If our bodies were simplistic mechanical devices, then I might agree with you, but our bodies have an innate intelligence that regulates all the processes in our bodies better than the computers in the Space Shuttle.
Fortunately for those who have been flung out into space, NASA's computers are way more precise
The human organism is absolutely a marvellous thing, but it is also pretty flawed.
The body seems good with ranges (e.g temperature or acid/alkaline balance) and relative values, but not so good in terms of absolutes. An apropos (for this forum, at least) example would be false hypos i.e When one's body has made a 'new normal' of elevated blood sugars, such that a low can occur even when sugars are actually still really high (I've experienced false hypos at approx 170 mg/dl - 10 mmol/L). Weight set-point theory is another probable (I'd suggest definite) set of feedback mechanisms that will cause the body to defend a particular weight, even if that weight happens to be hundreds of pounds heavier than a healthy weight for our body. I actually believe that set-point is the root cause of why most diets are doomed to fail, unless the appropriate weight is held for long enough for the body to recalibrate to the improved 'normal'. I mention SPF because it relates to something you write, further down in your reply.
While our food transits the intestines and colon, our body controls how and when it collects nutrients from the gut and usually this process is responding to another set of demand./ saity enzymes that alter what we accept from the food stream. Most of our meal appears at the other end and gets discarded. We know this contains the macronutrients from the meal since it can be placed into a sludge digester and produce methane which we can burn for energy, It also can be used as a fertilizer so has other nutrients still in the mix that did not get grabbed. So the calories-in does not represent what actually gets passed into our body for use.
Which is what I tried to explain in my answer. Calories-in is the measurement of what actually does get passed into our body for use. The confusion comes because everybody seems to measure it at our mouths. Our bodies have no interest in the fact that the pizza box says that the meal is 1000 calories. It only knows what is left as usable energy once all these other processes have played their role.
An analogy would perhaps be the difference between our quoted salaries and the amount of cash in our wallets, that we can actually spend. Someone may earn £2000 a month. But after Tax, National Insurance, Investment plan costs, monthly insurance deductions and payments of rent, utility bills etc. only then can we see how much available money (or financial calories) are usable by us.
We have now established that this is true of glucose, and also for lipids, so that fat in does not usually equal fat gain for example.
Of course it doesn't. No single macronutrient, or combination thereof, will result in fat gain. Fat gain will only occur if you eat more energy than you burn
If it did then I would be severely obese instead of skinny.
Right. But there are an awful lot of people that believe that carbs cause weight-gain, regardless of quantity.
I eat a high-fat diet, but my weight has been static for 5 years now and my BMI is 21.
"High-fat" is a relative term. Nothing about eating high(er) fat would suggest weight-gain, unless you were exceeding your energy needs. A cup of olive oil has 1900 calories of fat. For a six-foot-seven male in his twenties, that amount of energy would lead to weight-loss. For a 15-year old, male at five foot, it would lead to weigh-gain.
I am a pensioner who is a couch potato in lockdown, but my mass is still unchanging whereas the CICO mantra states I should be ballooning.
No! CICO is simple energy equation. Couch-potato-ism is irrelevant, unless it leads your output to fall below your input.
I used to weigh nearly 17 stone, but after I started LCHF for my blood glucose control, I have dropped to 10 stone and stay there within 1 kg. My wife shared my diet, and she dropped from 18 stone to just over 9 stone at the same time. My waist lost 6 inches of girth as a result. So for me, Low Carb lost weight and gave me control of my BGL. I have never counted a calorie in my life. I do not even count carbs and I do not starve myself.
When asked which is the best exercise to take up to become fit and healthy, experienced PT's will often answer "The one you are able to stick to". It's really the same with diet. There is no magic bullet or metabolic advantage that makes a high-fat or keto diet more effective for weight-loss. We have examples of people losing weight and improved markers on a diet of McDonalds (
https://metro.co.uk/2018/11/13/man-...ng-nothing-but-mcdonalds-for-30-days-8134563/) or on cakes , Doritos, Oreos etc. (
https://edition.cnn.com/2010/HEALTH/11/08/twinkie.diet.professor/index.html). And many, many people who have ditched pounds and regained health by eating less and moving more.
If you are eating less than you burn, your body will start plundering reserves, whichever diet you choose. Ketosis can, for many, help by quietening/dampening hunger signals, but this really only speaks to compliance. And it doesn't always work for all people.
As for your effortless weight-maintenance? I also have effortless weight-maintenance. I've been the same weight, give or take a few kilos, for well over a decade. I don't need to count calories (Though i have done occasionally as a part of ensuring macro and micronutrient sufficiency). If I eat to satiation, I maintain my weight. Unfortunately, the weight my body is fiercely defending, and leveraging hormones to do so, is 117 kilos. That's correct. My body is doing everything it can to keep me at a weight that would, without an ongoing concerted effort, eventually kill me. So much for our bodies having an "innate intelligence"
Either way, I'm glad you and your wife have got down to a healthy weight. I'll be there in no time, albeit using a diametrically-opposed approach.