Women who exercise regularly and have constant niggling injuries are being warned about running themselves down.

An Otago University study found 44 percent of women are expending more energy than they are consuming.

Study author Doctor Katherine Black said when insufficient energy intake is combined with exercise the body acts to conserve energy in a way that can be harmful to health.

She said not eating enough can have detrimental effects on reproductive and bone health, and also cause more injuries. 

But, she said the effects can also be a little more subtle:

"You may see increases in colds and flus and other sorts of infections and you'll probably feel more tired than normal."