Neither Weight Loss nor Weight Gain Are Magical. They Happen Because of Energy Balance
Conservation of energy is a natural law. It is also called the "First Law of Thermodynamics". Natural laws are based upon observation; they remain true from time to time and place to place and everywhere. The first law of thermodynamics tells us that energy is never created from nothing nor is it destroyed although it can change form. This is true for cars, planes, physics experiments, trees, bees and human bodies. Energy is never created from nothing nor is it destroyed.
Let's apply this law to the energy balance that controls human body weight. What it means is that energy we consume must go somewhere. It cannot simply vanish into nothing. So where can food energy go? Here is a partial list:
It can be stored as body fat
It can be burned as heat
It can be used to perform work (muscular contraction)
It can be used to make chemical bonds
It can be lost to malabsorbtion
Energy can do many things, but it cannot vanish.
Now, the body is generally very good at storing energy. If it weren't, our ancestors would have died in famines and we wouldn't be here. This means that generally we make fat out of any calories that we absorb and do not immediately burn. Fat is how we store energy. Fat also has mass; it has weight. Therefore, when we store excess energy, we gain weight as fat.
Conversely, if we receive too little energy by absorbtion (we don't get enough calories to meet daily demand), then we make up the difference by converting some of the fat in our body into energy to power all our vital life functions (like heart beat, breathing, metabolism and running from predators). Thus when calorie intake drops below our delay need, we lose weight as fat.
Finally, when we absorb exactly the same number of calories as we burn, our fat stores are neither increased nor depleted; they remain stable.
The three paragraphs above represent plain-English descriptions of how energy balance is related to human fat storage. They are more traditionally stated as
When the rate of calorie intake exceeds output we gain weight
When the rate of calorie output exceeds intake, we lose weight
When the rates of intake and output are equal, weight remains stable
If we turn the middle statement around what we get is this:
To lose weight, our calorie output must be greater than our calorie input. To reiterate: this simple fact is a direct consequence of a law of nature. It is incontrovertable.
And finally, to state things with utmost clarity: when we lose weight it is because we burn more calories than we consume- this is true when we lose weight by counting calories, by exercising, by following a low fat diet or a low-carb diet or starving or after bariatric surgery or because we are on the Cambridge diet, the
Atkins diet, the Drinking Man's Diet, The Cabbage Soup Diet, The Ice Cream Diet, The Raw Brains Diet, the Eat Nothing Diet....you get the point. No matter what else is true of any weight control plan, if it works, it does so because it causes a higher rate of calorie output than input.
Does This Mean Calorie Counting is the Best (or Only) Way to Lose Weight?
As we shall see, calorie counting is torture; it causes hunger. There are better (easier, healthier) ways to lose weight BUT.....BUT in the end, from the perspective of basic energy balance, any diet that produces weight loss ultimately does so by causing calorie deficit. So the REAL question in weight control is NOT whether there is any way around the basic facts of energy balance, but rather, what is the easiest, healthiest, most livable long-term way to achieve a net calorie deficit.
The answer(s) to THAT question is the subject of everything else written here.