How can motivation be increased by improving brain chemistry with nutrition and activities? I'm reading some material about motivation and brain chemistry.
One of the interesting articles is "7 Ways to Increase Motivation by Improving your Brain Chemistry". I'm interested in a more scientific treatment of the topic. Thus,


How can motivation be increased by improving brain chemistry with nutrition or various activities (sport, exercise, relationships etc.)?
What scientific articles exist on this topic?


It would be great if there was some article targeting this exact topic with references.
The best thing you can do is avoid nutritional deficiencies and exercise.  In general, it's the same as for the rest of the body.  Many people have subtle nutritional deficiencies that they may never know about their whole life just based on eating habits.  As Chuck Sherrington said, neurotransmitter-based treatments are subject to homeostatic compensation, but if you have a deficiency, it may be more complicated.

Nutrition

Particularly, the same things that increase pulmonary/circulatory health are going to increase brain health[1,2].  A large contribution to health in the developing fetal brain comes from the mother's diet.  There are lots of diseases associated with maternal nutrtional defecits[3,4].  

Exercise

Running and other physical activities increases neurogenesis [5,6].  But does it do it "intelligently"?  I don't know.  More neurons isn't automatically more motivation or intelligence.  But it is a stress-relief technique, and too much stress can certainly kill motivation.

Behavior

You didn't ask, but behavioral practice is probably another strong factor.  For example, breaking jobs into smaller tasks: getting the first small task done can give a motivational boost towards getting the second done (and so on).  They've actually found a correlation between paying smaller loans off first (even though not mathematically more efficient in terms of costs) is more likely to lead to loans being paid off all the way because of the human element[9]