What is the definition for bad health as a result of a lack of food or eating unhealthy food?
Bad health resulting from a lack of food could be termed 'dietary deficiency,' 'malnutrition,' or 'lack of nutrients.' Severe lack of food could be starvation, whereas mild lack of food could be hunger or eating meals slightly less regularly.
Bad health resulting from eating unhealthy food says very little at all except that there is a claim that the unhealthy food is either partly or fully to blame for whatever health problem is apparent from symptoms.
Deficiencies in well-known vitamins (including A, B, C, D, and K) are associated with well-known symptoms, which is why those biochemical substances are termed vitamins.
'Unhealthy food' is a modern parlance. The older term 'malnutrition' spoke of lack of basic vitamins, arising from eating only scraps and not being properly cared for by society. 'Unhealthy food' results from having too many choices about what to eat. People with the means to eat whatever food they choose can still choose 'unhealthy food.' Better-off members of society often fault chronic diseases and other health issues to diet. Cardiovascular disease, diabetes, and allergies are some examples. Scientific research is ongoing as to whether 'nutrition' (establishing what is and what isn't 'healthy food') can significantly help tackle chronic diseases, and also slow progressive diseases. Acute disease caused by 'unhealthy food' would mean the food is dangerously toxic or poisonous, beyond a level tolerable by the body. 'Unhealthy food' of that nature would be much better described as simply poison.