I think you have to eat well, exercise lots, do some strength training and hope for the best.

You WILL lose some lean body mass during a large weight loss, it is inevitable. But that doesnt mean your fat to muscle ratio will actually alter. You wont need as much muscle to support your weight and you will have less fat so it will remain at a healthy ratio if you do it right.

Fast weight loss is generally not all fat loss. You just dont lose 4lb of fat per week, sorry to say. Very low calories tend to encourage lean tissue loss as well. Slow, steady and with plenty of exercise is the way to go.

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Jacqui