What is Wholesome and What is Unwholesome food?

एवंवादिनम् भगवन्तमात्रेयम् अग्निवेश उवाच ---- 

कथमिह भगवन् ! हिताहितानाम् आहारजातानाम् लक्षणम् अनपवादम् जानीमहे ; 

हितसमाख्यातानाम् अहितसमाख्यातानाम् च 

मात्रा-काल-क्रिया-भूमि-देह-दोष-पुरुष-अवस्था-अन्तरेषु विपरीतकारित्वम् उपलभामहे इति॥३२॥

तमुवाच भगवानात्रेयः -- यद् आहारजातम् अग्निवेश समान् चैव शरीरधातून् प्रकृतौ स्थापयति 

विषमान च  समीकरोति एतद् हितम् विद्धि ; विपरीतम् तु अहितम् इति

इत्येतद् हिताहितलक्षणम् अनपवादम् भवति॥३३॥

Translation : When Aatreya said this Agnivesha asked him " Oh Lord ! How we are to know the definition of wholesome and unwholesome foods without exception? And how are we to know the reversal of wholesomeness and unwholesomeness of described foods with changes in amount, time, preparation,region state of body, state of Dosha, individuals,and stages (of diseases) ?

Aatreya said to him " Agnivesha ! Whatever food that will enhance only the dhatus which are in equilibrium and restores equilibrium of the unbalanced dhatus is wholesome and the opposite is  unwholesome. This is definition of wholesome and unwholesome without exception.

Commentary (chakrapani) :  Agnivesha is pointing to difficulty in knowing the wholesomeness and unwholesomeness due to the factors which modify the nature of food. Wholesome becomes unwholesome and unwholesome may become wholesome due to factors like amount etc.For example

Wholesome foods like Red rice if taken in excess or in very small quantities become unwholesome.

The light foods like red rice etc. will become unwholesome in Hemant ritu when Agni is very strong.

Of the two types of Time here universal time is to be assumed as the other time i.e. stages of disease etc. is stated separately as Avastha.

Kriya is processing. Improperly washed or boiled red rice becomes unwholesome. 

Red rice produced in Aanup desha ( Wet and humid regions) becomes unwholesome.

In an obese person red rice becomes unwholesome due to its lightness It is stated in chapter 21 that for obese people food should be heavy and reducing.

Red rice is unwholesome for Vata. Dosha also includes diseases here. 

In childhood there is predominance of Kapha and bitter foods are wholesome but the same become unwholesome in old age due to predominance of Vata. Change in status relates to all factors like amount etc.

Similarly unwholesome food becomes wholesome depending upon quantity etc.

Aatreya is stating the principle. He is not assigning wholesomeness or unwholesomeness to any specific food article but saying that food becomes wholesome or unwholesome due to factors like quantity etc.

Commentary (Vishnu) :   So far most of the verses in this chapter were एकीयसूत्र or opinions of others. Here we are considering शिष्यसूत्र  and गुरुसूत्र (Query by disciple and explanation by the Guru). This indicates that with Aatreyas resolution of the discussion the symposium is over and normal teaching learning process has resumed. Agnivesha is asking a very specific question and Aatreya is answering unequivocally.
In the query Agnivesha is demanding a definition which is without exception. 
While giving such definition 

(H2B) लक्षण

n. accurate description , definition , illustration  Monier Williams Sanskrit-English Dictionary

Aatreya is not specifying any food item. Wholesomness and unwholesomeness is to be decided by the effect of the food on Dhatus which include Dosha, Dhatu, Mala, Agni, Kriya(The meaning here is physiological processes) Sense organs and mind and even soul. Any food which will maintain the equilibrium of all these is termed wholesome and any food which disturbs equilibrium of these is termed unwholesome. Here we should consider two more verses stated elsewhere. 
किञ्चित् दोषप्रशमनम् किञ्चित् धातुप्रदूषणम् ॥

स्वस्थवृत्तौ मतम् किञ्चित् त्रिविधम द्रव्यमुच्यते॥

Matter is of three types: 
1. That which restores equilibrium of Doshas
2. That which pollutes or vitiates Dhatus
3. That which is wholesome for a healty person for long term use


First type is called as drug or medication as it is useful in conditions where equilibrium of Doshas is disturbed (Diseases)
Second type is called a poison as it has deleterious effects on health
Third type is called food as it can be used as staple for a long term (sometimes lifelong)


Same article can be used as any of the three types.
For example: Yogurt or curds 
It becomes a drug when used in Vishamajwara. Peenasa, Atisaara, Sheetajwara, Aruchi, Mootrakrichhra and Thinness  (Ref. Charaka Sootrasthan 27/226) 
It becomes a poison when used in Sharad, Greeshma and Vasant Ritus, Raktapitta and all Kapha diseases.
It is usually used as food and increases Agni, used for weight gain, Aphrodisiac etc.


Another example Vatsanabha or Aconite
Normally it is a poison
When used after Shodhana and Marana it is an excellent drug used in many ayurvedic formulations.
Ashtang Sangraha prescribes it as an additive to foods in a very small dose as a Rasayana


Bhallataka
It is poisonous every time to those who have allergy to it.
Used as drug in many conditions and best for dry piles.
Charaka describes its long term use as Rasayana,
Its nut collected in specific conditions (Called Godambi) is very nutritious dry fruit used alongside almonds and apricots.


In short any material can be used as food, drug or poison and classification depends upon the effects produced in the body. 


This approach to  food materials is very different from moder nutritional approach as the latter depends predominently on food articles or Dravya and not on its Karma or outcome.

Second important verse 
समदोषः समाग्निश्च समधातुमलक्रियः॥

प्रसन्नात्मेन्द्रियमनाः स्वस्थ इत्यभिधीयते॥ Sushrut Sootrasthana 15/44


One whose Doshas, Agni, Dhatus, Malas, Bodily functions are in equilibrium and whose sense organs, mind and Soul are clear , bright , pure and perspicuous is called as Swastha or healthy person. 
Therefore Wholesomness and unwholesomeness is to be decided by the effect of the food on Dhatus which include Dosha, Dhatu, Mala, Agni, Kriya(The meaning here is physiological processes) Sense organs and mind and even soul