I have a client in her late 20s. She has been slightly underweight for some time, but is very athletic and lean. She came to me with the desire to increase her overall functional/core strength and to correct her many biomechanical imbalances of the kinetic chain. She did not desire to gain any more weight, but it certainly would not hurt her!
She has made great gains in the desired areas in our work together. However, in a month, she has gained about 10 pounds, mostly muscle and some fat, but she claims is eating no differently, less if anything and exercising more cardio-wise. She is not happy about this unexplainable weight gain. It is my feeling that this weight gain is beneficial for her, since she looks and feels healthier. However, it seems odd that for years she has been the same weight, and all of a sudden this has changed. I suspect it is because she is doing more cardio, eating less and slacking off on her resistance training, and therefore, her metabolism has slowed.
I explained this to her, advising her to cut back on her cardio to 6 hours a week as opposed to 10, increase her resistance training frequency and increase her caloric intake by about 500 a day. I also advised her to maintain her weight because she was slightly too thin prior to the weight gain. Could she have slowed her metabolism enough to contribute to this sudden weight gain? Will she be able to speed up her metabolism by the regimen I have suggested? Any suggestions or insights you have will be much appreciated!
It sounds like you are on the right track. I will cover a few areas that you can look into more deeply.
First, contrary to conventional wisdom, less calories doesn't always mean less weight. It seems you are at least partially aware of this. The problem is, the less you eat (within reason, going from 1800 to 1300 is a good example) the slower your metabolism gets. Food is actually thermogenic; some foods take more energy to digest than others. Sugar/starches take very little to digest while whole food proteins take more energy to digest.
This leads to my second point, which is its not just how much you eat but what you eat. Many "experts" will tell you that 2000 calories of candy is the same as 2000 calories of chicken. Those of us who live in the "real world" know that is simply not true. If you don't believe me, buy two mice, feed one candy and the other chicken and see what happens.
What does this all mean? It means you must find out what she has been eating and how much. I tell all of my clients that I would rather have them do 300 calories worth of exercise than eat 300 less calories. I also have yet to see anyone overeat on vegetables, meat and fruit (in the proper combination).
The last thing I'd like to mention is muscle. You said she gained weight, but most of it was muscle and a little fat. One thing that can happen when you increase your lean muscle mass is that your skin becomes more taut and harder to get an accurate body fat measurement. This can lead to the apperance of body fat increase.