The simple but difficult physics of losing weight

Following up this CT post on health living,, everyone has their own story and their own health. That’s true, but we are all subject to the same physical laws. So, here’s my story and some thoughts on the physics.

I managed to lose about 12 Kg over a couple of years, almost entirely through exercise.

The basic physics is simple
(1) weight loss = (kilojoules burnt – kilojoules consumed)*k,
(2) kilojoules burnt = base metabolism + work done

where k ≃ 0.025 is a constant reflecting the rate at which your body converts kilojoules of food energy into kilograms of fat. If you can alter the right hand side of (1) through any combination of diet and exercise then you will lose weight.

The problem is that altering either of these, or even altering while holding the other constant is really hard. Dieting makes you tired and slows your metabolism. Exercise increases your appetite, and also encourages you to flop once you stop exercising. All that’s because your body isn’t evolved to lose weight easily. Hunger and fatigue are both adaptations to stop you doing that. And, even if you can shift (1) enough to lose some weight, (2) puts a limit on how much you can lose. Balance is restored by the fact that your lighter body takes less energy to maintain and move around.

The crucial thing is to find some change for which you have both the willpower to adopt it initially and the willingness to maintain it indefinitely. For me, as I said, that’s been exercise. I aim to burn 4000 kj (about 1000 calories) a day in addition to base metabolism, which implies about 100 minutes of vigorous aerobic exercise. That’s logistically feasible for someone with flexible working hours and no kids at home, but very difficult otherwise. And it takes a long while to get to the point where you really enjoy it. That’s why the experts mostly recommend working on diet. But, if you can manage it, I think exercise is the better way to go.

28 thoughts on “The simple but difficult physics of losing weight

  1. my experience worked and was rather simple. I maxed out 10 years ago at 93 or 94 kilos. Seeing my self alongside my grand daughters was not a pretty sight. I first cut our sugar, but lost no weight.However I had found on the net a way to stop weight gain. I topped out at 94-5 kg and 18 months later i topped out at 69 kg. that lasted until a year ago. But i have regained half the original loss as having a new home sealed vigh

  2. There is no doubt that exercise is to be seen as ‘better’ than dieting since the general consensus seems to be that exercise is important for the cardiovascular health (as well as emotional well-being for many of us) independently of weight (of course provided exercise is an option) , and dieting is not usually associated with the dieter achieving their long term goal or at least that was my impression.
    Personally, during periods of exercise my body craves and I will simply choose ‘healthier’ foods to eat without trying. My body reacts well to exercise and adjusts naturally within weeks and its fitness level usually remains good during breaks from exercise so that there are no great issues with slightly unhealthier way of eating during these periods. Dieting or thinking of how and what to eat has and would make me anxious and unhappy (and I’d have to eventually go for a run).
    But again everyone is different and not everyone has the time or the willpower to exercise (or they may be unable for a number of reasons). Some people simply don’t like the idea and will not exercise no matter what in the same way that I dread the idea of a diet to follow.
    It is likely true that there are different ‘body types‘ and that what works for one person does not work necessarily for another. So I think it is best never to give advice to anyone about this and try and support them in whatever they choose to do.

  3. I think JQ is correct, it’s simple science, even if individual genetic factors are involved. I lost about 10 kg over a couple of years by eating less sugar, more ‘sensible’ foods e.g. tuna and salad for lunch instead of ham and cheese croissant / chocolate muffin, and exercising more – cycling and walking, if only at golf (9 slow Km) or with the dog on the beach, and a little resistance training at the gym. That’s only 100g per week – slow and fairly constant. I haven’t done the energy numbers on the exercise, or on food intake. Seems like the fat is staying off as long as I don’t overdue consumption of kJ e.g. at Christmas or Feb’s Bali holiday (just in time before COVID-19). I had to be disciplined for a ‘week or three’ after that to reduce the about 1 kg regained. I remember someone saying that to lose weight, a person needs to feel “slightly hungry or slightly physically fatigued” most days. There is some sense in that, and it’s a matter of determination to maintain it. What do you want most?

  4. Having lost about 10kg in the last two months or so, I would say that a crucial factor is not allowing yourself to be hungry. The key is to fill up on nutritious but low-calorie foods such as vegetables, and to hydrate (I aim for about 5L of water daily). Programs such as MyFitnessPal will calculate your calorie intake requirement for a given goal (say, lose a kilogram a week), and I’ve found that it’s best to eat almost exactly that and treat exercise calories as a bonus. In terms of exercise, I find that walking works well, since it has a lower risk of injury than running and tends to have less of a “flop” factor than running.

  5. I’ve lost ~12kg over the last decade. Would have been faster but for my love of beer 🙂

    It is definitely calories in = calories out but I also think it’s important to remember the wise words of James Fell: telling an obese person to eat less and move more is like telling a person drowning to drown less and swim more. Losing weight is some you have to learn to do.

  6. A number of observations and a question:

    1. I cannot believe J.Q. was ever 12 kgs overweight. All the photos I have seen of him in the public domain have been of a slim person.

    2. Does the “k” constant imply that the body converts excess calories to stored fat at 25% efficiency and that it converts fat back to energy at 25% efficiency? If so, that’s a round-trip efficiency of 6.25%. That seems quite low to me. Are my assumptions and/or maths wrong here?

    3. I’ve found that depending on exercise alone is a strategy vulnerable to injury. If you “do” a knee or an ankle, as I have done in the past, then the whole strategy falls down (at least if one is wholly dependent on jogging).

    4. We are adapted to low and insecure food supply from our prehistoric evolution: adapted to feast and famine. It’s hard to resist modern foods (type and quantity) in continuous supply. Come the inevitable global collapse that won’t be a problem for most of us.

  7. Further to my previous comment, protein provides better satiety value than carbohydrate. So for me it’s eggs and salmon for breakfast, not cereal and toast, and hunger is delayed. There are essential amino acids and advantageous, if not essential, oils. There are no essential carbohydrates. There is medical evidence that some oils, for example omega 3, provide health benefits for the skeletal system, the cardiovascular system, and for the eyes. I believe there is also some evidence of benefits for brain function, as some oils are though to be precursors for the production of neurotransmitters. I therefore eat fish regularly.

  8. Timely post and something I am grappling with at the moment. Isolation has caused me to put on many kilos and so a diet has become something of a necessity. I don’t want a few extra kilos around the middle to become a permanent reminder of the COVID-19 anymore than pulmonary fibrosis (scarring of the lungs) as a result of contracting the virus.

    I have maintained the 80 percent diet, 20 percent exercise rule. Most weight loss will come via behavioural modifications to your diet. Yes, you can lose weight without changing your diet, but the amount of exercise required to do this is beyond most people, especially for those of us raising children and working full-time. But due to the “starvation effect”, you can’t lose weight via changes to diet alone.

    I use what I call the Odysseus dieting principle. Recall in Homer’s Odyssey, the eponymous hero tied himself to the mask of his ship so that he wouldn’t succumb to the seductive song of the sirens. I have a blanket prohibition on sugar, cheese and anything high in saturated fats. If it comes into the house, it goes straight in into the bin. I also focus on smaller meal sizes and avoiding snacking. As Thoreau exhorts “simplify, simplify, simplify.”

    Also, NO CHEAT DAYS. Take a neurological approach to your diet and remember that eating things like sugar will produce a surge of dopamine and your brain will require more dopamine. Periodic sugar eating will leave your brain craving a dopamine hit. It’s as foolish as quitting cigarettes and allowing yourself to smoke for one day per week. Find new ways to produce beneficial neurotransmitters, such as sleep and exercise.

    In respect to exercise, I do both cardio and weights. Cardio is great for the cardiovascular system and has other benefits such as improved mood, but, in terms of weight loss, lifting weights is more important. You don’t need to go to the gym because most weights can be done at home (push-ups, squats, lunges, planks). These exercises, especially core exercises, will improve your cardio performance as well.

    Try and avoid weighing yourself too regularly. If you find you’ve lost more than you expected in a small amount of time, you may be tempted to abandon your diet and exercise regime; if you haven’t lost as much as you anticipated, you may give up in frustration. I try once per month because this allows me to make modifications (also weigh yourself several times over a 1 to 2 day period to account for fluctuations in body weight). A month may seem like a long time, but my final piece of advice is that proper, sustainable weight loss takes time.

  9. Cardio good. Chuck in a couple of sessions per week of weight lifting targeting the large muscle groups, and you can get the best of both worlds. It takes a lot of food and burns a cartload of calories to build a kilo of muscle, and that extra kilo is using energy all the time, just to maintain it. They also tell me it looks better than a kilo of fat…so, guess I should try my own advice :-0

  10. There is a whole literature on how different forms of exercise affect the base metabolic rate (BMR). Anaerobic exercise and resistance/strength training, I think, induce quite an ‘afterburn’ — keeping the BMR elevated for hours after the exercise stops. Aerobic/ LSD (long slow distance) induces much less afterburn.

    Moreover, strength/resistance training builds muscle, and a body of given weight but more muscle burns more calories than a fattier body of equivalent weight.

    The moral? Do anaerobic exercise (where you build up an oxygen debt), as well as aerobic, to get an afterburn effect. And do strength training as well as cardio, to become more muscular and thereby increase your BMR.

    The phenomenon of the ‘skinny-fat’ LSD addict is a widespread one. Their bodyweight might be 60 kg, but their fat percentage is 20%.

    Remember also the adage, ‘You can’t out-train a poor diet’, especially as you age and your BMR declines.

    Finally, don’t lose sight of the importance of incidental activity — walking or cycling to and from the shops or work, vaulting over fences, taking the stairs with heavy shopping, doing a set of chin ups on the Hills hoist after you’ve pegged out the washing etc. This will probably produce a greater calorie deficit than the 90 mins/day of LSD that the recreational athlete does, who then reverts to couch potato for the remaining 22.5 hrs.

  11. Thanks John for your blog that I have enjoyed reading for quite a while. The issue becoming overweight has interested me for a number of years although I fortunately do not have a personal problem. The question is why do we become overweight? – too much food and not enough exercise may well be the wrong answer. Another way of asking the question is – is there something in our food chain that is doing this? . When you look at the literature it does seem that a change in our diets occurred several decades ago with the introduction of seed oils – such as canola etc. These seed oils contain linoleic acid which is converted to arachidonic acid which is a major ingredient in our endocannabinoid system. This system is the main driver of our body weight. So my idea is that we are chronically overstimulating our endocannabinoid system with a food chain item and this is the real issue that needs to be addressed. Also we seem to have a set point for body mass in the hypothalamus and as far as I know no one knows how to dial that down. We all know of the person who loses weight but it goes back on aver a period of time and then some. The best reference on this is “Dietary Linoleic Acid Elevates Endogenous 2-AG and Anandamide and Induces Obesity” by Alvheim et al . It is a little old now at 2012 and is free article on pubmed. It contains some pretty compelling pictures from rat studies. As a former GP I used to suggest to my patients to avoid seed oils and use extra virgin olive oil, coconut oil ( reservations about triglyceride effect) and butter instead as they have little or no linoleic acid.

  12. During iso we baked our own bread – after a few dud loaves we had the technique tweaked and they were so good, we were eating far too much bread.

    I’ve also noticed my portion sizes creeping up, after weighing muesli etc I’ve now downsized.

  13. eat right,sleep tight and a couple of times a week make the body exert itself strenuously.

    eat your fresh greens and herbs from your small or large or balcony or potted

    avoid the micro doses of cumulative toxins found in agricorp offerings.

    don’t be a glutton.

  14. Thanks for this interesting suggestion Thomas. In non-scientific terms, however, I’ve always thought that those extra atoms in those fatty acid carbon chains need to some from somewhere, so diet has to be part of the story!

  15. Also, I think exercise is very useful for losing weight. Mostly because it’s difficult to eat while exercising.

  16. Thomas, “Also we seem to have a set point for body mass in the hypothalamus and as far as I know no one knows how to dial that down.”

    A cautionary tale…
    I know a woman around 40 who started gaining weight. 2 yrs of above advise – excersize, good diet, no sugar etc. And still the wieght went on. 4 x visits to male endocronologists who kept saying early menopause, its natural, diet – she had tried 3, come back in six months.

    As the endocrine experts were male and a program on abc rn with a female endocronologist said her male counterparts need to experience life in a woman’s body.

    She booked appt with female endi, did tests – again – and was told, be in neurology atn2pm for mri, and then told she had a brain tumor on her hypothalamus and “Go home and get things in order. We will operate the day after tomorrow “.

    They did. Pressure on hyothalmus within in a month off effecting eyesight and motor coordination. Scary stuff but 3 years on, after going ketogenic for a year and slowly relaxing back to a ‘normal’ high protien diet she is trim taught and terrific now. But always waiting to see if tumor returns.

    The keto diet I thought was extreme but for her, a near death exlerience gave her the resolve to stick to it with doctors monitoring.

    I, as was her family and friends, wondering how someone could put in so much weight in such a short time considering the way she watched her diet and excersize.

    Amazing to see a body change so radically back and forth.

    And Thomas, the endocanabinoids – I assume cannabis has an effect ala ‘the munchies’. Does it trigger craving or satiated? Could thc, cbd etc be used in managing wieght or appitite?

  17. A lot of protein and some fat from red meat is nearly guaranteed to achieve normal weight in a healthy manner without hunger. Then add some vegetables, and avoid seed oils at all costs is probably a good idea. This memoir from 1962 remains timely advice although the tone of the writing may offend some. Go to page 61 for the diet.

    Click to access strong-medicine-blake-donaldson.pdf

  18. There seems to be a six month window where you can put on weight and then take it off ok but after that the hypothalamus resets at a higher body mass. I can find that reference (hopefully) if requested. Re CBD – cannabis is well known as an appetite stimulant so I doubt CBD would be useful. Rimonabant was a successful weight loss agent that is an endocannabinoid receptor agonist but unfortunately caused adverse mood changes and was withdrawn from clinical use several years ago. The search for useful drugs that work via the endocannabinoid system is ongoing- as an interesting aside Paracetamol and Ibuprofen exert analgesic action via the endocannabinoid system and the “high” from exercise is also the endocannabinoid system not “endorphins” at play

  19. John I would like to offer some counter-evidence to your experience. I have lost 5 kg in 4 months while almost completely immobile. In February I dislocated my kneecap and in April I had some fairly serious surgery to fix historical damage in my knee (a child attacked me when I was 13 and destroyed my knee, but crappy Aussie doctors never bothered to fix it, so I’m missing a ligament and have some misaligned bones). I spent a week in hospital and much of the last 4 months I haven’t been able to walk (I started using a bike on an absolutely pissy level at the gym a week ago, for 10 minutes). I lost that weight simply by reducing what I ate, first because I thought the dislocation was due to being overweight and also because I couldn’t drink alcohol for a week or two here or there. But when I was in hospital in April for 1 week I was kept to a strict 1800 calorie a day diet and I was really shocked at the tiny size of the portions. So when I came out I restricted my portions, and forced down my food size, and now even though I’m regaining muscle my weight is stable or even dropping. I have lost 5 cm on my waist in 4 months without doing any exercise (to be fair I restarted sit-ups a month ago).

    Before I did this I was kickboxing two times a week and walking about 40 minutes a day, eating and drinking whatever I want, and not losing weight. I know I could have lost weight if I didn’t drink, but I like drinking. Two years ago I did two weeks of 2 hours a day of kickboxing and lost 1 kg a week, but it’s almost impossible to do that in a normal life (I was in a unique position in between jobs).

    While I agree that the simple physics means you can lose weight by exercise, I think in practice it’s much easier for ordinary people through diet. This is how most Japanese women manage to retain a healthy weight (which westerners call “too skinny”) and it is much more viable for the majority of the human population. It’s not, I discovered, even that difficult if you really want to do it.

    The problem is that there’s an enormous amount of resistance to dieting in the west, based on a lot of false notions about fat-shaming and what-not. So people pretend that exercise is the “healthy” way to do it. It isn’t more healthy (witness my dislocated knee) and it isn’t a better way. I was in Okinawa on holiday last week and there were so many overweight Americans running on the beachfront, achieving nothing because they’re consuming 3000 calories a day. For the majority of people the best and easiest option is to restrict calories, but our entire society is geared around making sure they don’t.

  20. About 4 years ago I was working a desk job in management and carrying 102kg plus. Then I was pushed sideways in a reorganisation and decided to do things my way before I took early retirement so I started riding a bike to and from work instead of driving a company car to various work centres for meetings. I choose to ride on A bike trail to work that took around 35 minutes each way and because the trail had 6 Pedestrian crossings in 16km plus I was able to replicate the high intensity interval training method by going flat stick between lights knowing you get a minute or so breather at the lights. The result was I lost around 12 kg in six months and got down to 90kg. Three years later and now retired I am still 90kg although I thought I might have lost a bit more during the COVID 19 lockdown recently as I was riding for several hours every morning for exercise as the only excuse to be out of the house. But that exercise was probably cancelled out by the extra wine I was consuming perhaps. In short I think it is more about exercise than diet if you physically can do the exercise that is. For many people dieting may be the only option of course.

  21. A marginally on-topic advert for my patented one-line diet plan for the lazy rich. Simply: only eat airline meals. Some entrepreneur would have to strike deals with airlines to deliver their meals door-to-door in cities with international airports. The meals are designed for reheating anyway, so there isn’t much of a technical problem.

    The key point is that airline meals are small – under half the size of a plate in a restaurant. Airlines compete on quality in business class, so the food is usually palatable, and available to meet a range of cultural preferences.. But to an airline, weight costs money. The portions are calculated by professionals to be the minimum needed to deter completely sedentary passengers from mutiny out of hunger. For a more active city dweller, it’s an automatic diet.

    The other half of the airline mutiny prevention programme is booze. This is Not Recommended for dieting anyway. Note that aircrew are rarely obese, even the completely sedentary pilots and copilots. Presumably they eat the same food as passengers – and I hope omit the booze.

    Feedback (sic) welcome.

  22. James, the airplane food idea isn’t bad, although I believe there are already people doing something similar where they send you a reheatable food but not enough to maintain your weight.

    I think this can be improved on. People can have a fit bit or smartwatch or whatever and the amount of food they get sent depends on how active they are. Sure, it’s mostly just a gimmick, but when it comes to diets it appears to be the gimmick that works. None of the actual rules about what to eat differ from what your mother taught you, people just need some sort of emotional investment to stick to it.

  23. I fixed my bicycle today and I’ll try riding it to work tomorrow. This isn’t exercise as I am just doing to to be cheap. But if anybody asks, feel free to tell them I’m doing it to be environmentally friendly. If regular riding results in me losing weight then I may no longer have to avoid weak points in the earth’s crust.

  24. OK, I don’t really know what it is like because I have never been overweight. However, after a 4 month trip to Europe where we cycled, skied, paddled or walked every day, I did lose about 3 kg, which was 5 percent of my body weight. And I ate like a horse. When you exercise a minimum of 5 hours a day, no worries at all.

  25. Fight weight with weight? And where is my ‘gravitostat’ and what are it’s settings?

    “Increased weight loading reduces body weight and body fat in obese subjects – A proof of concept randomized clinical trial

    Recently we provided evidence for a leptin-independent homeostatic regulation, the gravitostat, of body weight in rodents. The aim of the present translational proof of concept study was to test the gravitostat hypothesis in humans

    Increased weight loading reduces body weight and fat mass in obese subjects in a similar way as previously shown in obese rodents. These findings demonstrate that there is weight loading dependent homeostatic regulation of body weight, the gravitostat, also in humans.”

  26. I am in May’s camp. I am not convinced that “weight” is a problem, or “the” problem. I say, if you sleep well, eat fruits and vegetables, and have enough energy to engage regularly in physical activities – preferably ones you enjoy and which involve high intensity intervals – that is probably good enough. Well, except also do something weight-bearing and try not to smoke. I am not convinced that the number is the problem.

    Moreover, I don’t at all believe it is a math question of calories. There have been times when I ate whatever I wanted and lost weight, and what was even better, I suddenly had so much more energy. I am female and I suspect that it was building muscle that made this change happen. So, I agree with the exercise part of the thesis – I just don’t care about the scale. (Caveat if your ancestors had heart problems … I don’t know diddly about that. Whole different scene.)

    Faustusnotes, I am sorry about your knee! I got injured too and now I am flabbier, which is too bad.

    So, are we talking about health or mere thinness? In the US, pretty much every single person and every single study described in the paper conflates these two different things. (And yes I know about Type II … we don’t even need to go there.) Insulin resistance, the microbiome-whatsits in our guts … these fields are in their infancy, it seems to me. There is so much we don’t know.

  27. I like JQ’s economists’ version of the basic equations.

    My own experience is that my success at sticking to a weight loss regimen is dependent on how I feel, and how I feel is influenced by what I eat, as much as by the quantity.

    High GI carbs, sugar and salt, and fatty protein make me sluggish. A hunter-gatherer diet, fruit, veges (excl. potato) and lean animal protein for a couple of days and I’m much more energetic.

    DQ and Garis mention avoiding hunger. I welcome hunger. Hunger is my friend. I mean real hunger, not just a craving for oral stimulation and a carb hit. Best of all, on the hunter-gatherer diet I know that I can eat without guilt.

    We evolved on this diet and it remains the best for us. It contained little or no grain, and I find that eliminating grain-based foods entirely helps significantly.

    After a couple of days on this diet, exercise is not such a frightening prospect, and going for a long walk is a pleasure. Preferably up and down hilly streets for 90 minutes or so. Anyone who is significantly overweight should walk at a comfortable pace at first, and be careful not to overdo exercise at the gym.

    And don’t condemn yourself severely if you occasionally lapse, it will take months to lose weight if you do it gradually, and gradually is best and more sustainable over time.

    Thanks to entry restrictions to the CYP Bio-security Zone, I spent 10 weeks alone on our remote property, so the ideal diet was unavailable and I put on a few kilos. It’s a relief now to have access to supermarkets and a gym, but I visited a doctor before starting my exercise program, and just as well. Blood pressure a bit up, low density cholesterol a bit high, but heart is fine so the doc was happy for me to get into the gym. At 69, I’m feeling pretty good.

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