In summary
Evidence shows that responses to identical diet and exercise interventions vary dramatically between individuals, some lose significant fat, while others see minimal change despite full adherence. Differences in weight loss outcomes are not primarily about willpower or discipline, they are deeply rooted in biology.
The “average” is misleading and masks reality. Group-level success in studies conceals a wide biological spectrum of “high responders” and “low responders,” meaning many individuals are wrongly labelled as failures when judged against averages.
The body actively resists weight loss. Humans are adaptive systems, not static machines. Caloric restriction triggers metabolic slowdown, increased hunger, and long-term reductions in energy expenditure—biological mechanisms designed to preserve energy stores.
Metabolic adaptation can be long-lasting. Evidence shows that even years after weight loss, individuals may burn significantly fewer calories than expected, predisposing them to weight regain independent of effort or discipline.
Genetic “thrifty vs spendthrift” phenotypes shape outcomes Some individuals are biologically predisposed to conserve energy (thrifty), making weight loss far harder, while others naturally expend more energy (spendthrift), making weight loss easier.
Hormones regulate appetite, energy use, and fat storage. Hormonal responses—including insulin, leptin, and ghrelin—vary widely between individuals, influencing hunger, satiety, and the ability to access stored fat.
Metabolic flexibility determines how easily energy is accessed. Individuals differ in their ability to switch between burning carbohydrates and fat. Poor metabolic flexibility makes dieting harder, increasing fatigue, hunger, and likelihood of relapse.
The gut microbiome alters how calories are processed. People extract and utilise energy differently from the same foods due to variations in gut bacteria, meaning identical diets can produce very different physiological outcomes.
The immune system can act as a ‘brake’ on fat loss. Emerging research shows immune responses may actively slow fat breakdown under stress, adding another layer of resistance in some individuals.
Even advanced treatments show variable effectiveness. Modern weight-loss drugs demonstrate the same pattern of high responders and non-responders, reinforcing that biology—not just behaviour—drives outcomes.
One-size-fits-all approaches are fundamentally flawed. Standardised diets and wellness programmes fail to account for biological diversity, leading to inconsistent results and disengagement.
The future is personalised, data-driven health. Advances in wearable tech, continuous monitoring, and AI are enabling tailored approaches based on individual biological responses rather than population averages.
Implication for organisations: rethink wellness strategy Corporate wellness programmes must evolve from generic interventions to personalised, adaptive systems that recognise biological differences and broaden definitions of success beyond weight alone.
To fully understand the mechanisms, evidence base, and implications for both individuals and organisations, the full article provides a far richer and more nuanced exploration.
Introduction
I am a proponent of viewing performance in the workplace as a function of thinking about the whole person. It’s easy to write about things like sleep, diet and exercise but the reality of controlling these things can be very hard (but very important). Some people lose weight easily and others really struggle. Why is this?
If the new frontier of performance in the workplace is the application of wellness (look after yourself in order to optimise your performance), then we need to take a considered approach to how we help people be the best version of themselves.
Let’s look at the evidence.
The Illusion of the Average
A pivotal 12-week clinical trial investigating high-intensity interval training (HIIT) combined with a diet set at 75% of daily energy requirements in overweight and obese adult women vividly illustrates the differences between people. At the aggregate level, the exercise and low calorie diet was deemed a success, producing an average group reduction in absolute fat mass of 7.8% and significant total body mass reductions. However, analysing the group average completely masked a profound spectrum of individual physiological responses. When researchers classified the participants based on the magnitude of their fat mass reduction, two distinct biological cohorts emerged: high responders and low responders.
The divergence in outcomes across identical interventions is striking. High responders, representing 33% of the cohort, achieved an 18.3% reduction in fat mass, dropping an average of 4.7 kilograms. Conversely, low responders, who constituted the majority of the cohort at 66%, achieved a mere 3.6% reduction, losing an average of only 1.1 kilograms, despite adhering to the exact same rigorous, supervised exercise protocol and dietary compliance. The study found that your starting weight or fat level doesn't predict how much weight you'll drop on a diet. It challenges the old myth that "the more you have to lose, the faster/more you'll lose" when you start eating less.
A fundamental insight from this study is that just because you don’t significantly change one aspect of your physiology, e.g. weight loss, doesn’t mean you won’t benefit in other ways. Subjects classified as low responders for fat loss frequently demonstrated high-responder characteristics in measures of fitness. Across all the participants in the study, regardless of how much fat they lost, they all demonstrated significant improvements in blood pressure (a 5.1% reduction), power output during exercise (a 19.8% increase), and in measures of fitness (a 14.0% increase). You might not have lost weight, but you benefited significantly from the exercise.
It seems your body prioritises different physiological adaptations based on individual genetic and metabolic programming. For some people, the primary adaptive response to dieting and high-intensity exercise is the rapid use of stored fat; whilst for others, the biological adaptation is localised almost entirely to enhanced fitness, but leaving fat tissue largely intact. If success of the combination of high intensity exercise and a diet low in calories was defined solely by the metric of weight loss, the 66% of individuals who were low responders would be incorrectly labeled as intervention failures, despite having achieved profound, health-altering improvements in cardiovascular risk factors.
The Biological Reality of Adaptation
The standard "calories in vs. calories out" idea is too simple because it assumes your metabolism stays the same. In reality, your body acts like a protective manager trying to save its energy "savings" (fat). When you eat less, your body doesn't just keep burning fat at a steady speed; it treats the diet like an emergency. To protect itself, it aggressively slows down your metabolism much more than you’d expect, making your body run on as little fuel as possible to stop you from losing more weight.
How much your metabolism slows down is different for everyone, and it’s the main reason people hit weight-loss plateaus or gain weight back. A famous study on "The Biggest Loser" contestants showed how extreme this can get. Even years after their massive weight loss from intense dieting and exercise, their metabolisms stayed much slower than they should have been, proving that the body can "lock in" a lower calorie-burn rate to fight against weight loss. Researchers tracked the body composition and resting metabolic rates of individuals who underwent massive weight loss through extreme caloric restriction and high-volume exercise, measuring them at baseline, at the end of the 30-week competition, and six years later.
Researchers found that after extreme weight loss, participants' resting metabolic rates dropped by an average of 610 kcal/day. Alarmingly, even six years later, these rates did not recover, with participants burning 499 kcal/day less than predicted for their weight.
This biological adaptation forces people to consume fewer calories just to maintain their weight, creating a two-pronged counterattack of lower energy expenditure and increased hunger. These findings indicate that weight regain is a biological response rather than a lack of willpower.
It’s a Genetic Lottery
Some people are naturally more resistant to weight loss than others, and this is something they are born with rather than a result of their diet. Modern science, using specialized rooms that track every calorie burned, has found two main body types when it comes to losing weight:
The "Thrifty" Type: These bodies hold onto energy tightly, making it harder to lose weight.
The "Spendthrift" Type: These bodies burn through energy quickly, making weight loss easier.
Researchers can figure out which type a person has by seeing how their body reacts to two simple tests: fasting for a day and overeating for a day.
Why Standard Diets Often Fail
The reason "one-size-fits-all" diets don't work is that our bodies handle the maths differently:
- For a Spendthrift: If they eat 500 fewer calories, their body actually loses 500 calories worth of weight.
- For a Thrifty Person: If they eat 500 fewer calories, their body panics and slows down so much that they might only "lose" 150 calories. Their biology "closes the gap" to protect their fat stores. These people demonstrate exceptional metabolic efficiency.
The same thing happens with overeating. Thrifty people don't "burn off" extra food naturally; their bodies prefer to store it as fat immediately. In the past, having a thrifty metabolism was a massive survival advantage. During a famine, these people were the ones who survived because their bodies knew how to stretch a tiny bit of food for a long time. Today, however, we live in a world where high-calorie food is everywhere. In this environment, that "survival gift" has become a disadvantage, making it much harder for thrifty individuals to lose weight compared to spendthrifts.
It’s your hormones as wellnbsp;
The traditional “Calories In, Calories Out” (CICO) model assumes that all calories are equal, regardless of whether they come from carbs, fats, or protein. But another perspective, the Carbohydrate–Insulin Model (CIM) argues that what you eat changes how your body stores and uses energy. According to this view, eating fast-digesting carbohydrates (like sugar or refined grains) causes a sharp rise in insulin. Insulin is a powerful storage hormone. It encourages the body to store energy as fat and makes it harder to access that stored fat for fuel. The result? Energy gets pushed into fat cells, leaving less available in the bloodstream. The brain interprets this as a shortage and responds by increasing hunger and slowing metabolism. From this perspective, people don’t gain weight simply because they overeat, they overeat because their biology is pushing them to.
People respond very differently to the same food.
- Insulin-sensitive individuals handle carbohydrates efficiently. Blood sugar and insulin rise modestly and return to normal quickly.
- Insulin-resistant individuals experience larger, longer-lasting insulin spikes. This promotes fat storage and can lead to energy crashes and increased hunger.
So a high-carb diet might feel effortless for one person but leave another constantly hungry and struggling to lose weight. Losing weight isn’t just about discipline, it triggers real biological resistance. As body fat decreases:
- Leptin (fullness hormone) drops → you feel less satisfied
- Ghrelin (hunger hormone) rises → you feel hungrier
At the same time, food becomes more rewarding and as explained above our body burns fewer calories than expected. This creates a powerful internal drive to regain weight. All of which means weight management isn’t just a maths problem, it’s a biological system under hormonal control. And crucially, hormonal responses vary massively between individuals. Some people are fighting mild resistance. Others are fighting a full-scale biological headwind. That’s why the same diet can produce radically different outcomes, and why willpower alone is often not enough.
Metabolic Flexibility
“Metabolic flexibility” describes how well your body can switch between different fuel sources, burning carbohydrates after eating and burning fat when you haven’t eaten for a while. Some people do this seamlessly, while others struggle. This difference goes a long way in explaining why certain individuals adapt easily to a wide range of diets, while others find the same approaches ineffective or difficult to sustain.
People with good metabolic flexibility are able to burn more fat at rest and then smoothly shift to using carbohydrates when they eat. When they reduce calories, their bodies can access stored fat to make up the difference, which helps maintain energy levels and keeps hunger relatively stable. In contrast, those with poor metabolic flexibility tend to rely more heavily on carbohydrates for energy and find it much harder to switch into fat-burning mode. Even when they are in a calorie deficit, their bodies struggle to tap into stored fat, which often leads to fatigue, strong cravings, and increased hunger.
This has important implications for weight loss. When someone with poor metabolic flexibility tries to eat less, their body cannot easily access its stored energy. As a result, energy levels drop quickly, hunger rises, and metabolism may slow down. In simple terms, their body is unable to “switch fuel tanks,” making dieting feel significantly harder than it does for someone whose metabolism is more adaptable. This helps explain why some people can tolerate a calorie deficit relatively comfortably, while others feel exhausted, constantly hungry, and more prone to overeating.
Much of this difference comes down to what is happening in the muscles, particularly in the mitochondria, which are responsible for producing energy. When these systems are working well, the body can burn fat efficiently, regulate blood sugar effectively, and maintain stable energy throughout the day. When they are not functioning optimally, the body becomes more dependent on quick sources of energy like carbohydrates, and both energy levels and appetite become more difficult to manage.
One of the most effective ways to improve metabolic flexibility is through resistance training. Strength training increases muscle mass, improves the body’s ability to use fat for fuel, and enhances insulin sensitivity. It also raises overall energy demand, helping to counteract the slowdown that often occurs during dieting. In practical terms, it helps the body become better at accessing and using the energy it already has.
Weight loss is not just about eating less. It is also about how effectively the body can access and use stored energy. Some people can switch easily between fuel sources, while others are more constrained. For those individuals, the priority is not simply reducing calories, but improving the body’s ability to burn fat, making any dietary approach more sustainable and effective over time.
The Guts Role
The gut microbiome—trillions of bacteria living in your digestive system—plays a major role in how your body responds to food and weight loss. These microbes are not passive; they interact with your metabolism, immune system, and even your hormones. As a result, two people can eat the same meal and extract different amounts of energy from it, experience different levels of inflammation, and see very different outcomes in terms of weight and health.
Large-scale research, such as the PREDICT 1 Study, has shown just how important this is. In this study, over 1,000 people, including many identical twins, were given the same foods and monitored closely. The results showed huge differences in blood sugar and fat responses between individuals, even among people with identical genetics. One of the strongest factors explaining these differences was the composition of their gut microbiome. In simple terms, your microbiome can matter as much as, or more than, the food itself.
Certain types of gut bacteria are associated with better weight loss outcomes. For example, higher levels of beneficial bacteria such as Akkermansia muciniphila are linked to improved fat loss and better metabolic health. These bacteria help produce compounds that support hormone regulation, strengthen the gut lining, and reduce low-level inflammation, factors that make it easier for the body to lose fat and maintain that loss.
On the other hand, an imbalanced microbiome can work against you. Some microbial profiles are more efficient at extracting calories from food, meaning that two people eating the same number of calories may not actually absorb the same amount of energy. This can make weight loss slower and more difficult, even when someone appears to be following a calorie deficit. It also helps explain why some individuals feel as though they are “doing everything right” but still struggle to lose weight.
Weight loss is not just about what you eat, but also about how your body processes it. Your gut microbiome acts as a powerful intermediary, shaping how many calories you absorb, how your body responds to food, and how easy or difficult it is to lose weight. This is another reason why a one-size-fits-all approach to dieting often falls short.
The Immunological Brake
Recent research suggests that the immune system also plays an important role in protecting the body’s energy stores. What was once seen simply as harmful inflammation in body fat is now understood to have a more complex function. In some cases, this immune activity appears to act as a protective mechanism, helping to slow down fat loss when the body is under stress.
When the body experiences stress, such as very low calorie intake or cold exposure, it activates systems designed to release energy from fat stores. Normally, this would increase fat burning. However, studies from institutions such as the University of California, San Diego have shown that this same stress response can also trigger immune cells to move into fat tissue.
These immune cells, particularly neutrophils, release signals that effectively act as a brake on fat breakdown. In simple terms, while one part of the body is trying to burn fat, another part is simultaneously working to slow that process down. This helps preserve energy, especially in situations the body interprets as threatening or extreme.
Interestingly, this response appears to be stronger in individuals with obesity. The biological systems that limit fat breakdown may be more active, making it harder to lose weight even when someone is eating less and exercising more. This suggests that, for some people, resistance to weight loss is not just about behaviour or willpower, but also about how the immune system is regulating energy use.
The body has multiple layers of defence designed to prevent rapid weight loss. The immune system is one of those layers, acting to protect stored energy when the body senses stress. This adds another reason why weight loss can be difficult and why simple approaches do not work equally well for everyone.
Pharmacological Non-Responders
While factors like the microbiome and immune system shape how we respond to food and weight loss, the underlying foundation is still largely genetic. Each person has a unique biological “starting point” that influences how their body stores fat, burns energy, and regulates appetite. Specific genes, such as FTO gene—are linked to a higher likelihood of gaining weight or regaining it after weight loss. This means that some individuals are biologically predisposed to find weight management more difficult, regardless of effort.
These genetic differences help explain why people can follow the same diet or programme and experience very different results. For some, weight loss happens relatively easily and is easier to maintain. For others, the body seems to resist change, making progress slower and regain more likely. In some cases, even structured and closely supervised interventions have limited success, suggesting that biology can set practical limits on how effective certain approaches will be.
Even the newest and most powerful weight loss medications do not remove this variability. Drugs like semaglutide and tirzepatide work by reducing appetite, slowing digestion, and improving how the body handles blood sugar. On average, they can lead to significant weight loss—levels previously only seen with surgery.
However, responses to these drugs still vary widely. Some people lose a large amount of weight, while others see only modest changes or very little effect at all, even when following treatment correctly. This pattern mirrors what is seen with diet and exercise alone: strong responders, average responders, and non-responders.
Weight loss is influenced by multiple overlapping biological systems, with genetics playing a central role. Even highly effective treatments do not work equally for everyone. This reinforces a simple but important point: differences in outcomes are not just about effort or discipline—they are deeply rooted in individual biology.
Implications for wellness campaigns
The growing body of evidence showing how differently people respond to diets and weight loss means we need to rethink how we approach health altogether. The traditional “one-size-fits-all” model, used in public health advice, corporate wellness programmes, and commercial diets, no longer holds up. The reality is that people do not respond in the same way, and strategies built on averages often fail large parts of the population.
This shift is already happening in the market. The rise of personalised nutrition reflects increasing frustration with generic advice that delivers inconsistent results. Instead of broad recommendations, there is growing demand for approaches tailored to the individual, recognising differences in metabolism, biology, and lifestyle.
At the centre of this shift is a move toward treating each person as their own unique case. Rather than relying on population averages, newer approaches use real-time data to understand how an individual responds to food, exercise, and recovery. Tools such as continuous glucose monitors, wearable devices, and microbiome testing are making it possible to track how the body behaves day to day, revealing patterns that would otherwise be invisible.
Advances in Artificial Intelligence are accelerating this trend. By analysing large amounts of personal data, ranging from genetics to daily activity, AI systems can help tailor nutrition and lifestyle recommendations in a far more precise way. In practical terms, this means adjusting diet and behaviour dynamically, based on how the body is actually responding, rather than following a fixed plan.
These insights have major implications for organisations. Most corporate wellness programmes still rely on standardised challenges, generic advice, and simple metrics like step counts. While these approaches may work well for some employees, they can be ineffective, or even discouraging, for others whose biology makes weight loss more difficult. As a result, organisations often invest heavily in programmes that deliver uneven results and fail to engage a significant portion of their workforce.
A more effective approach is to recognise biological differences and design wellness strategies accordingly. This means moving toward more personalised, data-driven support, giving employees access to better diagnostics, more tailored guidance, and a broader understanding of health that goes beyond weight alone. It also requires a shift in mindset: recognising that differences in outcomes are not simply about effort or discipline, but are often rooted in underlying biology.
The future of health, both individually and organisationally, lies in personalisation. Approaches that adapt to the individual, rather than forcing individuals to adapt to the programme, are far more likely to succeed.
Conclusion
The evidence is clear: people vary significantly in how easily they lose weight, and this variation is deeply rooted in biology. While the “calories in, calories out” model is technically correct at a basic level, it is too simplistic to explain what actually happens in the human body. People are not closed systems—they are complex, adaptive systems that actively work to maintain energy balance and resist change.
Research consistently shows that individuals respond very differently to the same diet or exercise programme. These differences are shaped by multiple biological factors. Genetics influence how the body stores fat and regulates appetite. The gut microbiome affects how much energy is extracted from food and how the body responds after eating. Metabolic flexibility determines whether the body can easily access stored fat or struggles to do so. On top of this, the body can slow its metabolism during weight loss, and even the immune system can act to protect fat stores under stress.
Taken together, these factors create a wide spectrum of responses. Some people respond quickly and effectively to standard approaches, while others see little progress despite doing the same things. This distinction between “high responders” and “non-responders” is not about motivation or discipline—it reflects underlying biological differences.
The implication is straightforward but important: weight management needs to move away from generic advice and toward more personalised approaches. Instead of relying on averages, we need to understand how each individual responds and adapt accordingly. This applies not only in healthcare, but also in policy and organisational settings.
There is no single solution that works for everyone. Effective and sustainable progress comes from recognising individual differences and tailoring approaches to fit the person, rather than expecting the person to fit the approach.
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