preposition + Noun without article

In the first sentence, you use food in general ( hence we don't use any article, like "the"). By food, you mean any food, unspecified. Whereas, the second sentence, which uses the only known definite article"the", you have knowledge of the type of food required. "It must be obtained from food (=any food)". In "it must be obtained from the food", you have a particular brand in mind. You know which brand to obtain what you are looking for.

If you referring to unspecified food or food in general, you don't need to put the definite article "the" in front of food. On the other hand, if you are referring to specific food, you use "the" in front of food. Look at the following sentences to know the difference:

We gave him food and drink.

We don't sell food.

The food at the restaurant was terrible.

The food you gave us has been eaten.

The food is very spicy.