It is 7am. You have hauled yourself out of bed, laced up your trainers, and dragged your body through a 45-minute run in the heat. You are sweaty, virtuous, and deeply pleased with yourself. You have earned something. So you stop at Starbucks on the way home and order a Venti Caramel Frappuccino.
That Frappuccino contains approximately 500 calories. Your 45-minute run burned somewhere between 300 and 400. You are now in a caloric surplus before 8am, wearing running shoes, feeling magnificent about your health choices.
This is not a character flaw. This is not laziness. This is the predictable, logical outcome of a weight loss model that has been wrong for decades and refuses to admit it.
The Fossil Idea
The calorie-in, calorie-out model — CICO — is built on the first law of thermodynamics. Apply that to the human body and you get the equation that has dominated diet culture since the 1960s: eat less, move more, lose weight. Simple. Elegant. And for the vast majority of people trying to use it for long-term weight loss, spectacularly unsuccessful.
Dr. Jason Fung, a Canadian nephrologist and one of the most important voices in obesity medicine today, has been dismantling this model with rigorous precision for years. His argument is not that thermodynamics is wrong. His argument is that applying the first law of thermodynamics to the human body as if it were a simple combustion engine is a category error — one with devastating consequences for millions of people who have spent decades blaming themselves for a failure that was built into the model from the start.
Here is the problem with treating the body like a furnace: furnaces do not have hormones.
The Doctor Didn’t Know Either — And That’s Not Their Fault
The doctors who told you to eat less and move more were not wrong because they were careless. They were wrong because they were taught incorrectly. Most physicians currently in practice received fewer than 20 hours of nutrition training across their entire medical education. Twenty hours. In a degree that spans a decade of training.
The nutrition content that was covered leaned heavily on the thermogenic model. These were presented as settled science. They were not settled science. They were hypotheses adopted as policy before the research had fully caught up, and in some cases shaped by the food industry’s considerable financial interest in keeping fat and calories at the centre of the conversation rather than sugar and insulin.
So if your doctor has told you for years to eat less and exercise more and you have tried and failed and tried and failed — please hear this: the failure was not yours. It was the model. And the model is being revised.
The Hormonal Truth Nobody Told You
The human body does not respond to a calorie deficit the way a spreadsheet does. It responds the way a nervous system shaped by millions of years of famine survival does — with considerable alarm and a suite of hormonal responses specifically designed to make you eat more and burn less.
Ghrelin rises. Your primary hunger hormone signals the brain to seek food and promotes fat storage. Caloric restriction causes ghrelin to rise significantly — and in people who have lost weight through caloric restriction, ghrelin levels remain elevated for months to years after the diet ends. Your body is biochemically insisting on the weight it has lost. This is not weakness. This is physiology.
Leptin falls. When you lose fat through caloric restriction, leptin drops. The brain reads this as an emergency. Hunger increases. Metabolism slows. You are being biochemically punished for dieting.
Insulin remains the central player. Dr. Fung’s core argument is that obesity is primarily a hormonal disorder driven by chronically elevated insulin. When insulin is high, fat cannot be released from fat cells regardless of caloric deficit.
Cortisol complicates everything. Prolonged or intense exercise — particularly undertaken in a caloric deficit, under stress, with inadequate sleep — elevates cortisol, which raises blood glucose, promotes abdominal fat storage, increases appetite specifically for high-calorie foods, and impairs sleep. For some people, exercising more makes the hormonal environment for weight loss measurably worse.
But Please — Keep Exercising
Before you use this blog as justification for cancelling your gym membership, let me be unequivocal: exercise is one of the most powerful health interventions available to human beings. Regular physical movement reduces cardiovascular disease risk by up to 35%, improves insulin sensitivity, reduces systemic inflammation, improves mood and depression more effectively than medication in some populations, preserves muscle mass and bone density, improves sleep quality, and extends healthspan.
Exercise is medicine. Extraordinary, accessible, side-effect-free medicine. It is just not, by itself, a weight loss strategy. And confusing these two things has caused incalculable harm to people who exercised faithfully, saw limited results on the scale, and concluded that something was wrong with them. Nothing was wrong with them. The model was wrong.
What Actually Works
The answer, supported by a growing and robust body of evidence, is hormonal regulation — and specifically, the management of insulin. Reducing dietary sugar and refined carbohydrates is the single most impactful dietary change for most people. Intermittent fasting works on the same principle: by extending the period between meals, insulin levels drop into a range that allows fat mobilisation. Addressing insulin resistance is foundational. Sleep and stress management are not optional lifestyle add-ons — they are core metabolic interventions. Protein and fat as primary macronutrients produce sustained satiety, minimal insulin response, and stable energy.
If weight loss is the goal — put down the shake, back away from the reward culture, and let us talk about what is actually happening in your hormones. That is where the real work is. And it is far more interesting than counting calories.
References: Fung, J. (2016) The Obesity Code · Sumithran et al. (2011) NEJM · Ludwig & Ebbeling (2018) JAMA Internal Medicine · Epel et al. (2000) Psychosomatic Medicine · Warburton et al. (2006) CMAJ · Adams et al. (2010) Nutrition in Clinical Practice.
This blog is for informational and educational purposes only and does not constitute medical advice.