There is a useful thought experiment for anyone trying to understand the modern weight conversation. Imagine the body of a person walking into a medical appointment in 1975. Now imagine the body of a person walking into a medical appointment in 2026. Biologically, they are the same body. The genetic toolkit, the metabolic regulation, the appetite hormones, the way muscle preserves itself, the way fat is stored: none of this has meaningfully changed in fifty years. Fifty years is not enough time. Human biology evolves on timescales measured in tens of thousands.
The environment around that body, however, has changed almost completely. The food it is offered. The work it is asked to do. The sleep it is allowed. The stresses it carries. The light it sees in the evening. The distance from the kitchen to the calorie. The number of food cues per hour of waking life. None of these were the same in 1975. Most of them were not the same as recently as 1995.
The simplest way to make sense of why obesity rates have risen, in Australia and almost every comparable country, over the same fifty-year period, is to take the change in environment seriously. The body has not failed to adapt. The body is doing exactly what it evolved to do: protect against famine, store excess energy, conserve calories under stress, and seek calorie-dense food. It is doing all of this in an environment that has, for the first time in human history, almost no famine, almost unlimited calorie-dense food, and engineered cues to eat it as often as possible.
Four shifts have done most of the work. They are worth being specific about.
Food
The most documented of the four. In 1975, the average Australian or British diet was around 25 percent ultra-processed food, by the standard NOVA classification used in the international research literature. By 2025, that figure is approximately 60 percent. The same trajectory holds in most developed economies. Ultra-processed food, in the technical definition, is not “junk food” or “convenience food” in the older sense. It is food formulated from industrial ingredients (emulsifiers, stabilisers, flavourings, modified starches, hydrogenated fats) and engineered, often through extensive consumer testing, to maximise consumption.
This is not an editorial framing. It is the design specification. The food industry employs sensory testing panels whose explicit job is to identify formulations that produce the highest rates of repeat consumption. Products that reduce repeat consumption do not reach market. Products that drive overconsumption do.
The Nature Medicine paper published in August 2025 from Chris van Tulleken’s team at University College London is the most rigorous evidence to date on what this means for weight. They ran a randomised, crossover, controlled-feeding trial of 55 adults with overweight or obesity, comparing two diets (one minimally processed, one ultra-processed), both following the UK Eatwell Guide, both eaten ad libitum for eight weeks. Both diets produced weight loss. The minimally processed diet produced approximately twice the weight loss of the ultra-processed diet, despite both meeting national nutritional guidelines, despite both being calorie-matched in availability, and despite participants choosing how much to eat. The implication is direct. Even within “healthy” guidelines, processing matters. The same nutritional facts on the back of the packet do not mean the same thing once they are inside a body.
Stephan Guyenet, the obesity researcher whose work focuses on how the brain regulates food intake, has been making a related point for over a decade. The architecture of the modern food environment (ultra-processed food, available everywhere, promoted constantly, engineered for hyperpalatability) operates directly on appetite-regulation circuits in the brain. The reward signals are amplified. The satiety signals are dampened. The result is consumption that exceeds what the body would otherwise have chosen.
Movement
The shift here is less about exercise and more about background daily activity. Recreational exercise rates in 2026 are similar to or modestly higher than they were in 1975. What has fallen is occupational activity: the calories burned in the routine motion of doing a job. Manufacturing has automated. Office work has gone from typewriters and filing cabinets to a fixed seated position in front of a screen. Domestic activity has shrunk as appliances have multiplied. Estimated population-level daily energy expenditure from non-exercise activity has fallen substantially over the period.
The implication, as covered in a previous piece, is not that exercise should now do the weight-management work. Pontzer’s research suggests that beyond moderate activity levels, the body compensates and total daily energy expenditure plateaus. The implication is narrower and more useful: the modern environment requires deliberately constructed movement, where the older environment provided it as a side effect of ordinary life. What used to be free now requires an investment of time and attention. Most people are not in a position to make that investment at the levels that would compensate for the structural shift.
Sleep
Average sleep duration in adult populations has fallen by approximately an hour per night over the same fifty-year window. The drivers are well-understood: artificial light, screen exposure, longer working hours, shift work, the colonisation of the evening by entertainment and email. The metabolic consequences are also well-understood and somewhat under-discussed.
Sleep restriction, as covered in our piece on the four levers, increases caloric intake by approximately 300 kilocalories per day, predominantly from carbohydrates, predominantly in the evening. Hunger hormones rise. Satiety signals fall. Insulin sensitivity decreases. Cortisol response to ordinary stressors the next day increases. None of this requires extreme sleep deprivation. It happens at the levels of sleep loss most working adults consider normal.
The shift in sleep, accumulated over years and across populations, has been a meaningful contributor to the modern metabolic environment. It is also one of the more leveraged places to intervene, because the technology to address it (restraint around evening light, attention to bedtime, limits on screens) is unusually accessible.
Stress
The stress profile of modern life is qualitatively different from the stress profile of 1975. Acute stressors, the sort the body is well-adapted to, have probably decreased. Chronic, low-grade, always-on stress has increased. Email. News. Notifications. Financial pressure under wage stagnation. Caring responsibilities under shrinking social support. The cognitive load of managing a complex life inside a phone.
The body’s stress response was designed for short bursts of adrenaline followed by recovery. It was not designed for a continuous low elevation of cortisol over years. The metabolic consequence (visceral fat deposition, insulin resistance, disturbed sleep, dysregulated appetite) is now well-described. The harder truth is that the relevant interventions are not pharmacological. They are structural. Time outdoors. Time without screens. Genuinely connected relationships. Sleep. Resistance training. The boring foundations.
What to do with this
The point of this piece is not to be defeatist about modernity. The conveniences of modern life are real conveniences. The food technology that has driven ultra-processing has also fed populations that previous centuries could not feed. The sedentary work that has reduced occupational activity has also lifted hundreds of millions out of physical labour that broke bodies. None of this is straightforwardly bad.
The point is that the conversation about weight, when it is conducted as if the environment is neutral and the individual is the relevant variable, is starting from the wrong place. The environment is not neutral. It is structured, in much of the developed world, in ways that make weight gain the default trajectory. The body responds to that environment exactly as the body has always responded to its environment: by storing energy, by seeking calorie-dense food, by resisting weight loss when it senses scarcity.
The honest conversation begins with this recognition. It then moves to the practical question: given this environment, what can be done. Some of the answer is individual: protect sleep, prioritise minimally processed food, build movement back into the day, manage chronic stress where possible. Some of the answer is structural: shop at the edges of the supermarket where the whole food sits, limit ultra-processed food at home rather than relying on willpower at the moment of eating, batch-cook on the weekend so the weeknight default is whole food. Some of the answer, for some people, is medical: the modern weight-management medications were developed in part as a response to the recognition that the environment is now misaligned with biology in ways that lifestyle alone cannot reliably correct.
What the answer is not, for most people, is try harder. The body is not failing. The environment has shifted. Knowing what shifted, and how, is the start of being able to push back.