Här är en artikeln som går igenom problemen med att utföra precisa studier när det gäller hälsosam mat.
Det beskrivs hur det förr i tiden var lättare att utföra enklare men fortfarande vetenskapliga tester när det gäller t.ex skörbjugg (vitamin C brist).

In 1747, a Scottish doctor named James Lind wanted to figure out why so many sailors got scurvy, a disease that leaves sufferers exhausted and anemic, with bloody gums and missing teeth. So Lind took 12 scurvy patients and ran the first modern clinical trial.

The sailors were divided into six groups, each given a different treatment. The men who ate oranges and lemons eventually recovered — a striking result that pointed to vitamin C deficiency as the culprit.

This sort of nutritional puzzle solving was common in the pre-industrial era. Many of troubling diseases of the day, such as scurvy, pellagra, anemia, and goiter, were due to some sort of deficiency in the diet. Doctors could develop hypotheses and run experiments until they figured out what was missing in people’s foods. Puzzle solved.

Nu för tiden är inte problemen så kortsiktiga. Nu gäller det att ta reda på hur hela dieter påverkar under flera decennier. De stora problemen som vi står inför idag är sjukdomar som växer fram under lång tid, hjärt och kärlsjukdomar, cancer, diabets etc.

Today, our greatest health problems relate to overeating. People are consuming too many calories and too much low-quality food, bringing on chronic diseases like cancer, obesity, diabetes, and heart disease.

Unlike scurvy, these illnesses are much harder to get a handle on. They don’t appear overnight; they develop over a lifetime. And fixing them isn’t just a question of adding an occasional orange to someone’s diet. It involves looking holistically at diets and other lifestyle behaviors, trying to tease out the risk factors that lead to illness.

Today’s nutrition science has to be a lot more imprecise. It’s filled with contradictory studies that are each rife with flaws and limitations. The messiness of this field is a big reason why nutrition advice can be confusing.

It’s also part of why researchers can’t seem to agree on whether tomatoes cause or protect against cancer, or whether alcohol is good for you or not, and so on, and why journalists so badly muck up reporting on food and health.

To get a sense for how difficult it is to study nutrition, I spoke to eight health researchers over the past several months. Here’s what they told me.

För att förstå problemen och varför det idag är så svårt att enas om slutsatser när det gäller vad som är nyttigt och farligt så bör du läsa artikeln.
Här är slutsatsen:

On questions of how to eat, none of the researchers talked about seeking out specific foods or cutting others. They didn’t make bold claims about the effects of particular fruits or vegetables or meats beyond simply suggesting that a ”dietary pattern” could be ”healthy.”

This broad advice was reflected by a consensus statement from a very diverse group of nutrition researchers, who recently got together to discuss what they agree on about food and health.

Here’s what they came up with:

A healthy dietary pattern is higher in vegetables, fruits, whole grains, low- or non-fat dairy, seafood, legumes, and nuts; moderate in alcohol (among adults); lower in red and processed meats; and low in sugar-sweetened foods and drinks and refined grains.

Additional strong evidence shows that it is not necessary to eliminate food groups or conform to a single dietary pattern to achieve healthy dietary patterns. Rather, individuals can combine foods in a variety of flexible ways to achieve healthy dietary patterns, and these strategies should be tailored to meet the individual’s health needs, dietary preferences and cultural traditions.

Anyone who tells you it’s more complicated than that — that particular foods like kale or gluten are killing people — probably isn’t speaking from science, because, as you can see now, that science would actually be near impossible to conduct.

I asked 8 researchers why the science of nutrition is so messy. Here’s what they said.