Is it really true that my actions in this life determine what kind of body I will be reborn with in my next life?

Lama Ole’s answer:

Yes! The body we have now is a consequence of karma. What we do in this life has four results. First, it determines the experiences we will have when we die—which subconscious impressions come up and whether we experience happiness or suffering. Second, if we are reborn as humans, it governs what kind of body and genetic constitution we get. Third, it determines the country we are born in—a pleasant or unpleasant one, a rich or poor one, and so on. And finally, what we do in this life molds the attitudes we develop when we’re reborn—whether we are basically friendly, like people, and want to help them, or whether we are generally difficult and bring people suffering and problems.

These are the four results of previous thoughts, words, and actions. One can say that the body is the solidified state of consciousness; it reflects the impressions that lie in the mind. And thus one can influence the mind through the body and the body through the mind.