Calories In, Calories Out
When weight loss psychology meets physics.
Posted October 16, 2019 | Reviewed by Devon Frye
When it comes to weight loss, no formula is more maligned or misunderstood than the calories in, calories out equation. Attempts to either defend or refute this equation in a public forum invariably result in heated debate, even among circles of people with advanced nutrition, biology, or even physics degrees .
On the surface, the calories in, calories out debate is difficult to understand. This equation is derived from the first law of thermodynamics, reading that “energy cannot be created or destroyed in an isolated system.” Across decades of increasingly precise scientific measurements of energy systems, no exceptions to this first law have been found.
Further, because the human body is a type of energy system, it follows that biological processes such as weight loss must also operate in accordance with this law. In reference to the calories in, calories out equation, this means that a human body can neither gain energy (i.e., gain weight, because energy also has mass) without a source to obtain the energy (e.g., increasing calories in by eating more) nor lose energy (i.e., lose weight) without a process that allows energy inside us to leave the body (e.g., increasing calories out by moving more; see Ruben Meerman’s TEDxQUT talk on YouTube for an excellent visual explanation of the chemical process of weight loss ).
Combine the calories in, calories out equation with estimates of the amount of energy stored in fat (about 3,500 kcal in a pound of body fat), and this becomes the source of common advice about losing a pound of weight a week by reducing calories in by 500/day, increasing calories out by 500/day, or achieving the same total through a combination of diet and exercise. Simple!
The problem is that when people apply the calories in, calories out formula to their weight loss experiences, their results rarely align with what is predicted. For example, many go on diets only to find that they lose less weight than their calories in, calories out equation suggests. Others exercise more, only to find that their increased calories out total exceeds the weight loss they achieve. And these are only the most common examples.
Observations of different weight loss results from different diets, weight gain side effects of medicines, hormone changes—such as those resulting from menopause and thyroid dysfunction—and even controlled studies where people gain or lose different amounts of weight despite being feed carefully measured portions, all seem to counter the calories in, calories out equation. When real-world weight loss results routinely fail to meet the formula’s predictions, the debate about the calories in, calories out equation begins to make sense.
It turns out that both sides of the calories in, calories out debate are correct in their own ways. Critics of the calories in, calories out equation, for example, rightly argue that simply eating less and moving more will not reliably predict weight loss when the formula is used in a static form. For instance, if a person only compares their calories consumed to their calories expended through exercise, their weight loss prediction may be very different from their results measured on a scale.
And, unfortunately, most people use the calories in, calories out formula only in a simple and static form for reasons that are predictable. Our minds crave simplicity, including for weight loss, seducing us into believing that there is a special diet, exercise, or supplement that has all the answers, even when the reality is more complex.
This is where the advocates of the calories in, calories out equation are correct: When the formula is used in a dynamic way, it accurately accounts for every weight loss result, including all the so-called exceptions that people think prove the formula wrong. The figure below shows an example of a simple-static application of the calories in, calories out equation versus a dynamic application of the same equation.
To expand from the figure in a practical way, consider how the dynamic application of the calories in, calories out equation explains each of the following results thought to disprove the equation.
1. If the calories in, calories out equation is true, then why do people lose more weight with keto or low-carb diets?
Perhaps the most common objection to the equation is the markedly different short-term weight loss results people observe when following low-carb diets. Even a smaller person may lose five pounds in the first week on a keto diet, for example, and a larger person may lose 10 or more pounds.
Compare this to a one- to two-pound weight loss by a person on a normal carbohydrate diet—with the same calories—and the calories in, calories out equation seems fatally flawed. However, this is also the most easily explained result; the rapid weight loss achieved by low-carb diets is the result of water loss rather than a loss of body fat. Fat loss does not differ between calorie-matched lower and normal carbohydrate diets, as shown in many clinical trials.
2. If the calories in, calories out equation is true, then why can some people “eat whatever they want” and never gain weight, while others must watch every calorie?
Even in two people of the same size and gender, following the same diet and exercise routines, weight loss results can still differ in important ways explained by the calories in, calories out equation.
For instance, the kind of physical activity scientists call NEAT—referring to non-exercise movement outside of formal exercise—can differ substantially between people of the same age and size. Some people fidget, pace, stand, and gesture more, whereas others are just the opposite. Further, this source of energy expenditure is dynamic, changing in response to calorie intake.
Ever notice that some people eat a large Thanksgiving dinner and then crash on the couch (lowering NEAT)? Other people do the opposite, becoming more active after overeating. The famous 1999 study from Science  finding that overfeeding people resulted in different weight gain results, for instance, was explained by differences in NEAT, entirely consistent with the calories in, calories out equation—even though some try to use the study as a refutation of the equation.
3. If the calories in, calories out equation is true, then why can’t people with poor thyroid function lose weight even when they are eating very few calories?
This is also a common argument against the equation that, in fact, strengthens it. Poor thyroid function decreases the calories out side of the equation in two ways.
First, low thyroid function reduces body temperature. This is critical because body heat is the largest component of our basal metabolic rate. Lowering temperature even by a degree or two results in hundreds of fewer calories used per day. Secondly, low thyroid function results in fatigue, reducing calories out by lowering exercise and general movement. Combine these two effects, and the weight loss difficulties reported by people with poor thyroid function is entirely consistent with the calories in, calories out formula.
4. If the calories in, calories out equation is true, then why do people gain weight as they get older, even when they eat and exercise the same as always?
As people go from youth to older ages—even maintaining the same diet and lifestyle—their calories out usually decrease in ways that promote weight gain. First, most people lose muscle as they age. Muscle is high maintenance material and requires more energy to preserve than body fat. As muscle loss accumulates over time, the metabolic rate declines.
Secondly, general movement (NEAT) also usually decreases with age. Children can be in constant motion, whereas older-aged people move much less in comparison. This adds up, in the average person, to a calories-out total that may be hundreds less per day than among their younger self. As predicted by the calories in, calories out equation, the usual result is gradual weight gain even when calories and exercise are the same.
5. If the calories in, calories out equation is true, then why do people hit weight-loss plateaus even when they keep up their diet and exercise programs?
Plateaus are a great example of the dynamic nature of the body’s energy balance system. As a person loses weight, their calories out decrease even when they maintain the same diet and exercise routines. This is because, with a smaller body, their basal metabolic rate decreases, as does their energy expended during exercise and general movement.
Consider that a larger person is like a V8 engine in a car. A big engine requires a lot of fuel to function. As that person loses weight, however, they become a V6 engine that needs less and less fuel. Simply put, weight-loss plateaus are further evidence of the need to treat the calories in, calories out equation dynamically rather than statically. Plateaus are a reminder that we need to keep adjusting our weight-loss approach over time to keep getting results.
1. Defending Thermodynamics in a Diet Debate. https://physics.aps.org/articles/v12/47.
2. The mathematics of weight loss by Ruben Meerman. https://www.youtube.com/watch?v=vuIlsN32WaE.
3. James A. Levine, et al. Role of Nonexercise Activity Thermogenesis in Resistance to Fat Gain in Humans. Science 283, 212 (1999).