What’s An Energy Or Calorie Deficit And Why Does It Matter For Fat Loss?

What’s An Energy Or Calorie Deficit And Why Does It Matter For Fat Loss?

By Dr. Cody Haun


In the physics sense of the word, energy can be defined as the ability to do work.
According to the first law of thermodynamics, energy can neither be created nor destroyed but can change forms.
Energy can be categorized as kinetic or potential energy.
Kinetic energy is the energy of movement. For example, the motion of molecules.
Potential energy is stored energy which is of particular importance to this post.
In human biology, potential energy can be better understood considering the energy stored in chemical bonds of atoms within molecules.
An example of potential energy in the human body is the energy stored in the covalent chemical bonds of triglyceride molecules in fat cells.

A triglyceride is a lipid that consists of a glycerol molecule bonded with three fatty acids. Triglycerides are stored in lipid droplets of adipose or fat cells and the amount stored largely dictates the size of the fat cell.

Derived from Mobley et al., 2017

Triglycerides can be broken down and provide energy-yielding free fatty acids in circulation to be taken up by other tissues like muscle, and used to synthesize adenosine triphosphate (ATP) which is the energy currency of cells. When ATP is broken down, energy is made available to cells to perform work (e.g., muscle contraction). Consequently, this can result in a reduction in fat cell size over time, as shown in the picture above after a training intervention.

Energetic demands of the body can be broken down as follows: 1) resting metabolic rate (RMR), 2) non-exercise energy expenditure (NEE), and 3) exercise energy expenditure (EEE).

RMR is generally the largest component of total daily energy expenditure (TDEE) for most people (~60%). Resting metabolic rate is the energy expended at rest which results from the typical function of organ systems. See the table below from McClave and Snider derived from “Dissecting the energy needs of the body” for an example of various organ metabolic rates and how these contribute to RMR (REE in the table below).

While total RMR is quite easy to estimate by multiplying body mass in kilograms by 22 (or bodyweight in pounds by 11), or by using other estimation equations (e.g., Harris-Benedict, Mifflin, etc.), NEE consists of the thermic effect of food (TEF) and non-exercise activity thermogenesis (NEAT). TEF depends on how much food is consumed and what the food consists of (e.g., proportion of fat, protein, or carbs), while NEAT depends on a host of factors. NEAT can be thought of as the energy expended beyond RMR that is not a result of sleeping, eating, or exercise. For example, walking, cleaning your house, or fidgeting. NEAT can vary widely between individuals, but some general guidelines can be gleaned from the literature.

Consider this quote regarding NEAT from Christian von Loeffelholz, M.D. and Andreas Birkenfeld derived from the article: “The Role of Non-exercise Activity Thermogenesis in Human Obesity”.

“The importance of NEAT becomes apparent when considering the following: the variability in BMR between individuals of similar age, BMI and of equal gender ranges around 7-9% (39), while the contribution of TEF is maximally 15%. Thus, BMR and TEF are relatively fixed in amount and account for approximately three quarters of daily TEE variance. As EAT is believed to be negligible on a population level, NEAT consequently represents the most variable component of TEE within and across subjects. It is responsible for 6-10% of TEE in individuals with a mainly sedentary lifestyle and for 50% or more in highly active subjects (15, 19, 37).”

Here’s a way to estimate NEAT + RMR for yourself. On what you’d call a normal day NOT including exercise, select which description best fits your lifestyle.

(calculations above based on examples from this source)

Technically, EEE depends on the type and work rate of the exercise and should be directly measured for high confidence. But, I’ve found a reasonable estimate of calorie expenditure during most types of exercise for most people to be the duration of active work or exercise in minutes multiplied by the rating of perceived exertion (i.e., RPE) on a scale of 1-10 with 1 being extremely easy and 10 being extremely hard (subjectively). For example, 30 minutes of work at a 5 RPE equals to ~150 calories expended. 60 minutes of work at an RPE 8 equals to ~480 calories and so on. This is not a perfect estimate and more formal equations are available. Wearable technology can be used to provide this estimate and plenty of calculators and scientific publications provide alternative approaches. This is just an example method of estimation. Monitoring energy intake, exercise volume (EEE), and subsequent bodyweight change can provide improved precision. Of relevance, people who weigh everyday tend to adopt more weight control behaviors and lose more bodyweight over the course of months.

These calculations in addition to the assumption that TEF contributes to TDEE via an additional ~10-15% of the calories actually consumed. For example, if I were to consume 2000 calories in a day, in general, a safe estimate is that ~200 calories would be expended to effectively digest and absorb the food. So, let’s say by RMR + NEAT = ~2500 calories. If I consume 2500 calories, TEF results in an additional ~250 calories expended. On a day involving no exercise, this results in a TDEE of ~2750 calories (i.e., RMR + NEAT = 2500, TEF = 250, RMR + NEAT + TEF = 2750).

To simplify for now, these components (i.e., RMR, NEE, EEE) can be used to calculate total daily energy expenditure or TDEE.

To liberate stored energy from fat cells and realize decreases in fat cell sizes (i.e., “fat loss”), consuming less energy from food than TDEE provides a powerful signal for fat loss and, objectively, is the most foolproof way to lose body fat.

To revisit the question in the title of the article, a calorie or energy deficit is simply consuming less calories than TDEE.

If less calories are consumed than TDEE, the resultant calorie deficit can result in remaining energy demands of the body (i.e., RMR + NEE + EEE) being primarily met by free fatty acids liberated from storage in fat cells.

Using the calculations above (or similar ones) and monitoring energy intakes, energy expenditures, and resultant bodyweight change to regularly create a calorie deficit can allow for fine-tuning of the process since numbers won’t be static as variables change (e.g., decrease in bodyweight, change in training volume, etc.)

Considering this, aiming to lose ~0.5-1 % bodyweight per week is a nice target for most individuals to avoid the loss of lean body mass while concurrently making notable fat loss progress.

Check out this blog and this blog for more information on the construction of calorie and macronutrient targets to support this goal.

To provide a bit more clarity, consuming ~500 calories below TDEE per day will typically result in ~1 pound of fat loss per week since there are ~3500 calories stored in 1 pound of fat tissue (454 g in 1 pound, ~85 % of fat cell composed of triglycerides, .85 * 454 = 386 g, 386 g * 9 calories in 1 gram of fat = 3,473 calories).

Using this method, numbers can be fine-tuned over time to ensure continued progress. Future posts will be dedicated to proper durations of fat loss dieting, or massing, but a general guideline is to carefully interrupt prolonged deficits with increased intakes insofar as to avoid burnout or unnecessarily slow progress at a certain point. Generally, a few months of dieting can be interrupted with 1-2 months of maintenance dieting followed by a shift back to cutting or massing. However, certain situations dictate different approaches and this is another reason why hiring a qualified coach might be worthwhile if you’re uncomfortable setting this up for yourself.

To summarize, a calorie deficit provides the requisite signal that elicits physiological responses which result in the mobilization, transportation, and eventual oxidation of fat for energy since energy consumed through food isn’t meeting the energy demands of the body.

The loss of body fat boils down to these calculations with the level of adherence to an intervention or diet, psychological variables, and other factors also playing vital roles in successful outcomes. This post is simply intended to highlight some of the concepts and calculations behind the argument that creating a calorie deficit is the most important factor for fat loss, and is not meant to delve into too much detail on the psychology of dieting (although this is equally if not more important since these calculations won’t really help someone if they aren’t adhering to a plan).

Of course, a diet structure for fat loss which can be adhered to long-term is critical.

Numbers might be similar in a variety of dietary structures but if someone doesn’t adhere, failure ensues.

Being armed with this information and the various formulas is important, but, to ensure long-term progress, so is creating a dietary structure that is enjoyable and adherable. Otherwise, it’s very unlikely that the approach will be successful.

Considering this, many people benefit from hiring a qualified coach to help set up numbers and an appropriate dietary strategy at least in the initial stages of pursuing a body composition goal.

If you’re curious about the psychology of dieting and strategies to better adhere to a plan, Sohee Lee has some really good content over on her site regarding the psychology of dieting and a flexible dieting approach that I’d recommend checking out.


I hope you found this blog post helpful!

Best wishes and be on the lookout for more writing soon!


-Dr. Haun


“I’m a scientist first and a coach second. I have a passion for positively impacting the lives of people through providing critically thought-out, data-driven, scientifically-sound nutrition and training programming services that equip individuals to successfully achieve their performance and/or physique goals. I seek to offer the best service within my power and I am confident, given my background, education, experience, and relentless pursuit of knowledge pertaining to human physiology and the training process, that I can provide you with programming to realize great results. Feel free to contact me with any questions.”


Cody Haun, PhD, MA, CSCS
-APLYFT Science Consultant