It is easy for me to make the connection between nutrient deficiencies and physical illness. The first thought that comes to mind is how scurvy, a deficiency in vitamin C, was cured by adding lime juice into the daily grog ration. Then I think of something like Rickets. It was previously a medical mystery and seen in both people and animals, including a historic report of lion cubs with Rickets at the London Zoo. Finally, it was found to be a deficiency in vitamin D.
Nutrient deficiencies can be linked to mental health as well. Correcting these issues can be an easy way to boost mood.