Ah, agriculture – that wonderful development that gave the human species a (semi-)reliable source of food.
For the tens of thousands of years before that, our prehistoric ancestors had to rely on the vagaries of fate to cross paths with suitable prey (and kill them, rather than being killed) or to come across edible plants.
They never knew when their next meal would be, and thus, had a body adapted to maximise whatever food they could obtain.
While science and technology has advanced by leaps and bounds ever since the Industrial Revolution, our body remains genetically and physiologically the same as our hunter-gatherer ancestors.
Today, we are surrounded by food, but our body is still programmed for those cavemen days of scarcity. And that is our downfall.
Signals from the body
“The general process of how our brain influences food intake revolves around the fact that there are circulating cues – hormonal or nutritional – in the blood, and they signal to the brain.
“What they signal to the brain is your general nutrition status, and the brain translates these signals into influencing intake,” says University of Cambridge, United Kingdom, Metabolic Research Laboratories Genomics and Transcriptomics director Dr Giles Yeo, adding that this is a simplification as there are a number of complex underlying mechanisms and factors involved.
From below the neck, the signals come from two major areas, he says.
“The first are your long-term stores, which are coming from your fat.
“What is absolutely critical from an evolutionary point of view is for your brain to know how much fat you have.
“And the reason that is critical is because how much fat you have is how long you can last without food,” he says.
“However, that is still not enough information to actually influence your food intake.
“The second thing you need is to know meal to meal variation, i.e. what did you have last and how much did you have.
“For that, your brain has to rely on signals from your stomach and your gut.”
He explains: “When you’re eating, your body is very good at informing the brain – unconscious to you – exactly what’s going on.
“So it knows how many calories you’re eating, it knows the macronutrient content of the calories – what percentage of carbs, fat, protein etc.”
The body does this by producing different hormones in response to how much protein, fat, carbohydrates or other nutrients we eat.
The subtle changes in the amount or ratio of the different hormones are what allows the brain to know exactly what and how much nutrients we have consumed.
“It then integrates all the signals – how long can I last without food, what did I last eat and how much did I last eat – and the next time you come across food, it influences your interaction with food.”
Food and the brain
When it comes to the brain itself, Dr Yeo says that it can be broadly divided into two systems: the homeostatic system and the hedonistic system.
The homeostatic system, which he equates to the body’s fuel gauge, calculates how much energy we have spent in physical activity or physiological processes, and informs our body as to how much we need to eat to replace that energy,
In today’s context, the homeostatic system is actually sufficient to control our food intake as most of us have regular, reliable mealtimes.
However, before the advent of agriculture, the human species could not rely on a regular food supply; therefore, the homeostatic system, while critical, was not enough to ensure the survival of the species.
When you do not know when your next meal will be, you not only need to replace the energy you have already used up, but also build up your long-term energy stores, i.e. fat, so that you can survive for a long time without eating if necessary.
Says Dr Yeo: “Another part of the brain takes over in order to make sure you eat more than you need, and that part of the brain I call the hedonic or reward part of the brain.
“That’s the part of the brain that makes eating feel good.”
Many might be familiar with the concept of the “dessert stomach” or “extra stomach for dessert”. This is when, no matter how full you feel from the main meal, you can still find space for dessert.
Dr Yeo shares that there is a real physiological basis to the concept.
“It’s actually a different part of the brain.
“Going back to evolution, what happens is, you need a part of the brain that makes you want to eat more than you need, so that you store enough fat to survive the next famine.
“It doesn’t work with all foods because by the time you are full, what you need then are calories that are denser.”
In order to maximise whatever space is left in the stomach once it feels full and the body’s energy requirements are met, the food taken must be as full of calories as possible.
Therefore, what triggers the hedonic system in the brain are the foods with the densest and most easily digestable calories, i.e. foods high in sugar, also known as desserts.
And that is why sugary foods taste so good, and why we can always squeeze in dessert.
This pleasurable feeling also had a secondary effect for our hunter-gatherer ancestors; it helped reinforce their memory of where such foods – usually fruits – were located, so that they could find them again – providing a more reliable source of food.
Tan Shiow Chin was a 2015 Khazanah-Wolfson Press Fellow at Wolfson College, University of Cambridge. This article is part of a series from her fellowship project on the subconscious cues that influence us to eat more and unhealthily. Her next article will be on the influence of our genes on obesity.