Both are important. Diet affects weight loss more than physical activity does. Physical activity, including exercise, has a stronger effect in keeping weight from coming back after weight loss.
Losing weight with diet alone and without physical activity can make people weaker. This is because of age-related losses in bone density and muscle mass. Adding resistance training and aerobic exercise to a weight-loss program helps prevent the loss of bone and muscle.
These are the exercise guidelines for most healthy adults from the U.S. Department of Health and Human Services:
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Aerobic activity. Get at least 150 minutes of moderate aerobic activity a week. Or get 75 minutes of vigorous aerobic activity a week. You also can get an equal mix of the two types.
Aim to exercise most days of the week. For even more health benefits, strive for 300 minutes a week or more of moderate aerobic activity or 150 minutes of vigorous activity. Exercising this much may help with weight loss or keeping off lost weight. But even small amounts of physical activity can be helpful. Being active for short periods of time during the day can add up and give you great health benefits.
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Strength training. Do strength training exercises for all major muscle groups at least two times a week. One set of each exercise is enough for health and fitness benefits. Use a weight or resistance level heavy enough to tire your muscles after about 12 to 15 repetitions.
Moderate aerobic exercise includes activities such as brisk walking, biking, swimming and mowing the lawn.
Vigorous aerobic exercise includes activities such as running, swimming hard laps, heavy yardwork and aerobic dancing.
Strength training can include use of weights or weight machines, your own body weight, resistance tubing, or activities such as rock climbing.