Why calories in minus calories out doesn't work

Many of us have been told by our friends, family, and doctors that losing weight is easy—we just need to eat fewer calories than we expend, right? If only it was that simple! Weight loss is actually far more complicated but by making the right choices can have a significant impact on your hormones, metabolism, and gut health, helping you lose weight effectively and sustainably. 
Napala Pratini
min read
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Quick summary

  • Calories in minus calories out is a classic "hack" to weight loss. But it's actually a massive oversimplification.
  • Different types of calories are processed differently in the body, meaning that their impact on our metabolism, hunger, and more varies
  • Metabolism, hormones, and gut health all play important roles in weight loss and are important to consider in addition to calories

It’s probably fair to say that anyone who’s tried dieting will have heard of “calories in, calories out” (which, if you can believe it, is sometimes abbreviated to CICO). Many of us have been told by our friends, family, and doctors that losing weight is easy - all we need to do is follow along with this simple formula:

Energy In (what we eat in the form of calories) - Energy Out (the energy our bodies consume) = Energy Stored (the reason for weight gain or loss).

According to this rule, if you consume fewer calories than you burn, you will lose weight. From a purely scientific perspective, it checks out. For the majority of people, consuming, for example, 1,000 calories per day will lead to weight loss. But the truth is that, for a long-term and sustainable diet, there’s much more to think about than CICO. 

Calories in minus calories out overlooks the critical fact that not all calories are equal.

The main issue with this formula is that it assumes all calories are equal. And while they are equal in the basic sense that a calorie is a unit of measurement for energy, the body reacts and processes calories across food groups very differently. The food sources you choose will affect your metabolism, hormones, and gut microbiome - each of which play a major role in how successful you will be in losing weight, and maintaining that weight loss over time.

All calories are not equal

When the CICO formula was created, very little research had been done into the way our bodies process and respond to various food sources. In the words of nutrition journalist Gary Taubes, ‘For the last 60 years, researchers studying obesity and weight regulation have insisted on treating the human body as a thermodynamic black box...all living organisms are ruled by biology, not thermodynamics’.[1] Today, we know that CICO is a very oversimplified idea. 

To illustrate how the body responds to calories from different sources, let’s compare types of sugars. Peas, beans, wholegrains, and vegetables contain something called complex carbohydrates, which are long sequences of sugars strung together in a complex chain. These have to be broken down in order to enter the bloodstream as glucose. Because these sugars are gradually processed and turned into energy, and also contain other vital nutrients like vitamins and fibre, they make us feel full and sustained over a long period of time.[2]

However, foods such as sodas and cookies contain refined sugar, which takes just minutes to move from the stomach to the bloodstream. If you’re familiar with blood sugar monitoring, you’ll already know that simple sugars moving quickly into the bloodstream will cause a blood sugar spike. It’s not the spike itself that causes problems, but the body’s response, which is to produce a surge of insulin. An insulin spike makes the body transfer all the glucose into cells very quickly. This leads to rapidly falling blood glucose levels, which makes you crave more sugar.[3] 

Over time, blood sugar spikes can have other negative effects. Elevated levels of insulin can eventually lead to leptin resistance. Leptin is the hormone which makes us feel full. If you’re resistant to it, you’ll never quite feel satiated - it’s no surprise that this often leads to weight gain.

Research indicates that ultra-processed foods make up around 60% of the Western diet.[4] These foods, full of refined sugars, have incredibly negative impacts on our bodies’ insulin response and hunger levels. Plus, they even create physiological changes which make us crave them more. Moving away from a processed diet is a crucial step for working towards sustainable weight loss.

The role of metabolism

When most people think about the CICO formula, they assume that “Energy Out” is based on how much we exercise. In fact, it’s more to do with metabolism. Your metabolism is a complex process in which your body combines what you consume with oxygen, turning food into the energy your body needs to function. 

Most people have a Basic Metabolic Rate (BMR) of around 1400-1800, which means they burn 1400-1800 calories per day just by staying alive. While additional exercise can boost the number of calories your body uses, the couple hundred calories you might burn during a workout pale in comparison to 1400+ calories burnt at rest. So much higher than the few hundred calories you might burn during an intensive workout! Making sure you have a healthy metabolism is an important part of ensuring that your body is losing weight as efficiently as possible.

Some foods require more work to digest, absorb, or metabolise than others. The amount of work it takes your body to process a food is measured by the food’s thermic effect, or TEF. The higher the TEF, the more energy it takes for your body to break it down. Protein has the highest TEF of any food group and fat has the lowest. Therefore, protein is often said to boost your metabolism more than carbs or fat.[5]

Considering the TEF of the foods you eat is just one way that you can optimise your weight loss. There are many other ways to boost your metabolism, such as lowering stress levels, weight training, and consuming caffeine and green tea.[6] Interestingly, while most of us think about exercise because of the calories you burn whilst exercising, some will argue that the more important impact of exercise is that it increases muscle mass, which in turn boosts BMR—and voila, all of a sudden you're burning more calories on a daily basis, without even trying.

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Hormones and gut health are important

The gut microbiome (the collection of bacteria in your gastrointestinal tract) is made up of trillions of bacteria. These have a big impact on metabolism, hormones, and how we absorb nutrients.[7] Your gut microbiome has many important functions, including:

  • Helping your body efficiently absorb macro and micronutrients
  • Aiding the regulation of hormones and neurotransmitters, such as oestrogen and insulin
  • Keeping your immune system healthy

Your gut health is responsible for regulating some very important metabolic processes, including sensitivity to insulin, fat storage, and appetite. Research indicates that plant-based diets, and diets high in fiber and protein, result in a much healthier microbiome. However, sugar and processed foods such as artificial sweeteners are shown to have a negative effect on the gut. Gut health is a frequently overlooked aspect of wellbeing, but can have a big impact on weight loss.[8] 

Weight loss is more complex than calories in vs. calories out

The CICO formula is appealing because it’s so straightforward but, in reality, weight loss just isn’t that simple. To make lasting changes to your health, it’s important to think about what foods you’re eating and why.


[1] Taubes, G. (2007) The scientist and the stairmaster, Available at: https://nymag.com/news/sports/38001/index3.html (Accessed: 18th May 2021).

[2] U.S. National Library of Medicine (2020) Complex carbohydrates, Available here. (Accessed: 15th June 2021).

[3] Hensley, L. (2021) Here's everything you need to know about fruit sugar, Available at: https://aaptiv.com/magazine/fruit-sugar (Accessed: 18th May 2021).

[4] Baraldi, L. G., Martinez Steele, E., Canella, D. S., and Monteiro, C. A. (2018) ' Consumption of ultra-processed foods and associated sociodemographic factors in the USA between 2007 and 2012: evidence from a nationally representative cross-sectional study', BMJ Open, 8(3) [Online]. Available at: https://doi.org/10.1136/bmjopen-2017-020574 (Accessed: 18th May 2021).

[5] de Jonee, L. and Bray, G. A. (1997) 'The thermic effect of food and obesity: a critical review', Obesity Society, 5(6), pp. 622-631 [Online]. Available at: https://doi.org/10.1002/j.1550-8528.1997.tb00584.x (Accessed: 15th June 2021).

[6] Jeukendrup, A. E., and Randell, R. (2011) ‘Fat burners: nutrition supplements that increase fat metabolism’, Obesity Reviews: An Official Journal Of The International Association for the Study of Obesity, 12(10) [Online]. Available at: https://doi.org/10.1111/j.1467-789X.2011.00908.x (Accessed 15th June 2021).

[7] Aziz-Scott, G. (2021) Hormones and gut health: the importance of gut health for hormone balance, Available at: https://www.mariongluckclinic.com/blog/hormones-and-gut-health-the-estrobolome-and-hormone-balance.html (Accessed: 18th May 2021).

[8] Martin, A.M., Sun, E.W., Rogers, G.B., and Keating, D.J. (2019) 'The influence of the gut microbiome on host metabolism through the regulation of gut hormone release', Frontiers In Physiology, 10(428) [Online]. Available at: https://doi.org/10.3389/fphys.2019.00428 (Accessed: 18th May 2021).

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