Do we subconsciously eat what our body needs?

New research has examined whether there's truth to the notion that we adjust our diet to top up necessary nutrients.

Bristol University’s Prof. Jeff Brunstrom has carried out several studies investigating the concept of nutritional intelligence – the idea that humans innately choose food based on how it will benefit them. While the research is in its infancy, in future he says it could help people improve their diet and health…

Nutritional intelligence was an idea Jeff had always discounted. So when Canadian journalist and author Mark Schatzker gave a talk about his belief in the concept at a conference, he wasn't convinced.

“I bumped into Mark when he gave a talk about how different wild and domesticated species show a remarkable ability to respond to micronutrient deficiencies by changing their diet accordingly,” says Jeff.

Mark thought the same could apply to humans, but knew he was in the minority. He tweets the full story: “So there I was at the at the annual meeting of the Society for the Study of Ingestive Behavior, in a hall filled with very accomplished scientists who tended to think “nutritional wisdom” is outdated, Goop-worthy claptrap. I was among some serious heavyweights.

“After the talk, I was approached by a friendly man with an English accent who said, “Great talk. I think you are probably wrong. Would you like to test it?”

Jeff went into the research expecting to find nothing. “I thought it was highly dubious, so we continued to discuss this idea and soon collaborated on a paper on micronutrients.” The resulting research surprised him.

How to test for “nutritional wisdom”

A study conducted in the 1930s by Dr Clara Davis went some way to support Mark’s view. It saw newly-weened infants, who didn’t have access to healthy food at home, given free rein to choose from more than 30 foods in a laboratory setting over a sustained period. Clara found the children instinctively chose a diet high in nutrients.

However, over time, this has become a hotly disputed study. “It's not really clear what was going on there,” says Jeff. “Maybe the children were just exposed to a whole load of really healthy foods, and that was sufficient.” But, says Jeff, “It did offer up tantalising evidence.”

Conducting a similar study now would not be possible. It would be immoral to intentionally make people lack in a type of micronutrient to see if they remedy it through their food choices, says Jeff.

Instead, Jeff and Mark used hypothetical choices instead. “We showed people pictures of fruits and vegetables in different combinations and then asked people to choose the combination [they’d opt for].”

Each pairing together offered up a range of micronutrients, some more complete than others. But the average person wouldn’t be aware which combination was better. Would the people in the study choose the pairings which offered the most micronutrients? “To my amazement, the first study we did showed just that. Now it's a small effect, but it was a reliable effect.”

Intrigued, the duo looked at the data provided by the National Diet and Nutrition Survey (NDNS). “This included thousands of people self-reporting a diet diary.” Jeff and Mark discovered that there was once again a link between preferred pairings of food and their nutritional value. Would common combinations provide a balance of micronutrients better than a random pairing? “Yes,” says Jeff, “there was a small but significant effect.

“The paper demonstrated that we humans have that capacity [to crave nutritionally dense food]. The size of the effect was kind of irrelevant. I saw it a bit like telepathy. If you can show that anybody has got telepathy, over and above it doesn't really matter about levels, that's an amazing finding.”

So, could our day-to-day cravings reflect the vitamins and minerals we’re lacking? For example, do you crave liver, spinach or pumpkin seeds when you need iron? That’s something different schools of thought can’t agree on, but with time Jeff thinks we will have the answer.

“We need to learn cause and effect relations about micronutrients, I suspect that we can under certain conditions,” says Jeff.

Likewise, another big question still remains, if we do have nutritional intelligence, why is it that groups in society have nutritional deficiencies?

In the paper both Mark and Jeff explain that questions such as these will require further research to be answered. “How can we reconcile nutritional wisdom with the long history of vitamin deficiencies in human populations, and can this be attributed solely to a lack of access to specific foods and/or poor nutritional guidance?” they ask in the paper. Hopefully, more research on the subject will follow.

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“One of the long-standing debates in our [scientific] area has been around whether or not people respond to calories in food,” says Jeff. The big question is whether, without being told, we can sense what the energy density is in food and then adjust the volume of food we consume accordingly. So, if something was high in calories, we would eat a smaller portion than something which had a lower amount.

“It seems odd as omnivores and as former hunter-gatherers that we would be insensitive as to how it works,” says Jeff, though he says, there are studies over the last few decades which have shown that both we do possess this ability and that we don’t. “I think it's left us shrugging our shoulders to an extent, and in many quarters, the prevailing view would be we are passive overconsumers of food.”

This view adds argues that we are vulnerable to eating more calories than we need in a world full of highly-calorific processed food.

“So, on that basis we need to change the foods that we're eating because we're poorly equipped to deal with these kinds of foods. That has a lot of implications for how we think about obesity…”

But as Jeff and co-author, Annika Flynn investigated, they found the opposite was true; people did – seemingly subconsciously – adjust their portion size depending on the calories in them.

“For years we’ve believed that humans mindlessly overeat energy-rich meals. Remarkably, this study indicates a degree of nutritional intelligence whereby humans manage to adjust the amount they consume of high-energy density options,” explains Annika.

They used pre-existing data for the study. Under controlled conditions, over four weeks, 20 people chose from a selection of everyday meals of different calories. None of the people were trying to lose weight. People had smaller portions of food with higher calorie content.

Annika and Jeff then examined more data from the NDNS to see if the same was true when people were making their own meal choices. Once again, people ate smaller amounts of high-calorie foods. “For instance, people ate smaller portions of a creamy cheese pasta dish, which is an energy-rich meal, than a salad with lots of different vegetables which is relatively energy-poor,” explains Annika.

“This isn't about explicit awareness of 'I know how many calories are in different dishes',” adds Jeff, before adding: “When we talk about nutritional intelligence, we're not saying somebody has just read a textbook on nutrition, it's an unconscious appreciation of energy content.”

What could nutritional intelligence mean for our health?

“What this tells us is we don’t seem to passively overconsume these [high calorie] foods and so the reason why they are associated with obesity is more nuanced than previously thought,” explains Jeff.

Previously, he says it’s been thought we eat by volume and that in turn has been linked to obesity. “If I ate 300 grams of a one calorie per gram food, but then ate 300 grams of a 2 calorie per gram food then, while it’s the same amount of food by volume, it’s twice as many calories. That's why this idea of passive consumption has gathered momentum. It's a way of thinking about why we overconsume. What our research has shown is we need to potentially rein that in.”

Jeff says more research is needed and as it stands, they can’t rule out whether nutritional intelligence is impacted by our learned knowledge of calorific and nutritionally dense food. Future research also needs to examine why we have an obesity crisis if we do have an innate nutritional intelligence that both sees us choose nutrient dense food over empty calories, and consume smaller portions of food that's high in energy.

Mark Schatzker offers up one potential answer: “The research throws up important questions, especially in the modern food environment. For example, does our cultural fixation with fad diets, which limit or forbid consumption of certain types of foods, disrupt or disturb this dietary ‘intelligence’ in ways we do not understand?”

Mark continues: “Studies have shown animals use flavour as a guide to the vitamins and minerals they require. If flavour serves a similar role for humans, then we may be imbuing junk foods such as potato chips and fizzy drinks with a false ‘sheen’ of nutrition by adding flavourings to them. In other words, the food industry may be turning our nutritional wisdom against us, making us eat food we would normally avoid and thus contributing to the obesity epidemic.”

However, there are still many queries that need to be answered. Jeff finishes: “The next set of questions will need to be about the impact of this on individual differences on chronic health. We know some people are more susceptible to poor dietary health and obesity than others…. so what role does nutritional intelligence play, and can we understand how those interactions predispose us to over or under consume on that base? That's an exciting area as well.”

Originally published July 2022