What is the meaning of "gross or subtle" physical food as nutriments? In the Sammaditthi Sutta it describes the 4 nutriments, and the first one - physical food as nutriment - is said to be "gross or subtle". What is the meaning of these terms?

I read that the term for nutriment (ahara) should be understood akin to paccaya (condition). So I am guessing that "gross or subtle" has something to do with the nature of food as conditionings?

Edit: The commentary adds that "And here, in a basis that is gross, the nutritive essence is limited and weak; in one that is subtle, it is strong" and "it is the basis that dispels fatigue, but it is unable to preserve; but the nutritive essence preserves, though it cannot dispel fatigue. But when the two are combined they both dispel fatigue and preserve."
Food is manifold, Mr/mrs. Shai106.

There is food which sustains the physical body and there is food which sustains the mind. Food for the eye - consciousness, food for the ear - consciousness, nose - consciousness, thong - consciousness, body - consciousness, intellect - consciousness. These six kinds of food can be considered as subtle food.

If one for example compares the nutrition of certain gross food (normal burger) so ones joy will not last long. If one put the burger an sublime image on it, let be say "Shai106-honey burger" one would have both, the gross and the fine food and this will last long. If there is just the food of "Shai106" it does not last long. So when mind get good inter-nurished between sense and intellect, its a long lasting nourishment not so like it is if there is just one sense impressed. 

That's why they do not sell cars any more but images. Not gross food but identification, so that it last long. This is but a simple practical explaining.

When we talk about moderation in food, all kinds of food are meant with it. Simply to maintain the life and not for the pleasure of it.

A straight explaining in regard of food, how to use food to abound food, can be found in the Puttamansa Sutta: A Son's Flesh

You may also, of course more in detail, read The Four Nutriments of Life - An Anthology of Buddhist Texts

Also a good talk, for your daily and practical use (since not only the food but also how to gain it is most relevant), in regard of food and livelihood, from the gross to the fine art of livelihood is: Right livelihood - The Noble Eightfold Path

In MN36 the talk is also on the matter of mental and bodily development and catches also the point.

If it should be more lit. Abhidhamma, this work might be of interest: Kalāpas – Groups of matter

(Note: this answer has not been given with the agreement to be means of trade or the purpose of/for trade and/or keep people trapped and bound. How you handle it lies in your sphere, but does not excuse the deed here either.)