It makes sense to me that the healthiest writer would also be one of the most prolific as well. Let's think about this for a second. If you have health problems, such as being overweight or having sleep issues, it's more than likely to affect the efficiency of your writing. On the other hand, if you're eating well and you exercise frequently, you probably have the energy you need whenever you're inspired to put your words on paper. Those are the most obvious points, but let's dig a little bit deeper.
The healthiest writer will probably be the most creative as well. A lot of creativity is innate, but what good is that creativity if you can't access it? The healthiest writer will probably exercise more often and have less of a plaque buildup in the heart, allowing blood and oxygen to flow more freely throughout the body, including to the brain. Studies have shown that having more oxygen in the brain can help memory and your speed of thinking. As a result, a healthier writer would be more able to access those creative thoughts. If that is true, an extremely creative person not getting into decent shape is like a talented person refusing to refine and work on his or her gift.
Exercise and a proper diet also have a major effect on mood. Let's face it. A writer who is mired in depression, despite the success of Edgar Allen Poe and Ernest Hemingway, are much less likely to believe in herself than one who feels generally positive about the profession. Exercise increases the level of endorphins in your body, which act as little happiness boosters and pain relievers. High amounts of sugar, fat and caffeine in an unhealthy diet can make you feel major highs and major lows. If one of those lows occurs during a writing session, you might be out of luck. A balanced diet helps you to feel more balanced while you're writing. You may not have the jolt of caffeine and the boost of productivity that comes with it, but in my opinion, the crash makes it not worth it.
Writing can be a lonely career, but the healthiest writer is much less likely to spend time alone because of the social activity of exercise. Playing a sport gives you an opportunity to hang out with the members of your team. If you are training for a race or some other event, you are able to train with a partner or a group as well. Keeping yourself in shape, even though you have a sedentary career, is one of the best ways to increase the number of people in your life.
Trying to be the best writer you can be is important. Equally important is to be the most creative, happy and social writer possible by being the healthiest writer as well.