With the holiday times approaching (have you finished your gift-buying chores yet?) one of the concerns that regularly happen to trouble the festive atmosphere is, “how much extra weight am I going to put on this time, after January 1?”. And there goes the ominous count of calories, friends and family around the table, drowse on the couch with that silly Christmas movie, and the guiltful projects to start losing weight immediately, starting January 2. Everybody has a quite good understanding about how to gain weight, even if leaving out some important technical details. However, given the widespread, generalized (and healthy) need to lose weight in our western society, there is surprising ignorance and confusion about the metabolic process of weight loss, both among the general public and health professionals. Widespread misconceptions about how humans lose weight are carried lips to ear (or rather – in the internet age – keyboard to screen) among medical doctors, dieticians, and personal trainers, and make their way to the general public. If you take the time and fun to go in the street to ask questions to strangers (if you do so, at least pretend you are some kind of journalist or free-lance youtuber, to avoid backfire hate…), you will find that most people believe that “fat is converted into energy or heat”, something which openly violates the law of conservation of mass (more on that at the end of today’s story). It is likely that this misconception is caused by the “energy in/energy out” mantra and the custom of measuring food on the basis of “calories”. Other common misconceptions could be that the metabolites of fat are excreted in the sweat, urine and feces or, according to some people, even converted to muscles (maybe believing that muscle is less “massive” than fat, when it is just the opposite, as far as tissue density: fat is 0.92 g/cm3 while muscle is 1.06 g/cm3).

As we all more or less know, the energy in our bodies can come from carbohydrates (about 4 kcal per gram), proteins (same 4 kcal/g), and fatty acids (about 9 kcal/g). Your morning croissant (in southern Europe) contains officially about 180 kcal, its 25 g of sugars providing more than 60% of the total, and the rest shared by the about 5 grams each of proteins and fat. If you are from the north, your eggs-and-bacon 100 g average serving counts for about the same caloric value, but coming for more than 75% from its 15 g of fatty acids, and the rest from its 10 g of proteins (carbohydrates here being negligible). Any excess amounts of carbohydrates or proteins in your daily food is eventually converted to triglyceride molecules (that is, groups of three acid (CH2)n long chains joined at a glycerol) and ends up stored in the lipid droplets of special cells, the adipocytes; excess fat instead needs no conversion other than lipolysis (that is, breaking down into smaller molecular units) and re-esterification (that is, reattaching a glycerol to the acid chain, after it has passed the cell membrane). Fiber and other waste from food are not transformed and cannot be stored, they will be just excreted through urine and feces. Therefore, when you p** or p** you are not ditching any accumulated extra weight, you are only getting rid of trash. When in January you will wish to lose weight while maintaining a healthy fat-free body mass, biochemically speaking you are attempting to metabolize the triglycerides stored in your adipocytes at Christmas and New Year’s dinners.

Looking at the dials on your cyclette, stepper or treadmill, they will usually indicate that you should have fried away about 10 kcal per kg of (your) weight, per hour of earnest exercise. That is, about 700 kcal/hour for an average body of 70 kg weight, the equivalent of a crispy chicken breast at KFC. However, our body is not a thermonuclear reactor which could convert mass into energy. While we ingest grams (or rather, kilograms…) of food, we cannot possibly loose this mass as energy. Mass goes in, mass must come out. As surprising as it may be to some, the only actual way to lose weight is to breathe.

To put it in the simplest form, the mitochondria in our cells burn glucose and oxygen in the cellular respiration cycle, giving off CO2 and water as residues, according to the (grossly approximated) formula C6H12O6 + 6O2 → 6CO2+6H2O. The water remains in the body (some of it will be expelled much later, with sweat and urine, in separate processes). Hence, we breath in an amount of oxygen molecules and we breathe out the same amount of CO2 molecules, which however weight about 40% more. The carbon attached to the O2 is the actual weight we are losing. So, the first rule is breath more! While at rest or sleeping, an average 70 kg person exhales about 200 ml/minute of CO2, in about 12 breaths; during a light activity that rate may double. Since 1 mole of gas is 22,4 liters at STP, each breath gives away about 33 mg of CO2, that is, 9 mg of pure carbon atoms that your mitochondria had cracked out of the food. In 24 hours of such light breathing, you breathe about 22-24,000 times, and lose about 220 grams of body weight as carbon in thin air. When exercising, the breathing rate can raise up to 6 or 7 times that at resting, as well as almost doubling the air volume intake, meaning that 1 full hour of steady exercise can add about 60-70 more grams of carbon to the daily excretion. If you want to convert it into glucose, this corresponds to about one mole, or ~700 kcal. That fits with the about 700 kcal/hour indicated by your treadmill, and it is the equivalent of just about 150 g of bread. Hence, the second rule is eat less!

In fact, every time something burns under rich oxygen atmosphere, such as a wildfire or a wax candle, the combustion products fix the oxygen, and are therefore heavier. There is a famous story about the sailor and statesman Sir Walter Raleigh and his pipe. Being one of the foremost who popularized tobacco smoking in XVI century England, he was once called by Queen Elizabeth (the first…) who reportedly asked him (some sources say she made a bet) if he could weigh the weight of the smoke from his pipe. As the story goes, Sir Walter filled up his pipe, weighted it, and started calmly smoking in front of the court reunited. When the tobacco was over, he weighted the pipe again and triumphantly announced the weight of the smoke being equal to the difference of weights. While undoubtedly smart, he was only partly right, since its carbon and hydrogen in the smoke had turned into CO2 and H2O: he had forgot to add the weight of oxygen to his smoke. We can easily forgive him, since oxygen would be discovered only about two centuries later. Who did not forgive him was King James, who had him beheaded for breaking the peace treaty with Spain. After the execution, they found in his cell an elegant tobacco box with engraved the Latin words “Smoke, my companion in this most miserable time” (my translation).

For all of its functions the animal body needs energy, just the right amount and avoiding any excess (we all know, that’s the difficult part). Get food and oxygen in, get carbon and water out, while storing a fraction of the energy available. Each cell’s mitochondria are working hard to extract energy from food, and this is the reason why in order to get the count of total calories burned, you need to know your own weight: all the cells in your body are participating in giving off the 33 mg of CO2 in each breath! The complex chemical mechanisms defined in millions of years of evolution are quite well optimized, even if moderately efficient. As far as the energy efficiency, the calculation boils down to how many ATP molecules are regenerated from ADP, per gram of glucose from carbohydrate and protein metabolism – or gram of fatty acids. The energy stored in ATP is 7.3 kcal per mole. A simplified calculation tells that a mole of glucose with its 6 carbon atoms in each molecule, contains 686 kcal; in the mitochondrial respiration cycle, these can be turned into a maximum of 36 moles of ATP (plus some other waste molecules), that is 6 ATP molecules per carbon atom removed. Hence the thermodynamic efficiency of the respiration cycle is (36×7.3)/686=38%, which is not bad considering that a gasoline engine in your car makes between 30 and 35% (while a diesel cycle can reach up to 50%). Directly burning fatty acids, although metabolically slower, is even better. In a process called Lynen’s helix, each two carbon atoms removed end up in 14 ATP molecules, that is 7 per carbon atom, which makes for 17% better energy efficiency when comparing one gram of fat vs. one gram of sugar.

A final word to adjust my previous statement about the venerable Lavoisier’s principle of mass conservation in chemical reactions. Obviously, it is mass-energy that is always conserved and not just mass. Einstein’s equation works also for chemical transformations, the only problem is that the mass variation is so small that no balance will (probably) be ever able to measure it. Sticking to our friend glucose, the release of 686 kcal, or 2872 kJoules per mole corresponds to a mass loss m=E/c2 of about 30 picograms for each 180 grams of glucose transformed, that is a variation of (let me write it down…) 0.000000000016667 per cent. This is so because of the small energy implied in breaking and making chemical bonds, something radically different from nuclear reactions, in which the mass-energy equivalence becomes much more evident. When someday in the future we will be able to fuse 180 grams of deuterium and tritium together, there will be a transformation of about 0.35 per cent of that mass into energy. The same amount of energy would hypothetically be released by 4000 tons of glucose. By comparison, in the nuclear fission of U-235, “only” the 0.1 per cent of the initial mass is turned into energy. If you think on this basis that fusion energy is better than fission you could be right, in principle. However, consider that at current prices, 108 grams of tritium cost nearly 3 million dollars – the cost of deuterium’s 72 grams is negligible – whereas 180 grams of enriched uranium are worth about 5,000 dollars.

It’s all about carbon

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