请问保持身材用英语怎么说？

keep fit 比较正宗，适合用于口语或者不正式的书写

保持身材 keep my figure; stay in shape

我跑步是为了保持身材。
I am running so that I can stay in shape.

我正在节食以保持身材不变(不发胖).
I'm dieting to keep my figure,ie in order not to get fatter.

keep fit

to be in good figure

to maintain your body shape/figure

keep a slender figure
keep a slim figure