Calories In vs Calories Out: What the Science Really Says

Does Weight Loss (and Gain) Ultimately Come Down to CICO?

In the world of weight management advice, few formulas are as well-known (or as controversial) as “Calories In vs Calories Out”

— the idea that if you eat fewer calories than you expend, you’ll lose weight (and vice-versa). But how accurate and useful is this model in real life, especially long-term? Below we dive into the science — looking at arguments in favour, and arguments contrasting the simplification.

TLDR: Science supports Calories In vs Calories Out being the driving factor for change in body mass. However, it is much more complex in real-world application than such a simple statement.

The Argument For CICO: The Energy-Balance Foundation

First-law of thermodynamics & the energy balance equation

At a basic level, the body is an energy system:

Energy in — Energy Out = Change in Mass

1st Law: energy cannot be created or destroyed, only transformed into another form e.g. stored fat, heat or movement)

https://www.precisionnutrition.com/calories-in-calories-out  

“When you consume more calories than you burn, you are in a state of positive energy balance which can lead to weight gain. Conversely, when you burn more than you consume, you create a calorie deficit resulting in weight loss.” https://healthcentre.nz/the-science-of-calories-understanding-energy-balance-for-weight-loss/

Energy in includes all the food and drinks you consume, which can be measured (though is often under reported)

Energy out includes your BMR-Basal Metabolic Rate (what you use to live), TEF-the Thermic Effect of Food (what you use to digest what you eat), and energy burned in physical activity: exercise and NEAT-Non-Exercise Activity (moving around, working, etc.)

So, eat less and/or move more to lose weight? – If only it were that easy…

Many diet plans, clinical weight-loss programmes and fitness interventions successfully use calorie deficits (or surpluses) to drive change

https://www.webmd.com/obesity/cico-diet

Calculate your calorie requirements

https://www.eatforhealth.gov.au/nutrition-calculators/daily-energy-requirements-calculator

Do cardio (it burns energy), do resistance training (it increases your ongoing expenditure), walk more…

But… Your body doesn’t like change… It wants to survive, and it doesn’t know about your goals.

• Increased physical activity will increase your hunger and make you more likely to overeat
• Fasting, and low-calorie diets will decrease your BMR through hormonal changes
• Improved cardio fitness makes you more effective at exercise, meaning you need to train harder, faster, longer for the same results
• Stress and poor sleep can adversely effect weight loss and muscle gain
• Hormonal changes can increase or decrease BMR and cause water retention
• Different medications can have effects on hormones and therefore weight loss

Metabolic adaptation does not negate energy balance

When the body adapts (slows its metabolism), the energy-balance equation remains true. It’s just that the numbers have shifted (i.e., your “calories out” drops)

The other Side of the argument:

The carbohydrate–insulin model

The carbohydrate–insulin model (CIM) of obesity posits that a diet high in processed, high-glycemic-load carbohydrates leads to hormonal changes, particularly elevated insulin, which shifts the body to store excess calories as fat. This process results in a state of “internal starvation”, causing increased hunger and decreased energy expenditure, which then drives the positive energy balance that leads to weight gain.

https://www.sciencedirect.com/science/article/pii/S0002916522002428

https://pmc.ncbi.nlm.nih.gov/articles/PMC6082688/

The Carbohydrate-Insulin model was declared to have been disproven in a study exploring the effects of a plant-based, low-fat diet versus an animal-based, ketogenic diet. In the study people on the LF diet consumed less calories, contrary to insulin led model predictions and a review of other studies. https://www.nature.com/articles/s41591-020-01209-1 https://www.nature.com/articles/ejcn2016260

Meta analysis of 32 studies favoured low-fat diets, compared to low carbohydrate diets when compared for the same calorie intake, in direct contradiction of the CIM model, and overwhelmingly found that decreased calorie intake combined with increased calorie expenditure (exercise) provided the best outcome.

https://pmc.ncbi.nlm.nih.gov/articles/PMC5568065/

While another systematic analysis of 12 reviews of multiple studies reported similar results regardless of carbohydrate proportion of diets and similar degree of longer-term adherence.

https://onlinelibrary.wiley.com/doi/10.1111/obr.12744

Gary Taubes and David Ludwig (Authors of the former paragraph’s articles) remain strong on their stances in support of the CIM.

https://www.sciencedirect.com/science/article/pii/S0002916522000697

https://pmc.ncbi.nlm.nih.gov/articles/PMC8634575/

The Oversimplification Argument:

Complex physiology

Critics argue that while the equation is physically true (you cannot create energy), in real-life the variables “in” and “out” are highly dynamic and influenced by many systems (hormonal, neural, gut microbiome, etc). They propose that the CIM and CICO models do not have to be all one or the other. For example: Hormonal fluctuations influence hunger and changes in BMR https://www.sciencedirect.com/science/article/pii/S0002916522002428

Also, commentary from the University of Sydney noted:

“It’s nearly impossible to calculate accurately” with BMRs varying person to person and digestive processes leaving calories in some foods undigestible, the TEF, genetic effects and the body’s adaptation to maintain homeostasis in response to weight loss. https://www.sydney.edu.au/news-opinion/news/2023/07/05/its-time-to-bust-the-calories-in-calories-out-weight-loss-myth.html

The reduction in resting metabolic rate may be a compensatory action of the body to adapt to loss of body mass, through hormonal changes (known as adaptive thermogenesis – AT). However, since this varies so much between people, they have been unable to confirm this definitively.

https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/does-adaptive-thermogenesis-occur-after-weight-loss-in-adults-a-systematic-review/726FC60518DA67349B9C3EC1D75A7156

Errors/inaccuracies in estimating “in” and “out”

A study titled: Discrepancy between Self-Reported and Actual Caloric Intake and Exercise in Obese Subjects showed participants underreported their actual food intake by an average of 47–16% and overreported their physical activity by 51–75%

https://www.nejm.org/doi/full/10.1056/NEJM199212313272701

Even if the equation is valid, in practice it is very hard to measure accurately: calorie intake estimation is often off; trackers for expenditure are imprecise; individual absorption and metabolism vary.

“The most accurate calorie estimates from wrist-worn trackers were off by 27%, while the least accurate were off by 93%.” https://nypost.com/2025/01/31/health/why-youre-not-losing-weight-in-a-calorie-deficit/
Also: Not all calories are absorbed when eaten… high-fibre foods may allow some calories to pass through unused. https://www.sciencealert.com/weight-loss-involves-more-than-calories-in-calories-out-heres-how

Not all excess weight is fat:

Typically, fat mass accounts for around 60-70% of weight gained, the rest being water, glycogen, gastrointestinal contents etc. Commonly, early dieting weight loss can be attributed to water loss, which then slows or returns leading to decreased long term adherence.

https://pmc.ncbi.nlm.nih.gov/articles/PMC5786199/

Healthy Habits

While short-term weight loss via bold calorie deficits may work, many people struggle to maintain the changes. The “body regulation” systems push back. Purely counting calories without behaviour/habit/context is less sustainable, you need to find something you can stick to long term.

Moreover, if you create large deficits without adequate protein, strength training or nutrient density, you may lose muscle (reducing “calories out”) and violate body composition goals.

If your goal is fat loss, we can help with group fitness classes or a nutrition and exercise program to balance cardio and resistance training so you’re fit, strong and healthy—not just lean.

If your goal is increased muscle size, tailor a nutrition and training program to gain muscle mass at the right rate, so you don’t wind up with excess fat.