There is no denying that a PhD can take a toll on mental health. Anxiety, depression, and imposter syndrome are things which many of us (if not all of us) have battled with during the course of a PhD. Yet, I think that the physical toll a PhD can take on the body is less frequently discussed.
Personally, the sedentary nature of a humanities PhD which involved long periods of time sitting at my desk at home and desk at the library was not conducive to maintaining a healthy lifestyle. As the PhD progressed and I entered the home stretch of writing up, I found I was taking the easy, time-saving options with respect to exercise and diet. Instead of walking or cycling to the library, I took the bus. Instead of going to the supermarket for groceries, I would order take-away. And don't even get me started on the amount of sugary, caffeinated drinks I was consuming on a daily basis.
It was only after I had come out the other side of the process that I could see the effect all this had on me. I had unknowingly gained just under 20 pounds and become horribly addicted to sugar. A lifestyle reset button needed to be hit, and I am happy to say that I am (almost) back to my pre-PhD weight through walking and pilates, and cut those sugary, caffeine-laden drinks out of my diet (and no doubt, there is the correlation between reducing sugar and reducing weight).
But this isn't a post to garner pats on the back, it's simply a warning about how easy it can be damage your body when the stress of a PhD masks it so effectively. My advice would be to be aware and listen to your body. And undoubtedly, good exercise and a healthy diet would likely boost PhD performance, so added bonuses!!