On the face of it, the equation “eat less than you burn so you’ll lose weight” seems beautifully clean and logical. The so-called calories in, calories out (CICO) model has been the backbone of countless weight-loss programs: Calories consumed (in) minus calories expended (out) equals net energy change.
But in real human biology, the story isn’t so tidy. Our bodies are not mechanical machines ticking off energy units; they are dynamic, adaptive systems, shaped by hormones, metabolic feedback loops, and environmental cues.
As a nutritionist, I’ve seen how the simplistic math of CICO often fails clients and why we need to broaden our lens.
The appeal and the beginning of the trouble
The core idea of CICO is almost irrefutable in principle: If you consistently consume fewer calories than you expend, weight loss must follow. As one review notes: “To lose weight, your “calories in” need to remain fewer than your “calories out.”” However and this is the crucial “but” the human body introduces complexity at multiple junctures and undermines the neatness of the calculation.
For instance, the CICO‐framework assumes that we can reliably measure both sides of the equation: Calories in (what we eat) and calories out (what we burn). In practice, both are fraught with error. Studies show substantial under‐reporting of food intake, over‐estimation of activity, and large individual variability in basal metabolic rate.
Biology fights back
A growing body of science reveals how the body resists simple calorie deficits. According to researchers at University of Sydney: “The biggest failing of the “calories in, calories out” formula is that it ignores that the body adjusts its control systems when calorie intake is reduced.”
Key mechanisms include:
Metabolic slowdown: When intake drops, the body reduces its energy expenditure not just because there’s less mass to support, but via hormonal changes and reduced non-exercise activity. One study found a further ~15% drop in metabolism beyond what body-mass loss would predict.
Hormonal shifts in appetite and satiety: Lower intake can reduce leptin (a “satiety” hormone) and increase ghrelin (a “hunger” hormone), making sustained calorie deficits harder. Some critics comment that CICO “doesn’t account for hormones.”
Energy compensation: Higher exercise or activity doesn’t always lead to proportionally higher total daily energy expenditure because the body may compensate by reducing other energy uses (like non-exercise activity or resting energy). This phenomenon undermines the “burn them off” half of CICO.
Quality matters
Another major caveat is that not all calories are equal in their effects on the body. For example, 100 kcal of broccoli affects hunger, insulin response, and metabolic pathways differently from 100 kcal of candy. According to the Harvard TH Chan School of Public Health: “This idea of “a calorie in and a calorie out” … is not only antiquated, it’s just wrong.”
The implication for dietary practice is that focusing solely on numbers can obscure the critical roles of nutrient density, food composition (protein, fibre, processed vs whole foods), and metabolic health. A person could remain within a calorie target yet fail to improve (or even worsen) health markers such as insulin sensitivity, lipid profile or inflammation.
Why this theory matters in a real-world context
For practitioners like us in the nutrition and health field, the implications of an over‐reliance on CICO are profound. Many clients get demoralized because they “count calories, increase activity, still don’t lose weight” and conclude that the failure is theirs (lack of discipline), not the inadequacy of the model.
Moreover, from a public health perspective, as one review in the Nutrition Journal put it: “Growing evidence suggests that the calorie imbalance concept may not be sufficient to manage and reverse the obesity epidemic.”
When CICO becomes dogma rather than a starting point, we risk overlooking critical levers: Sleep patterns, gut microbiome, hormonal disorders (eg, thyroid disease, polycystic ovary syndrome), food quality, stress, and socio-environmental factors (ultra-processed food access, food insecurity, obesogenic surroundings).
What should change? A more holistic, nuanced lens
1. Accept the deficit concept, but contextualize it. Yes, energy balance matters yet the path to achieving and sustaining it depends on more than simply “reduce calories, increase activity.”
2. Focus on food quality and metabolic health. Encourage nutrient-dense, minimally processed diets with sufficient protein and fibre to support satiety, lean mass, and metabolic rate.
3. Monitor non-dietary factors. Address sleep, stress, hormonal health, medication use, and underlying disease. These often derail weight change even when diets seem “on track.”
4. Recognize adaptive physiology. Set expectations realistically — plateaus and metabolic adaptation are not failures of the client, but features of human biology.
5. Each body adapts differently. Integrate individual variability in metabolism, activity compensation, genetics, and lifestyle.
Treating the body as a simple “calorie machine” ignores its complexity. While the CICO model holds a kernel of truth that energy balance matters, it collapses under the weight of human biology, behaviour, and environment. For lasting change, we must move beyond arithmetic and honour the living, adapting organism that each client inhabits.
In our work as nutrition professionals, this means helping clients understand: Yes, energy matters but so do hormones, hunger, food quality, movement, recovery, and environment. The math might be simple, but living it isn’t.
Raisa Mehzabeen is Founder and CEO, Nutrition For Change.
