Think of a house in winter. The thermostat is set to 21°C and the central heating works to hold it constant. Open a window and heat escapes, the thermostat senses the drop in temperature and the central heating kicks in again until the temperature is restored. The temperature is defended: whatever pushes it off target, the system works to bring it back.
Nobody would think the temperature is controlled by the boiler, because they understand it is the setting on the thermostat that regulates the temperature in the house.
The body regulates fat mass in a similar way. Energy balance and calories in versus calories out is arithmetically true, but it describes the room temperature rather than the regulation of the thermostat. The more useful question is what sets the dial and what happens biologically when we try to force weight down.
The Body Defends Fat Mass, Not Body Weight
Physiological pathways regulating fat mass resemble those governing core body temperature, blood glucose and blood pressure. All sense the internal and external environment and respond homeostatically to keep the regulated variable within a narrow range1,2.
In steady state, fat mass is typically held within roughly 1-3% of its defended level1. The defended level is the amount of fat mass the body works to maintain, resisting sustained loss or gain. Importantly, the body appears to regulate fat mass rather than body weight itself. This becomes evident when astronauts enter microgravity and they do not change their bodyweight because weight is not defended but fat mass is.
How the Brain Senses Calories, Not Just Volume: The Gut-Brain Axis
When nutrients reach the intestine, enteroendocrine cells (i.e. specialised hormone-secreting cells within the gastrointestinal tract) release a co-ordinated pattern of hormones including GLP-1, PYY, oxyntomodulin, cholecystokinin and amylin, while ghrelin (i.e. an appetite stimulanting hormone) secretion from the stomach and duodenum is suppressed1,2.
These signals reach the subcortical areas of the brain through the circulation and vagal pathways and their magnitude is influenced by the calories contained in the meal1. They contribute to satiation, which brings a meal to an end and to satiety that determines the period between meals2.
Much of this happens without conscious involvement. Calories rather than volume of food appears to be important because far more satiety is achieved with two litres of full fat milk versus two litres of water.
How the Brain Senses Fat Mass: Leptin
The second input is the size of the body fat store itself. The circulating signal that communicates the size of fat mass to the brain is leptin, which was identified in the early 1990s3,4.
Leptin acts on hypothalamic pathways including the melanocortin system and interacts with melanocortin signalling to influence food intake and energy expenditure1.
The importance of this circuitry is illustrated most clearly in rare genetic forms of obesity. Children with rare genetic variants affecting this pathway (for example, in LEP, LEPR, POMC or MC4R ) can develop severe early-onset obesity accompanied by profound hunger and impaired satiety5.
Many genetic variants linked to obesity affect how the brain and nervous system regulate hunger, fullness and eating behaviour6,7.
Obesity can therefore be understood as a disease characterised by dysregulation of brain-based systems interacting with the gut, adipose tissue, genetics and the environment.
Leptin appears to be the most impactful signal that can activate or deactivate the fat mass defence mechanisms of the subcortical areas of the brain unless the disease of obesity is controlled.
Leptin links the size of the adipocyte mass to the brain systems that regulate appetite and energy expenditure and falling leptin levels during fat loss help activate the biological responses that oppose further loss.
Unwanted Weight Gain Reflects a Rising Fat Mass that the Body Tries to Defend
The pathophysiology of obesity can be understood as involving impaired leptin signalling, similar to insulin resistance in type 2 diabetes2,8. When central leptin signalling is impaired, the counter-regulatory responses that would normally limit further fat accumulation become less effective.
Fat mass expands until sufficient signalling is restored, at which point a new higher fat mass becomes biologically defended1,2. Increased food intake during this process is thus a consequence of the underlying disease biology. Once a higher fat mass is established, the body defends it against calorie restriction in much the same way it defended the previous lower level1,2.
Why “Eat Less, Move More” Does Not Tell the Full Story
When calorie intake falls and fat mass is reduced, leptin levels decrease substantially, triggering a co-ordinated and durable biological response until the fat mass is restored to the defended homeostatic position.
Energy
Studies spanning four decades have shown that after weight loss the body becomes more energy-efficient, with total daily energy expenditure falling approximately 15–20% below what would be expected for the person’s new body size and composition2,9.
This metabolic adaptation makes weight loss more difficult to maintain, because the body defends against further weight loss. The adaptation persists for as long as the reduced fat mass is maintained and may diminish as fat mass is regained1,10.
Hormones
Following substantial weight loss, thyroid hormone concentrations fall without the expected compensatory rise in Thyroid-Stimulating Hormone (TSH)2,11. Satiety-related gut hormone responses weaken while ghrelin increases and some of these adaptations persist for at least a year2,12,13.
Appetitive Behaviour
Appetitive behaviour refers to the behaviours and the motivations underlying them that lead people to seek out, choose and consume food.
After weight loss, people often feel less satisfied by the same portion and become hungry sooner and this is frequently accompanied by greater preoccupation with food, increasingly described as “food noise”2,14.
This can result in a stronger drive to obtain food, which may also be accompanied by an increased preference for energy-dense foods.
What the Diabetes Prevention Program Showed
The Diabetes Prevention Program15,16 illustrates both the effectiveness of intensive lifestyle intervention and the difficulty of maintaining the initial weight loss over time.
Participants in the lifestyle group lost an average of approximately 7kg in one year. Over subsequent years, some of the lost weight returned and after 10 years of effort the group was only 2kg below their baseline weight15,16.
The pattern of initial weight loss followed by partial weight recurrence is consistent with the metabolic adaptations that oppose sustained fat mass reduction2,16. People who experience weight recurrence after diet-induced weight loss should therefore not be viewed as non-adherent or insufficiently motivated.
Understanding their biology may be far more productive2.
Where does Food and Diet Quality Fit
None of this diminishes the role of good quality nutrition. It places nutrition within the biology of appetite regulation.
An instructive example comes from an inpatient randomised trial comparing highly-processed and unprocessed diets designed to be matched for presented calories, energy density, macronutrients, sugar, sodium and fibre. Participants consumed approximately 500kcal per day more during the highly-processed diet and gained weight over two weeks17.
Food texture has been shown to influence eating rate and post-meal satiety, with harder-textured highly-processed meals consumed more slowly and associated with greater fullness and lower subsequent snack intake, compared with soft textured highly-processed meals18. The findings illustrate how food’s composition and texture can influence spontaneous energy intake without deliberate calorie counting.
Higher-protein dietary patterns can likewise improve satiety and reduce overall calorie intake in some settings19. Some aspects of eating including hunger, meal size, meal frequency and satiation are strongly physiologically driven and may respond to nutritional therapies, pharmacotherapies and/or surgical therapies.
Others including food choice, meal structure, culture and the household food environment remain important targets for nutritional and behavioural intervention2, albeit the target is not weight loss but health gains, especially in quality of life gains that may occur without weight loss . The two are complementary rather than competing.
Obesity Medicines Change Adipocyte Mass Regulation
With the exception of orlistat, current obesity medicines are understood to act through subcortical brain pathways involved in fat mass regulation2. Improvement in appetitive behaviour often occurs before substantial weight loss becomes apparent20.
This may be explained by considering the obesity medications as putting the disease of obesity under control, thus creating a lower fat mass that needs to be defended. Therefore, the only way for the subcortex of the brain to achieve the new defended fat mass is to make the person less hungry, feel fuller and reduce portion sizes.
Consequently the person may experience a reduction in appetitive behaviour, often interpreted as a reduction in “food noise”2. Discontinuation of effective obesity treatment commonly leads to relapse of the disease of obesity and a higher fat mass that is defended. This results in renewed hunger and weight recurrence, supporting the concept that obesity often requires long-term treatment2,21,22.
Surgery Works Through Similar Biology
Metabolic-bariatric surgery was once viewed primarily as a mechanical treatment. We now understand it acts through changing the homeostatic regulatory mechanisms similar to those targeted by pharmacotherapy.
Gastric bypass and sleeve gastrectomy alter gut-derived signals including GLP-1, PYY and ghrelin, improving appetite control and reducing the biological drive that would otherwise promote weight recurrence1,2.
Some return of hunger as weight loss reaches a plateau does not necessarily represent treatment failure; it may reflect a new homeostatic balance being reached2.
Table 1. Biological Responses to Fat Mass Reduction by Treatment Approach
What This Means for Nutrition Practice
Obesity should be viewed as a chronic disease not simply because excess adiposity causes complications, which alone would make it a risk factor, but because the system that defends fat mass itself becomes dysregulated1.
Framing obesity primarily as a failure of restraint overlooks this biology and only serves to reinforce stigma24.
Nutritional quality, physical activity and behavioural support remain important for every person with obesity and are sufficient for some when it corrects the dysregulated biological system.
But for many people with obesity, medications or metabolic-bariatric surgery are necessary components to control the disease of obesity rather than optional additions once lifestyle has “failed”2.
At the same time, food quality, protein adequacy, food structure, nutrient composition, sensory properties, dietary patterns and the wider food environment remain central to good nutrition practice that may increase health gains independent of weight loss. Their importance does not diminish if these approaches cannot effectively treat the disease of obesity, because the impact of quality of life is often weight loss independent.
Improved biological appetite control may create more favourable physiological conditions in which good nutritional interventions can be sustained.
The thermostat in the house is set too high. What we put into the room still matters a great deal, but it is not a substitute for resetting the dial.
