One of the oldest Yoga scriptures on Yoga, Bhagavadgita states that a yogi should take “appropriate diet (yuktaahaara)”. The appropriate here means Sattvic diet given in its chapter on Gunas, also it means appropriate amount. However, Sattvic diet mentioned in Bhagavadgita is not a nutritionally healthy diet. At the time of Bhagavadgita, the air, water and earth quality was definitely better. So, the types of nutrients and toxicity are not mentioned. Also, the Yoga mentioned along with diet, in Bhagavadgita is the Meditation practice and not Hathayoga practice.
Similarly, Yoga Sutras of Patanjali do not talk much about diet. This also has the similar reasons, meaning Meditation is indicated as Yoga.
In Hathayoga, where bodily practices have grown, diet is given more consideration. But, in Hathayoga also, the nutritionally correct diet is not mentioned. Here, certain environment and discipline and dedication for Yoga practice is mentioned.
In the modern world, however, yoga practitioners are mainly looking for health and fitness, which definitely needs a nutritionally correct diet and also non toxic diet as the toxicity has increased. Again, stress has increased multiple times. Thus, a Yoga practitioner of today needs to look at diet in a very different way. Knowledge of Ayurveda is no sufficient, one needs to look at the modern research. But, unfortunately, due to the commerciality of the modern world, the real information is rarely available. So, the proper sources of not only diet, but diet education are also needed.
Thus, a modern Yogi has to spend a very good amount of time in study of diet and nutrition in order to obtain health and fitness. And, in my opinion, it is even more important than the actual Yoga exercises. However, all actions of a human depend on the status of the mind. Whereas, the stress factor in today’s human life has increased multiple times. Therefore, mental peace and mental health/fitness are most important today. That needs proper mental nutrition. Therefore, the modern Yogi should start their practice with considerations of Mind and not of body. This is the interesting shift in Adhiyoga, as compared to the previous methods.