Energy balance and thermodynamics is the primary factor involved in increasing, maintaining, and decreasing body fat. When you look at the full energy balance picture, it helps explain why the simple idea of calories in vs calories out can actually be complex, and why different people of the same size may have different calorie needs. Sometimes we hear that bodies are machines. But we are much more than that. We are multifaceted systems that have the amazing ability to self-regulate and respond dynamically to external and internal factors. So, the following information starts out seeming simple, but as you continue to read about the factors involved on both sides of the energy balance equation, you'll quickly see why determining an individuals calorie needs is not black and white.
There are two sides to the equation when it comes to maintaining your current body weight:
Calories In —— Calories Out
If those things are equal than your body weight stays the same (but water weight and current food in the system will fluctuate throughout the day and week- so even if you are in a maintenance state, your scale weight will fluctuate by at least a little)
Some people get really worked up about this and say that this isn’t how it works- that there is much more involved then simply Calories in and Calories out. And to a degree, they are very much right. But those other factors they have in mind are all subsets of Calories in and Calories out. Let’s review the factors that affect both sides of this energy balance:
Factors affecting Calories/Energy In
Appetite: this can be greatly affected by your hormones. Thus why many women see a spike in appetite when on their period. Also, when cortisol is elevated your appetite increases. Cortisol will likely elevate when your stress level rises and you are low on sleep. Bottom line: when appetite increases, it’s more difficult to keep control of your food intake.
Environmental Factors: These are often things that effect mindless eating- such as long duration social functions surrounding food, distractions of watching tv and sporting events while snacking, quick food availability at home and convenience stores, etc.
Psychological Factors: hyperpalatable foods are in abundance nowadays. These are foods that are fatty, salty, sweet, and/or with refined carbs. Foods such as donuts, chips, sugar sodas, fries, etc are very calorie dense (low volume, and high calories) and lead us to crave more and provide little nutritional benefits. They tend to actually increase our desire to eat and thus quickly overeat without us knowing (unless we are being very attentive to our daily intake). Also, our mental and emotional state will quickly affect our ability to make good food choices. When depressed, lonely, stressed, we often reach first for the most hyperpalatable foods.
Metabolizable Energy: This is a big factor that can seem to muddle the energy balance equation. But in fact, this is PART of the equation. Every individual metabolizes the food and types of food they intake differently. Meaning- some people have very efficient bodies that USE and absorb more of the food they intake than others. So, ironically, more EFFICIENT bodies may also struggle more with weight loss. Because their body actually digests and metabolizes more of their food than someone else with a less efficient system. Also, some types of food are more digestible than others. A Twinkie, for example, will be quickly used and metabolized by the body (so good for quick energy but not much else!). While leafy greens will be slower to be metabolized and not all of it is able to be used by the body and thus excreted. Bottom line: not all of the calories you eat are actually used by the body, but still your body requires less metabolizable calories than you burn in order to lose body fat. So, the more precise way to view weight maintenance energy balance is METABOLIZABLE Calories In = Calories out.
Factors affecting Calories/Energy Out
BMR (Basal Metabolic Rate): 60-70% of the calories you burn are burned at REST or all the calories you would burn if you were in a coma. This often surprises people- but over the course of a day you burn far MORE calories sleeping and keeping your body functions working over a 24 hour period, than you do during 1-2 hour bouts of exercise. Those with more muscle mass will have an increased BMR- one of the many reasons why resistance training while working on fat loss is so important. Hormones can also affect a person's BMR- and since your Basal Metabolic Rate is such a huge chunk in the calories out equation, you can see how hormone issues and a lowered BMR can result in frustrating attempts at weight loss.
NEAT (non-exercise activity thermogenesis): 15-50% of the calories your burn are through your daily life activities- how much you move in your job and general lifestyle. This isn’t just walking, but also fidgeting. You may notice that leaner people have trouble sitting still- are always fidgeting, rocking their feet, shifting their weight, etc. The percentage range of this factor is large - because this varies A LOT between different people. This is why I encourage tracking steps and maintaining and possibly increasing your daily steps when in a calorie deficit. Your NEAT will be quick to decrease when you feed your body less.
Exercise: 15-30% of the calories burned come from actual exercise (cardio, weight lifting, gym classes, etc).
TEF (thermal effect of food): 10-15% is the energy burned from the food you actually eat. You burn calories in the digestion process- and protein has the highest thermic effect (20-35% of protein calories are burned through processing). Next is Carbs (5-15% of carb calories are burned through processing)- and last is fats with only 0-5% being burned. This is one of many reasons why I encourage clients to have high protein diets- particularly when focused on fat loss. It's also important to keep protein adequate for your body, paired with resistance training, to help maintain muscle as you lose fat (because we want to lose fat and NOT muscle!)
In other words, what types of food, how much you eat, how efficient your body uses food, how much your body burns at rest, and how much you move can all effect the calories in and calories out equation. Since this can feel convoluted, the best way to move forward, is to use an individual trial and error approach over the long term to see what your true day to day needs are for YOU. You can then adjust as needed depending on YOUR progress. Do you need to track calories to lose weight? Definitely not. But it's a logical step to take to make progress if you aren't seeing fat loss change even though you eat healthy. Eating "healthy" or "clean" is incredibly subjective. And the bottom line is that although the quality of your food matters greatly for health, the quantity of your food also matters for weight management (and is a big part of finding your appropriate energy balance). Tracking calories is simply ONE method of finding your bodies needed food intake.