One must care for both his body and his soul, but he need give only that which is necessary to the body, whereas he must provide amply for the needs of the soul. One who cares only for the body weakens the soul.

One must be aware of the needs of both his soul and his body:

Your soul and body require care and consideration. The nourishing and proper care of the soul occur by making it accustomed to accepting admonishment and wisdom, guiding it wisely, teaching it good character traits, and preventing it from [acting upon] its animalistic desires. But for nourishing and proper care of your body, you must regularly provide it with good-quality, pleasant food and drink that suit its nature, bathe in tepid water, and constantly take notice of what is good for it and what its needs are.

We need to make sure that our bodies are able to survive, but with regard to the soul, we must give it even more than it requires:

If your thoughts are on care of your body and you give it all your attention, you will overlook the care of the soul. Likewise, if your thoughts are turned to enlivening your soul, your attention to it will cause you to overlook many matters of your body. If you are devout, you will give priority to your eternal soul over your mortal body, regarding and paying attention to it. But do not disregard matters that are essential to your body. Do not overburden it and weaken it, as this will cause weakness in both body and soul. Rather, give your body the sustenance that it needs to function, and give your soul more wisdom and admonishment than is within its power [i.e., than it requires]. (Rabbeinu Baĥya ibn Pekuda, Ĥovot HaLevavot 8:3)

Further reading: For more on caring for the soul, and the connection between the body and the soul, see pp. 222ff.