Fasting, Calories & Metabolism
It’s can be confusing, so let’s sort it out.
The body does not recognize calories. Calories are it's only a way to measure energy of a food. Before you stop reading hear me out…
I love the way Dr. Jason Fung explains this. We have currency to buy things and things have value based on it’s price, right? Here in the US our currency is the dollar. Other cultures may use salt or silver for currency. When the body takes in two different foods they are metabolically different, it’s like having two different ‘currencies’ in the body… back to this in a minute.
I don’t want to get too technical here, but I really want everyone to understand this - I feel like when you get it, it’s easier to shed the ‘calorie’ mindset!
The standard Eat Less, Move More approach to managing weight:
Let’s throw some numbers into the mix to make things more clear. Let us assume the baseline situation of stable body weight (zero body fat gained or lost) and 2000 calories per day intake.
0 Body Fat = 2000 Calories In – 2000 Calories Out
Calories Out is not just exercise. This is composed of 2 things – resting energy expenditure, or basal metabolic rate (BMR) and exercise.
If you assume zero exercise, an average BMR is 2000 calories per day. This energy is used by the heart, lungs, kidneys, generation of body heat etc. Note that BMR is NOT under conscious control. You cannot ‘decide’ that your heart will pump more blood. You cannot ‘decide’ to generate more body heat. No amount of willpower will make your kidneys use more energy.
Exercise is generally a very small portion of the total daily expenditure, unless you are exercising multiple hours in the day. Consider a moderate exercise of 1 hour of moderate walking/ jogging, 3 times per week. Each walk burns approximately 100-200 calories. If you’ve ever exercised on a treadmill with a calorie counter, you’ll know how slowly that meter rises. That 100 calories used during exercise pales in comparison to the 2000 calories eaten on an average day. So, we can safely ignore the effect of exercise except for those who do in excess of 1 hour per day.
So, people suppose that if you decrease your caloric intake by 500 calories per day or 3500 calories per week, that you will lose 1 pound of fat per week assuming that 1 pound of fat contains roughly 3500 calories.
-500 calories = 1500 Calories In – 2000 Calories Out
Please take careful note that in order to lose body fat, Calories Out MUST remain stable. Must. Must. But this is what we know to be FALSE for at least the last 100 years. BMR may increase or decrease 30-40%. This was shown as early as 1917, when studies showed that a reduction of calorie intake by 30% is quickly met by a decrease in BMR by 30%.
Dr. Ancel Keys showed much the same effect in his famous Minnesota ‘starvation study’. Despite the title, subjects were given 1570 calories per day, more than most weight loss regimens being prescribed today. A drop in calories eaten by 40% is met with a 40% drop in BMR.
The reason for this is simple. Your body is very smart and does not want to die. If you do not alter your hormones (mainly insulin), you won’t be able to access your fat stores (this sentence is super important - read again!). If you can’t get energy from body fat, then then you cannot run an energy deficit forever. If you are only taking in 1500 calories, you can only spend 1500 calories.
So BMR drops. We’ve known this for over a century. If you cut a few calories every day, your body will burn less calories and you will not lose fat. Weight loss plateaus and then you start to regain weight. This is the reason why you stop a calorie restricted diet and gain the weight back. Our body was doing what it was designed to do - keep us alive - it doesn’t know the difference between being in the cave with rationed food for the winter and trying to get back into the jeans you love. So, counting calories, as a strategy for weight loss, has been proven over and over again to fail.
Strategies that lower insulin, however (Low Carb, Intermittent Fasting) are completely different. By lowering insulin, we tell our bodies that there is no food coming in. Therefore, the body switches from burning the calories from food, to burning the calories from our body fat. Our body wants to burn 2000 calories, but it just gets them from body fat instead of food. Instead of restricting energy (calories), our body is switching fuel sources, from food to stored food (body fat). But this can only happen if we correct the underlying hormonal problem of excessive insulin. So is ‘Calories In Calories Out’ totally useless?
Back to the calorie::currency analogy. We could say that the hot dog at Yankees Stadium is $10 or 20 salt cakes, but unless you’re a Tibetan Emperor, it just doesn’t matter…
Metabolism doesn’t = calories.
Source: Dr. Jason Fung