We all know overweight people for whom calorie-restricted diets are invariably recommended. This is bizarre because calories are a measure of energy, not of weight, and energy expenditures do not reduce mass except in nuclear reactors where huge energy production involves minimal mass destruction.
Weight reductions fundamentally have to involve weight and not energy. To maintain a static body weight, the combined input weight of nitrogen, hydrogen, oxygen and carbon should equal their combined output weight (inputs and outputs of other elements such as iron or calcium are either minimal or static). Carbon in carbohydrates and fats emerges as the only variable factor in weight regulation. Carbon intakes can be reduced by diets, and more recently by appetite suppression drugs, but homeostatic mechanisms supervene to minimise the effect on weight.
Metabolism of carbon-containing compounds produces water and carbon dioxide (CO2) with almost all this CO2 being excreted in the breath. Overactive or underactive metabolism does not directly cause weight loss because weighty atoms are not destroyed by metabolism. People with overactive metabolism will produce more CO2 and breathe more and people with underactive metabolism will produce less CO2 and breathe less. Thus, anything that affects CO2 in the breath will affect weight.
Most studies comparing normal and overweight people suggest that those who are overweight eat fewer calories than those of normal weight. This and other inconsistencies prompted Taubes to ask in his book
The Diet Delusion: why have clinicians always believed it necessary to semi-starve them (overweight patients) with 1,200-1,500 calories or even feed them very low-calorie diets of 800 calories or less, to achieve any significant weight loss?
He then made the perceptive remark: 'Something else is going on here, and it is nothing to do with calories'. This something is, I suggest, the variability of excretion of CO2 in the breath. The amount of carbon lost in the breath is highly significant. The average American male ingests about 183 grams of carbon every 24 hours in carbohydrates, fats and proteins, and breathes out about 161 grams of carbon every 24 hours. Only about 22 grams of carbon is excreted in faeces, urine, flatus, sweat and aromatic compounds every 24 hours.
I suggest that people who gain weight when they eat the same as those who do not gain weight have higher tolerance respiratory centres to CO2 levels, thus breathing less and breathing out less carbon, and in consequence gain weight. The wide variation of normal ventilation (between five and eight litres per minute at rest) provides evidence of variable sensitivity of respiratory centres. People with lower tolerance of CO2 levels would decrease their CO2 levels by breathing more, excreting more CO2, and would be thin.
There are genetic factors associated with obesity and relevant publications almost invariably allude to energy considerations without any suggestion as to the mechanisms of loss of carbon required.
Four circumstantial pieces of evidence support my theory that carbon excretion is of crucial importance in weight regulation. Firstly, weight loss occurs in those at high altitudes. When I visited Pikes Peak in the Rocky Mountains at a height of 14,115 feet, I was aware of more rapid and deeper breathing, almost certainly driven by the low oxygen levels that replaced CO2 as a drive for breathing. Indeed, exposure to low oxygen levels has been suggested as a treatment for obesity.
Secondly, despite sustained and excessive food intake, weight gains stabilise. Such stabilisations occur because people with respiratory centres of higher tolerance to CO2 levels initially allow lower levels of respiratory excretion of CO2 (with associated weight gain) but once their CO2 levels become higher, their respiratory centres then induce extra breathing such that carbon excretion is enhanced, and thus weight stabilisation occurs. In other words 'diets fail' after initial success. Similarly, people with respiratory centres of lower tolerance to CO2 levels will lose weight initially with diets but this will stabilise when their CO2 levels fall. With both situations, I refer to normal situations and not to grossly abnormal food intakes as found in anorexia nervosa and morbid obesity.
Thirdly, some diets, notably the Atkins diet, promote unlimited consumption of protein and saturated fats that induce acidosis with ketosis that causes increased breathing.
Fourthly, when a consultant in infectious diseases, I saw many patients with chronic fatigue ME/post-viral syndrome. In a proportion, usually following chest infections, I confirmed chronic hyperventilation as a factor. In retrospect, and admittedly anecdotally, nearly all these hyperventilators were thin.
You may well ask: why have these suggestions not been investigated? Carbon is usually only experienced as a constituent of solids because solids are visible. Carbon is not appreciated as a weighty substance because carbon dioxide is invisible. Photosynthesising plants are mostly made from carbon dioxide. Also, respiratory physicians and physiologists are not attracted to study weight problems and other specialties are not attracted to study respiratory problems. I hesitate to observe further that financial homeostasis operates. Calorie-restricted diets do not fail to earn large amounts of money for their advocates.
There are six conclusions:
1. Calorie-restricted diets will help with weight reduction but homeostatic mechanisms will reduce their effectiveness and patients should be warned that this will occur.
2. Respiratory centres are not under conscious control and tendencies to be overweight may not be the result of overeating or 'moral weakness'.
3. Energy expenditure exercise programs aimed at weight reduction will not be effective unless carbon excretion is increased by increasing breathing. Exercises, such as weight training, that do not significantly increase breathing, will not reduce weight.
4. Genetic factors affecting sensitivity to CO2 in respiratory centres are no doubt relevant and early screening for subnormal CO2 responsiveness and thus excretion capacity could identify those at risk of becoming overweight.
5. Calorie content of foods should ideally be replaced by labelling stating their weight of bioavailable carbon.
6. Finally, live at high altitudes. Your weight will reduce, pollution will be lower, and the view will be better.
Philip D Welsby is a retired consultant physician
