For decades, we were told that fat was the problem. Butter was pushed aside, egg yolks became something to limit, and supermarket shelves began filling up with products proudly labelled “low fat” or “fat free”.

But when manufacturers removed the fat, they still needed the food to taste good, which often meant adding sugar, refined starches, flavourings and various other ingredients to replace what had been taken out. Somehow, a naturally fatty food became the thing to fear, while a brightly packaged product containing a long list of ingredients could earn itself a health halo simply because it was low in fat.

Nutrition science is complicated, and this shift can’t be attributed to one study, one scientist or one organisation. However, a fascinating (and rather uncomfortable) historical analysis published in JAMA Internal Medicine gives us some additional insight into how the story of fat, sugar and heart disease unfolded.

The authors examined internal documents belonging to the Sugar Research Foundation, now known as the Sugar Association. During the 1950s and 1960s, scientists were trying to understand why rates of coronary heart disease were rising. Some believed that saturated fat and cholesterol were largely responsible, while others were beginning to question whether sugar might also be part of the problem.

In 1965, the Sugar Research Foundation funded Harvard researchers to review the available evidence. The review was published in two parts in the New England Journal of Medicine in 1967 and concluded that dietary fat and cholesterol were the main nutritional concerns when it came to coronary heart disease, while evidence implicating sugar was largely downplayed.

What makes this story concerning is the nature of the Sugar Research Foundation’s involvement. According to the historical documents, it helped determine the review’s objective, supplied some of the evidence to be considered, and received drafts before publication. None of this was disclosed to readers.

This is a powerful example of how commercial interests can shape a scientific story without necessarily fabricating data. Influence can begin much earlier, with the questions that are asked, the evidence that is prioritised and the conclusions that are ultimately presented to the public.

Once fat became the main dietary villain, the food industry had an obvious opportunity. If “low fat” meant healthy, almost any product could be reformulated and marketed as the better choice.

A biscuit was still a biscuit, but now it was a low-fat biscuit. A bowl of sugary breakfast cereal could be presented as heart-healthy because it contained no cholesterol. Yoghurt could have its natural fat removed, several teaspoons of sugar added and still be sold as the option for someone watching their health.

We stopped judging foods according to what they actually were and started judging them according to a single claim printed on the front of the packet.

Although the claims have changed, we are still doing much the same thing today. We have high-protein bars, vitamin-enriched cereals, plant-based substitutes, gluten-free snacks and “zero sugar” products, all competing for space in our cupboards. Some may be perfectly useful in the right context, but a product does not automatically become healthy because it contains extra protein, has a few vitamins added or has replaced sugar with something else.

One favourable feature does not tell us very much about the food as a whole.

This is perhaps where nutrition marketing is most effective. It encourages us to focus on the one thing a product contains (or doesn’t) while paying far less attention to everything else. We see “high protein” and forget to look at the ingredients. We see “sugar free” and assume that the product must somehow support health. We see “plant based” and rarely stop to ask just how far removed those plants are from their original form (amongst other things).

The claim on the front of the package may not be untrue, but it is very unlikely to tell us the full story.

When marketing borrows the language of science

Most people do not have the time, access or specialist knowledge required to examine every study behind every nutrition claim. We rely on researchers, healthcare professionals, policymakers and journalists to interpret the evidence responsibly. This makes nutrition particularly vulnerable when marketing begins to borrow the language and authority of science.

A product is no longer simply sold as convenient or tasty. It is “scientifically formulated”, “clinically proven” or designed to “support” some aspect of our health. The claim may be technically defensible, but it can still create an impression that extends well beyond what the evidence actually shows.

This does not always require a claim to be false. One favourable result can be highlighted while the rest of the findings receive very little attention. A statistically significant change can be presented without explaining whether it was large enough to really matter in real life. One nutrient can be celebrated while the overall quality of the product escapes scrutiny entirely.

Real food does not need quite as much explaining

There is not much marketing excitement in telling people to eat eggs, meat, fish, vegetables, dairy, nuts and other recognisable foods that suit their needs, culture and budget. Real food is not new, it cannot easily be patented, and it rarely comes with a celebrity endorsement or a newly discovered ingredient that promises to transform health.

It is also not automatically perfect simply because it is natural. People have different health conditions, metabolic needs, intolerances, preferences and circumstances, and no single way of eating will work equally well for everyone. However, choosing minimally processed food gives us a sensible place to start because it allows us to consider the quality of the whole food rather than becoming distracted by one carefully selected nutrient.

So, the next time a product tells you that it is healthier because it is low in one thing, high in another or fortified with something that has recently become fashionable, turn the packet around and see what you are actually buying.

Does it still resemble food? Would you recognise the ingredients in an ordinary kitchen? Are you buying nourishment, or are you buying a very clever health claim?

Nutrition science will continue to evolve, and commercial interests will continue to influence how food is researched, marketed and presented to us. That does not mean we need to distrust every study or avoid everything that comes in a packet, but it does mean that we should remain curious about who is shaping the message and what they might gain from it.

Because when the science becomes complicated, the marketing becomes convincing and everyone seems to have something to sell us, real food should always still win.

The historical analysis discussed in this article is available in JAMA Internal Medicine via PubMed.