I saw a question about a specific claim that a particular food is harmful (or unhealthy or worse than another food).
It seems to have received a negative reaction from the community, and hasn't been answered.
What is wrong with the question?
This sort of question sounds simple, but is actually very difficult to give a straight answer.
Pedantic note on behalf of the English prescriptivists: Food is better described formally as "healthful", not "healthy".
To explain, I will use an invented foodstuff as an example: Lemon-Lined Lima Beans. There are a large list of problems with the question "Are Lemon-Lined Lima Beans bad for you?"
A very small percentage of people are allergic to citrus. For those people, the lemon is harmful.
We could argue that Lemon-Lined Lima beans could kill if you are unlucky enough to be highly sensitive. However, this doesn't seem to be a satisfactory answer. It is not a reason to avoid Lemon-Lined Lima beans if you are not sensitive.
Many - perhaps all - foodstuffs are lethal if consumed in extremely large amounts in a short period.
We could argue that Lemon-Lined Lima beans could kill if you overconsume. However, this doesn't seem to be a satisfactory answer. It is not a reason to avoid Lemon-Lined Lima beans if you are eating them in moderation.
Some people have deficits in certain nutrients, which would normally be found in a balanced diet.
We could argue that Lemon-Lined Lima beans could save your life if you have scurvy (a deficiency in Vitamin C). However, this doesn't seem to be a satisfactory answer. Most people are not suffering scurvy, and if you were suffering scurvy, this wouldn't be the best source of Vitamin C.
A common problem in the first world is obesity. If Lemon-lined Lima beans are eaten instead of a diet of pizza and burgers, they may turn out to be healthful in comparison, without having any special nutritional power. If they are eaten in addition to a diet of pizza and burgers, they may be adding too many kilojoules to the diet.
Often, there will be studies on lemon juice, and studies on lima beans, but no studies on Lemon-lined Lima beans.
A trained nutritionist and chemist may be able to safely conclude those studies are likely to be applicable, but without that, we are in no position to know whether the contents of the lemon chemically react with the contents of the lima bean - or perhaps trigger hormones in the body that react synergistically. Perhaps, together, they have some physiological effect that they don't have separately.
Similarly, there might be some science about ascorbic acid, and it may or may not be reasonable to conclude that, given Lemon-lined Lima beans contain ascorbic acid, the results apply to them. (e.g. the ascorbic acid may be destroyed during the cooking).
It is much easier to measure the short-term effects on the human body of someone eating a single meal of only Lemon-lined Lima beans, or even to measure a two week diet very high in Lemon-lined Lima beans, than it is to measure the effect of slightly more frequent consumption of Lemon-lined Lima beans over the course of decades.
In vitro tests (e.g. exposing cells to substances in a dish) sound impressive, but are very limited evidence of the safety or danger of a food stuff.
Hence, it is easy for a pseudoscientist to claim a foodstuff causes cancer, but very hard for scientists to find persuasive evidence either way.
Some health foods are most commonly found in the diets of people dedicated to maintaining their health. If Lemon-lined Lima beans is pushed as a "superfood", it may correlate well to people who suffer less lung cancer, but the confounding factor might be that they also don't smoke.
Foods tend to be cultural. A food may be popular in a rich country, and thus its consumption be correlated with long life-expectancies, without it having any healthful effects. For the same reason, it may be correlated with higher rates of heart disease, obesity and cancer, as these are diseases of First World countries.
Proving causation is much harder than correlations. Often claims themselves are limited to correlation.
What if there was strong evidence that consumption Lemon-lined Lima beans decreased the risk of breast cancer, increased the risk of heart disease, decreased the risk of acne and increased flatulence?
Is it harmful or healthful? That is a false dilemma.
Epidemiology is hard. Different small studies may give quite different results. We need many studies, high quality reviews and a lot of time before we can achieve consilience. Epidemiology is a slow science.
Here are some suggestions on how to fix a poor question about food.
Do not ask if a food is "unhealthy"/"unhealthful"/"bad for you" or "worse than" another food. You need to be more specific.
Find a good source of the claim. It helps if it:
If a claim says that the food contains an ingredient that causes a disease, consider simplifying the question into the bit that you doubt. Do you doubt that the food contains that ingredient, or do you doubt that the ingredient causes the disease? Each of those questions is easier to answer than the claim that the food causes the disease.
Be sure to match the claim's association versus causation wording in the question.
Be sure to include the specific disease that you want investigated. Asking if it can cause any disease, or any of a large list makes the question intractable to answer.
Be ready to accept that there are a lot more claims about food than there is evidence about food, and the question may be unanswerable with today's research.