(Abu Hurairah reported Allah's Messenger PBUH as saying, "There is zakat applicable to everything, and the zakat of the body is fasting.")

[IbnMajah]

The human body gets healthier and stronger by Siyaam. Even if man does not  know the wisdom of Siyaam or the healthy benefits of it, he will reap them all upon observing it. 
Benefiting from something does not entail knowing it. The clearest example is using the most advanced computer which amazingly helps us without knowing how it works. Thus, you may reap the fruits of something without knowing how it works. Allah the Almighty orders us to observe Siyaam, so even if one does not know the wisdom behind this Divine Order, he will reap all the fruits of Siyaam and will have a healthy body.