Imagine you’re in a meeting room when someone brings out the biscuits – a packet of Jammie Dodgers, perhaps, or a nice little plate of custard creams. Maybe you want one and maybe you don’t, but the chances are the people around you are all responding differently: someone will grab a couple straight away, someone else will eat one without seeming to notice, another will barely be aware the biscuits exist, and someone will spend the whole meeting wanting one but not taking it. Our appetites and responses to food vary wildly – but what’s going on behind the scenes to govern them? And has modern food somehow hijacked the process? Grab a biscuit (or don’t) and settle in.

“First, it’s important to distinguish between hunger and appetite,” says Giles Yeo, a professor of molecular neuroendocrinology at the University of Cambridge and the author of Why Calories Don’t Count. “Hunger is a feeling – it’s what happens in the run-up to you deciding you need to eat something. Appetite is everything that surrounds why we eat – including hunger, fullness and reward, or how you actually feel when you eat. Those three sensations all use completely different parts of the brain, but they all work together.”

Hunger is regulated by the hypothalamus, which sits behind the bridge of the nose, at the base of the brain, monitoring your body’s levels of blood sugar and the hormones leptin and ghrelin to check whether you’re in an energy deficit. Fullness is regulated by the hindbrain, located roughly where your skull meets your neck: when your stomach stretches, the vagus nerve sends a signal to this area telling you that you’re physically full. Reward, meanwhile, is regulated by a diffuse network of neurons that sit higher up in the brain, driven by dopamine and its search for pleasurable activities.

“All those parts of the brain speak to each other, which is why if you’re really hungry, food that offers very little ‘reward’ – like rice or bread – can be delicious. Or why you can feel full but still feel ready for chocolate cake, because it’s activating your reward system even though your hindbrain says you’re full,” says Yeo. “It’s like a triangle that changes shape depending on your circumstances, with appetite in the middle.”

So what’s going on with the biscuits? Well, part of the reason we might respond differently to them is how hungry or full we are in the moment, but it’s likely that genetics also play a part. “We all know people who love food, and people who simply see it as fuel,” Yeo continues. “Food-is-fuel people will get hungry eventually, but it happens far closer to the time that they actually need to eat than for others. It’s also likely to be a matter of how much – or how little – food is needed to trigger the brain’s reward response. We know there are more than a thousand genes that influence our appetite, so it’s a very complex system.”

Another element in all this is that scent, sight and even sound cues activate the brain’s appetite circuitry independently of how much energy we have stored, resulting in what neuroscientists call “hedonic” hunger. “When we see food, sensory and olfactory input interacts with brain regions that regulate appetite, and temporarily increase dopamine signalling,” says Timothy Frie, a nutritional neuroscientist. “That heightens our motivation to eat, even if our physiological energy needs have already been met. The sensation of hunger isn’t coming from an empty stomach, but from a conditioned, cue-driven response where the brain and body are preparing for intake based on what you see. Sound can also play a part, with its influence coming primarily through learned associations, like the repeated pairing of a sizzle or a crunch with a desirable taste or sensation.”

One more complication is that all these systems can be confused, or at least disrupted, by stress. “When we’re stressed or experiencing some degree of cognitive overload or fatigue, the regulatory capacity of our prefrontal cortex is reduced, while appetite and reward systems remain active,” says Frie. “The brain’s demand for a rapid and reliable source of fuel also increases in response to stress. That creates a predictable imbalance: stronger drive to eat with reduced ability to regulate that drive.” Sugary, salty, fatty and especially ultra-processed foods rapidly increase glucose availability and light up motivation pathways in the brain, and when we’re stressed, the brain assigns higher priority to these foods because they provide quick and efficient energy.

Appetite can also be disrupted over time. When we overeat refined carbs, sugars and fats frequently over a long period, our receptors for insulin and leptin (which regulates energy balance and appetite) can become muted, reducing their responsiveness and making it harder for us to tell when we should stop eating.

Food companies, of course, know all this, and often respond to it by hijacking the systems that lead us astray: pumping delicious scents through the air in fast-food restaurants, say, or designing foods that pair hyperpalatability with sensory cues like a satisfying crunch. To make matters worse, although our in-built satiety systems are fairly good at roughly judging the energy content of foods that are mostly fat or protein, they’re terrible at estimating it in foods that mix refined carbs and fat, making it easy to enormously overeat things like biscuits, pastries and pizza.

Where does this leave us? Unfortunately, in a situation where our basic drives and biological mechanisms haven’t changed much since our hunter-gatherer past, but are being exploited by the endless food options available. “Many of us live in a supernormal, overstimulating and engineered food environment,” says Frie. “Our brains are saturated with cues to eat, but they aren’t necessarily equipped to respond to so many cues for a long period of time. The best thing we can do for ourselves is to develop what I call food-mind fluency: the ability to recognise what is driving the urge to eat in that moment and respond with awareness and conscious intention.”

This allows us to regulate and manage the sequence of events that occur between a food cue and a food response. In practice, Frie says: “That could mean inserting a brief pause before acting on the impulse to eat and asking a single question: ‘What is generating this signal right now: energy need, stress, habit or exposure to a cue?’ That step engages our prefrontal cortex, which allows us to shift our behaviour from automatic to intentional.”

But when the vast majority of non-infectious diseases we face as a species are diet-related, preaching personal responsibility probably isn’t enough. “Personal responsibility is fine and we need to talk about it and give people advice,” says Yeo. “But I also think it absolves policymakers and government from the public health decisions they need to take in order to try to improve our food environment. It has to be a holistic thing.”

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