Let's Simplify Calories in VS. Calories Out (CICO)

So what forms the basis - what are the necessary conditions for us to lose fat?  To lose fat you must create a calorie deficit.  Simply put, a calorie deficit refers to consuming less energy (i.e. food) than your body requires or expends in a given period of time (typically measured within a 24 hour period).  Now, the common MISTAKE made by dieters and CICO “deniers” is assuming that CALORIES IN literally refers only to the total (known) numerical value of calories eaten/drank and that CALORIES OUT literally refers only to calories “burned” through exercise - and for some slightly more acquainted with bioenergetics, resting metabolic rate.

CALORIES IN
Calories-In is relatively simple: it’s the food you digest.  However, the calorie content of food is imperfectly determined and calorie availability can vary between species/variety/source and even due to the degree of processing involved.  These are small inconsistencies, but inconsistencies nonetheless.  For example, dietary fiber cannot be digested through typical enzymatic action in our small intestine (we do not produce cellulase).  So even though fiber HAS calories, it doesn't mean we "absorb" those calories.  However, gut flora in our large intestine can - through fermentation - convert fiber into short-chain fatty acids.  Those SCFA can translocate from the intestinal lumen into circulation, yielding calories.  Thus, the composition of your gut flora influences your CALORIES IN, and that composition differs between people, and across your lifespan.

CALORIES OUT 
This gets more complicated.  CALORIES OUT is not simply represented by what the treadmill tells you.  You inhabit a living body that, whether you are conscious of the fact or not, is perpetually dependent on energy availability.  In short, CALORIES OUT may be summed up as:

• Basal Metabolic Rate (BMR)
• Non-Exercise Activity Thermogenesis (NEAT)
• Exercise Activity (EA)
• Thermic Effect of Food (TEF)

Most people only consider EA when calculating their calorie expenditure, but this is only a fraction of it.  BMR refers to the energy required to “keep the lights on” - the energy needed to keep your organs functioning, while conscious, but not including activity.  This accounts for most of your daily energy needs (~1200 kcals).  NEAT refers to all the low-level or general activities you may perform that don’t include deliberate acts of exercise or sports, such as chores, walking, fidgeting, errands, etc.  Lastly, TEF represents the energy demands of digestion, and these demands can change based on your food choices.  How much do each of these things contribute to your energy expenditure?

• BMR - ~60%
• TEF - ~10%
• NEAT (~20-30% if EA is low)
• EA (~20-30% if NEAT is low; you exercise, but are otherwise sedentary)

IMPORTANT CAVEATS:

• As you lose weight, your BMR decreases
• The longer you stay in a deficit or below your “bodyweight set-point”, the more your NEAT decreases
• Two people with identical genetics can demonstrate a difference in NEAT by up to 2000 kcals!

All of the above caveats make your “target calories” a moving target!  Embarking on a weight-loss journey without considering the marvel of your metabolism - which is designed to keep you alive in times of food scarcity - is possibly the greatest oversight in nutrition.  Now, despite all the underlying complexity of metabolism, the process of determining and narrowing down what YOUR calorie needs are is actually quite straightforward:

1. For 1-2 weeks gather the following data:
   1. daily bodyweight
   2. all foods eaten or drank daily
   3. daily activity (time, type, and intensity)
      1. "All foods" not only refers to the what, but specifically how much (e.g. 1 cup of cooked rice, 200g of raw chicken breast, etc.)
2. Use a reliable reference database for caloric values of common foods (e.g. USDA database, appendices of a nutrition textbook, MyFitnessPal, etc.) to calculate your daily caloric intake.
3. Is your weight decreasing, maintaining, or increasing?  Also take note whether the first 7 day average is within 100kcals or less as the last 7 days.  Generally, most people put on weight over the Fall/Winter holiday season.  In other words, you are likely maintaining weight on your current intake.
4. If maintaining, you are neither in a deficit nor a surplus for your CURRENT weight AND activity level.  Sometimes people's eating habits don't really change all that much from age 20 to age 30.  However, their activity level can reduce drastically upon leaving house league or varsity sports and entering a sedentary workplace.  People buy cars, start families, and ultimately begin to rely on the daily conveniences of fast-food, snack-foods, delivery options, and of-course... get-thin-quick schemes.
5. Now in order to lose fat, you must induce a caloric deficit, which if you'll recall has several moving parts: Calories-In & Calories-Out (and all its divisions).  So if you consume less calories but maintain your activity level and lifestyle, or conversely maintain calorie intake but become more active - a calorie deficit can be achieved.  Ideally, a combination of the two major determinants is best, however that may depend on the specific scenario. 

E.g. A previous client came to me with a markedly low-energy intake (according to the Harris-Benedict equation, she was already in a caloric deficit), yet was still 20-30lbs overweight.  The calories weren't adding up!  Clearly this meant the calorie model didn't fit!  YES!  CICO debunked! ..... or maybe something is being overlooked?  - what jumped out at me was her remarkably low-activity level.  SOLUTION: we kept her calorie intake where it was while increasing her protein intake moderately, but our ace-in-the-hole was getting her started with a progressive, resistance training program for home.  Without changing her calorie intake she lost over 20lbs and was her lightest since high school! 

So was this client actually in a caloric deficit when she came to me?  According to the equation, yes - according to real-life, NO!  By definition, because she was not losing weight she was NOT in a caloric deficit.  It's important to remember that equations are crude estimations of calorie needs, and just exactly how efficiently individuals digest and utilize/partition the food molecules they eat is unique to them.  Activities like resistance training stimulate muscle protein synthesis (MPS) to a large enough degree that we can actually see its effect in 1-2 months - which is pretty amazing to think about!  The food you eat literally forms the essential elements of contractile muscle!  Because we created a demand for protein and energy (in the molecular form of ATP), we robbed her adipose (fat cells) to maintain, build, and repair actively trained muscles.  Training created the deficit, but it also told her body what to do with her food: Burn the Fat, Feed the Muscle!