Explaining the successes of intermittent fasting with physics

Please read the evolutionary argument linked immediately below first.

Notes: prerequisite reading is the evolutionary argument, which can be read here http://bit.ly/3NtuSYF. Furthermore, the data in these plots is synthetic and thus are only approximately realistic, but from a teacher’s perspective, it gets the point across that we want to make.

The internet, as it pertains to intermittent fasting, is littered with forum posts about how eating one, or two large meals a day “helped with losing weight so much more than having six small meals a day”. If you look to find them, you’ll see posts where people still do this despite switching out six small healthy meals for something like an unhealthy pizza a day, which is eaten all in one sitting. While this is an extreme and is not suggested (because come on, the point of losing weight is to be healthy) a fact remains that somehow these posts exist. There are two reasonable hypotheses: 1) the people writing are lying and aren’t eating a pizza a day or 2) intermittent fasting is just better. We ascribe to the latter belief because we’ve personally done the “if it fits your macros” approach with one meal a day and it worked better than dividing the meal into a few smaller meals, while still fasting for the majority of the day.

This post will derive an easy-to-understand argument grounded in physics as to why intermittent fasting is just better for weight loss. Its figures also give intuitions for why intermittent fasting prevents type 2 diabetes, but real research on this will be highlighted in a future post. First, one must understand the current idea of a healthy day. Here is an approximate blood sugar over time graph for Average Joe, a fitness fanatic that religiously reads men’s health blogs online. He has been sold on eating more small healthy meals a day in order to “boost his metabolic rate”. This is a claim that no scientific evidence supports! In fact, all scientific evidence supports the opposite idea — that short-term fasts increase metabolic rate. This evidence will be presented in a future post rooted in biochemistry. For now, let’s look at a plot of Average Joe’s blood sugar while he eats 7 meals across a day.

Figure 1: A blood sugar vs time graph for Average Joe as he eats a total of 4 snacks and 3 large meals in a day. Note snacks bump his blood sugar upwards by less than large meals.

Delving into this, we see that Average Joe eats a snack upon waking at 5AM, breakfast at 7AM, and his blood sugar is elevated all day long until he goes to sleep around 9PM immediately after a tiny snack. It’s well established in the scientific literature that if blood sugar is above baseline fasting blood sugar, then the human body will use blood sugar as its main source of energy. Baseline is your fasting blood sugar, and is seen in the plot between 2AM and 5AM.

This isn’t a problem, but it changes how the human body functions. When you go for a walk (maybe it’s a walk to work, or it’s a walk to the bathroom), your body is going to primarily use the glucose in your blood (from your last meal) as energy. Additionally, insulin is constantly present in the blood. This means that cells will over time build a tolerance to insulin → this tolerance is called prediabetes, and then is later called type 2 diabetes once the constant presence of sugar has caused irreversible damage. Sugar is in fact, inflammatory, and if your blood sugar remains elevated, is actually toxic. This is why untreated diabetes results in nerve damage, organ failure, and eventually death. Furthermore, the constant presence of insulin suppresses the brain and prevents it from secreting human growth hormone, which heals connective tissue, ligaments, and joints.

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To contrast, let’s inspect a blood sugar graph vs time for someone that practices intermittent fasting, named Mr. IF.

Figure 2: A blood sugar vs time graph for someone who has protein pre-workout, protein post-workout, a big dinner, and then a bed-time snack. Note that the total calories consumed in the day, as well as the grams of all macronutrients (protein, fat, carbohydrate), are equal to Average Joe’s meals in Figure 1.

Before Mr. IF heads to the gym, he has a whey protein shake blended with a banana. While working out, his blood sugar remains elevated because blood sugar is secreted by muscles and the liver during exercise. After the gym, he has another protein shake, with more banana in it than the first, and begins to cook his largest meal of the day, dinner. Finally, he has a snack before bed.

So, compared with Average Joe, how is Mr. IF’s body functioning? All day long, instead of using blood sugar when he walks to work, walks to his car, or does whatever, his body is going to use free fatty acids in his blood (aka fat). This is a mechanism resulting from evolution to protect animals from losing their precious glycogen reserves. Glycogen is what humans turn the carbs we eat into for storage and it’s stored in both the liver and the muscles. It’s absolutely key for evolution to have preserved carbohydrate storage in animals — it’s our last resort or emergency energy supply. This is because carbohydrates can be broken down the fastest to supply energy to our muscles. While we are fasting, our bodies try as hard as they can to avoid using carbohydrates for energy because our bodies do not know when we may suddenly really need energy, e.g. to run away from a predator or defeat a predator/aggressor in a fight. The point here is that hundreds of thousands of years of evolution trained our bodies to use fat for energy while we are at rest.

Furthermore, if evolution prioritized the usage of fat while fasting, then wouldn’t it prioritize the minimization of the usage of breaking down proteins for amino acids while fasting? Almost certainly, amino acids would not be used for energy as a result of the evolutionary process our ancestors went through. The argument for this is treated here: http://bit.ly/3NtuSYF. So we can conclude that Mr. IF broke down fatty acids for the majority of his day.

Now we get into the juicy part — the physics. First, let us introduce the first two laws of thermodynamics: energy can not be created or destroyed, only transferred. The second law is that entropy is always increasing. Entropy can be thought of intuitively as the spread of energy. The important part, however, is that because entropy is always increasing, no chemical reaction can be 100% efficient. This means that every chemical reaction will transfer some energy to its surroundings in the form of heat (this is the spread of energy). We are now ready to make our extremely simple argument as to why Mr. IF, while eating exactly the same calories and macronutrients as Average Joe, will experience more weight loss if the two stick to these meal frequencies long term.

Quite simply, Mr. IF is wasting more energy in the form of heat. This is referred to as the “thermic effect of food” in biochemistry literature, but we take it a step further by considering the thermic effect in the event the majority of the day’s calories are all consumed in a single meal. As a result of eating so many calories all at once, his body must perform extra steps, i.e. extra chemical reactions, compared to Average Joe’s body. Since Average Joe’s blood sugar was elevated every time he moved or did exercise, his body used blood glucose for energy since his body thought the blood glucose was in ample supply (and indeed it was). In Mr. IF’s case, however, most of the time he spent moving around was while he was fasting, and resultingly, his body used stored fatty acids for energy.

A natural thought here is along the lines of “but then Mr. IF ate a huge meal and those extra calories, in the form of extra blood sugar, extra fat, and extra amino acids, were stored as fat”. You’re correct. The thing is, that most amino acids won’t even be converted to fat by our bodies. The process to do so is just too inefficient. Furthermore, breaking amino acids down for energy requires a process called deamination. A byproduct of this process is ammonia, which is toxic, so naturally, the process doesn’t end there and eventually converts ammonia into urea. Remember, every extra chemical reaction loses more energy to the environment as heat. In fact, the amount of calories in amino acids that are lost as heat when they’re converted to fat is estimated to be around 40%. Furthermore, the carbohydrates that are eaten are broken down as per usual and converted into glycogen which is stored in the liver and muscles. The thing is that glycogen storages fill up before all of the carbohydrates have been digested, and thus, the remainder must be converted to fat. Depending on the type of carbohydrate, the energy loss is estimated to be between 3-10%. Finally, fatty acids are the easiest to convert into human storage fat, and only 1-5% of the energy is lost, with the higher 3-5% coming from medium-chain triglycerides (coconut fat for example).

Taking this energy loss to an extreme, let’s maximize it and say Mr. IF ate one meal.

Figure 3: Mr. IF eats one big meal all at once, this spikes his blood sugar really high, and it comes down really fast because he is very sensitive to insulin, which then causes his blood sugar to go below his baseline. It then goes back up as his body secrets glucagon, a hormone that does the opposite of insulin — brings blood sugar back up.

Let’s run some numbers. Say they both ate 2000 calories, with 600 calories from carbs, 800 calories from protein, and 600 calories from fat. Then Mr. IF, because of his body converting macronutrients to fat, consumed 800*0.6 = 480 calories from protein, 600*0.9 = 540 calories from carbohydrates, and 600*0.97 = 582 calories from fat. This totals an effective 1602 calories for Mr. IF. Average Joe however, probably did convert some calories to fat, but because his blood always had more level concentrations of macronutrients and his body wouldn’t have been in a hurry to convert them into fat (because he has poorer insulin sensitivity, and lower absolute blood sugar to trigger the release of insulin).

The point of this article is not to advocate for an extreme of eating 1 meal a day. It is only to show a simple argument that obeys Occam’s razor, that as far as the author is aware, has never been made anywhere else. More articles will certainly be written on fasting and these will include citations, as everything in this article comes from my knowledge.

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