In this article Chris outlines his theory (backed by science and his own experimentation) how food intervenes with energy, thus productivity. I have to agree. It’s not just what we eat but how we consume food.
Try this yourself: eat your normal lunch (fast food, processed foods) in your normal habit (eating fast) and keep a journal of how you feel for the next couple of hours. Write it down. Be honest. Next, the following day bring your lunch (healthy-unprocessed food) and eat this mindfully (slowly, thinking about each bite) write down how you feel. You decide, which feeling is more energetic and productive.
Being more productive is hard. Change is hard. Make small consistent changes over time and you will become more productive.