What changes how many calories we burn?

In the never ending quest to simplify fat loss, some approaches tend to gloss over facts and physiology in favor of assuring us if you just do "X" that you can draw a straight line from where you are now to your goal.  Whether it's fat loss, performance or muscle gain, we as humans like to be able to predict what is going to happen and plan for that eventuality.  However, since we are also very adaptive creatures, every change we make in how much we eat or exercise results in an internal change in how the body expends energy.

We don't often think about how important it is that our physiology adapts to our nutrition and activity with limiters on what we can influence or force.  If you could mathematically predict fat loss then many more people would be walking around at 8% bodyfat and pushing the genetic limit of muscle mass. We have constraints on what we can do simply because choices we make in terms of nutrition and exercise are a homeostatic threat; essentially things that nudge or push our bodies out of their comfort zone.

To keep things as short, sweet and digestible as possible I'll go over what adapatations our metabolism makes when we diet and how much they impact our energy expenditure.

Total Daily Energy Expenditure (TDEE)

Consider this the total amount of calories you expend in a day.  However, most people think that exercise equals energy expenditure and that is in truth, a very small part of how many calories we burn.  Our TDEE is in fact made up of:

Resting Metabolic Rate (RMR): The number of calories you burn at rest. This is essentially your Basal Metabolic Rate (BMR) plus being awake.

• Your bodyweight has a huge influence on this with lean mass (muscle, organs) being the primary driver of how your RMR responds. As you lean out and decrease in size/weight, your RMR drops because a smaller body means fewer calories needed.
• There is an adapative component however that results from the loss of body fat. As body fat decreases hormones are affected including leptin, cortisol, adrenaline, thyroid and growth hormone. 
• You could predict the drop in RMR based off of body weight alone but the adaptive component skews that number meaning you expend 10-15% fewer calories than predicted based off weight changes alone.

Thermic Effect of Activity(TEA): How many calories you expend during structured and planned exercise.

• A larger body requires more energy to move (both muscle AND fat), so as you lean out, the energy expended during exercises drops a small amount
• Some muscular efficiencies may increase to help off set energy expenditure when eating less. Simply being on fewer calories (and carbohydrates in particular) may affect how intensely you can train and how much volume you can do.

Thermic Effect of Food (TEF): How many calories you expend digesting and assimilating nutrients.

• The energy it takes to digest and absorb the food we eat is about 10-15% of our total expenditure.  As the amount of food we eat goes down, this will go down with it based off the percentage.

Non-Exercise Activity Thermogenesis (N.E.A.T): How many calories you expend during any activity that is not exercise .  This not only includes fidgeting and tapping but also things like walking around, gardening, standing and sitting etc.

• This is the largest variable among individuals, dieting or not and the is the most influential factor in how many calories you expend while dieting (whether fat loss or muscle gain is the goal).
• NEAT is largely subconscious meaning during low-intensity activities you will expend less energy or simply do less of that activity without consciously making that decision.
• An example may be that while dieting you simply get up and move around at the office less and when you come home you garden for a shorter period of time, take the dog on a shorter walk and when you sit on the couch you don't fidget, tap or get up for as many bathroom breaks.
• Even particularly hard workouts where you train to exhaustion might result in less NEAT the rest of the day meaning the net change in energy expenditure is zero.

Hormone Changes

In addition to this, as you lean out your leptin drops which drives hunger in the brain and hormones released from the digestive system signal the hypothalamus in various ways.  Ghrelin, which is a hunger hormone increases as energy is reduced and tends to stay higher even after eating.  Ghrelin will be released before meal times if meals occur at the same times each day and this can make restricting a meal even harder. Fullness hormones like Cholecystokinin (CCK) Peptide YY (PYY) are fullness hormones that respond to ingesting protein, fats and fiber. With fewer of those nutrients coming in and less stretching of the stomach after meals, the sense of "fullness" goes down.

Your hedonic system relates to the biological reward for food and taking pleasure in eating it. It is driven by dopamine and how much you enjoy highly palatable foods sets you at a higher or lower level of wrestling with the desire to cram in some ice cream.  As restriction and fat loss increases, the hedonic drive to eat highly palatable and calore dense foods go up. You might tend to think about food more and enjoy how things taste to a larger degree.  As Escoffier said, "hunger makes the best sauce".

What this means for you

If we look at the above, we have a few things working against us.  Not all is lost though!  These adaptive changes are there merely to protect our system from threat and ensure survival.  In the modern world with unlimited access to food, especially calorie dense and highly palatable food, our ability to bypass fullness for pleasure or resist over-eating is challenged. You can eat when you aren't hungry, you can barely ever exercise or move and you have access to healthcare should your choices affect your health.  We have a lot of things in place that allow us to eat more, move less and make nutrient-poor but calorie-dense food choices.

There are a few key strategies to use to bypass these things:

• Eat a protein rich diet. 1.8 to 2.2g protein per kilogram or 0.8 to 1 g per pound.  Not only does this help ensure higher levels of satiety hormones but it will help protect against muscle loss, slow digestion and takes more energy to digest than other nutrients
• Eat mostly whole foods.  Eating whole foods compared to process foods results in about 15% more calories expended through digestion.  Not to mention the added fiber and stretching of the stomach from higher food volumes will contribute to fullness
• Keep strength training..  Do we even need to cover this?
• Off-set reductions in NEAT by ensuring you still move 8,000 to 10,000 steps per day.  Wear a Fitbit for a little while and see if you are hitting that goal.  Make sure you don't sit idle for too long, go on walks and in general stay active.  If you go to the grocery store, spend some time strolling around the aisles to get some steps in.
• Don't overly restrict carbohydrates too soon.  If you have less glycogen to fuel your high intensity training and your level of effort goes down, so does your calorie expenditure.  Even more of an issue would be losing any sort of training response because you feel so fatigued that you stop challenging yourself.
• Don't overly restrict calories too soon. As with carbohydrates above, a huge initial calorie reduction will obviously see fat loss but will also see a greater drop in calorie expenditure from all the means listed above.  A more moderate approach between 10-20% total calorie reduction is more sustainble at first and ensures it is manageable.  Increasing from there can be done on an individual basis.
• While this is a discussion for another time, taking a "diet break" whether it is one week back to maintenance every 3-5 weeks, having one to two maintenance days per week or an entire month at maintenance very 3-4 months, the average dieter needs not only psychological time to recoup from restricition but a chance to bring things back to baseline.  Increasing not only calories but carbohydrates back to maintenance can help off set all the hormonal reductions seen while dieting not to mention more normal social interactions and the ability to train a bit harder and see some faster progress in the gym.

Remember that these things are constantly in flux.  Simply assuming you can predict a pound a week weight loss because you cut 500 calories per day ignores all of your metabolic adaptations and will result in frustration, poor adherence and unrealistic expectations for rates of fat loss (or muscle gain).  Knowing that there WILL be adaptation which is completely normal and healthy means we don't have to try to rush the process or cheat the system using a fad diet.

These processes also highlight the same principles you've heard from me over and over again.  Protein, whole food and fiber, not cutting c arbs or calories too much, strength training, getting in daily steps, sleeping well and so on.  The things that help your progress overall are also the things that help reduce the side-effects of dieting.

Remember, calorie expenditure is not just the food you eat compared against how much you exercise.  One hard Crossfit workout or hill sprint, going keto or vegan does not influence your fat loss nearly as much as covering the basics above.