables and lean proteins.

If you are exercising for longer than an hour, consume easy-to-digest calories while you exercise. I like GU running gels, but you may prefer sport drinks, fruit juices, bananas or other simple carbohydrates.

Trainer's Tip: since you have a tendency to be slim, I suggest that you do some strength training as well. But abandon the typical endurance-based 12 to 15 repetition range most women tend to gravitate toward, and do 8 to 10 reps of each exercise with slightly heavier weights. Building some lean muscle mass could give your body some curves.

Send certified personal trainer Kathleen Trotter your questions at She will answer select questions, which could appear in the Globe and Mail and/or on The Globe and Mail website. Your name will not be published if your question is chosen.

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The content provided in The Globe and Mail's Ask a Health Expert centre is for information purposes only and is neither intended to be relied upon nor to be a substitute for professional medical advice, diagnosis or treatment.

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