Despite all the controversy surrounding weight loss, science has come exceedingly
close to having mathematical proof of how exactly weight loss occurs. This proof comes
from the laws of physics, specifically the first law of thermodynamics (the movement of energy).
Energy In Vs. Energy Out
Energy comes into the body in the form of food. Scientists have come up with calculations to estimate the metabolizable energy content of food. The most commonly used estimates today are from the Atwater general factor system. The Atwater factors consist of the four macronutrients that make up the total amount of calories consumed in a single food item. These are protein, carbohydrates, fats and alcohol and they have the following caloric (kcal) contents:
4kcal per gram of protein or carbohydrate
9kcal per gram of fat
7kcal per gram of alcohol
With these estimates we can simply and quite precisely calculate how many calories we consume based on what foods we eat. This is something I do quite often in practice with clients, by having them keep a food journal.
The body needs energy to move, produce heat and perform chemical reactions. The
following summarizes the contributions to the total daily energy expenditure (TDEE) of the average sedentary person:
Digestion (5-10%): From the foods that we eat.
Physical Activity (25-35%): Includes energy for movement and activity, including exercise.
Basal Metabolic Rate (55-65%): Energy for your body to function. This includes energy for your organs such as the kidneys, liver, brain, heart and all other organs. Maintenance of your current level of muscularity is also included here.
I'm sure you can begin to see a pattern emerging and that logically it makes sense that losing energy from the body equals losing mass. The stored chemical energy in tissue is lost from the body and transferred to the outside world in the form of heat or movement. On the other end, gaining bodily energy from the chemical energy in food equals gaining mass, in the form of fat or muscle tissue.
As such, we can formulate the principles of energy balance as:
If you consume more calories than you expend, your body stores energy and you gain weight.
If you are in energy balance, your body maintains the same amount of energy and you maintain your weight.
If you expend more calories than you consume, your body loses energy and you lose weight.
Weight loss simply comes down to tilting this scale in the right direction by consuming the appropriate amount of calories in our diet.
Cardio For Fat Loss?
Thinking about energy balance is far more useful than thinking about acute substrate
metabolism, like ‘fat burning’. For example, it's often said that cardio burns body fat and is therefore mandatory if your goal is fat loss. The truth is, cardio does not make you lean unless it results in a negative energy balance.
Throughout the day, you will at different points of time be storing and losing body fat. The only thing that ultimately matters is the total balance, which comes back to energy balance, since your body can convert carbohydrates and protein to fat and store them as energy. Here’s an example of fat balance across a day to show that cardio does not make you lean unless it results in negative energy balance:
*The above image shows actual fat losses and gains.
Fat Burning Diets?
Many diets claim to have special 'fat burning effects'. For example, during a ketogenic diet, your body will be burning a ton of fat all throughout the day because that is the main macronutrient that is consumed. This is easily advertised as ‘the keto diet turns you into a fat burning machine’, but it doesn’t mean you’ll actually lose fat mass from your body. Fat is just the substrate being burned, because it is abundant in your diet. Your body will now be burning relatively little of its own fat. It will only burn its own body fat if dietary fat is insufficient to cover the need for fat as fuel. So once again you’re back to looking at energy balance to determine actual weight change.
The same applies to low fat diets. Your body generally doesn’t convert protein or
carbohydrates to fat and prefers to store the fat from your diet as fat. However, even if
you don’t consume any fat, if you consume more protein and carbohydrates than your body needs for energy expenditure, they can be converted to fat. In every scenario, you end up looking at energy balance to determine whether your body has to burn its own energy.
When determining the effects of any diet on your weight and body composition, always keep total energy balance over time in mind, not just acute substrate metabolism, ie. 'fat burning'. The laws of physics do not bend. For any dietary intervention to improve fat loss, it must either:
Increase energy expenditure
Decrease energy intake
Is your lifestyle and diet causing you to be in a state of negative energy balance?