Genes do not determine obesity, but rather our response to overfeeding
Losing weight is trivially simple — in theory. The underlying physics is unquestionable, guaranteed by the Law of Conservation of Energy which states that energy cannot be created or destroyed, only transferred or changed from one form to another. It follows, therefore, that if our output is greater than our input, energy must be liberated from our stores. This begins with liver and muscle glycogen, and continues with fat. Voilà! Weight loss.
Formally, the Law of Conservation of Energy tells us that energy and mass are conserved in a closed system. In order to lose or gain energy or mass, the system must be open. The system, in this context, is our bodies. And our bodies are always open, perpetually losing energy to the surrounding environment in the form of heat and mechanical work. Thus, in order to gain weight — indeed, in order not to lose it — we must constantly be adding energy to our bodies via our mouths. It is not enough just to look at a cake; we must eat it.
Every cell in our bodies requires energy to function. If we are alive, our cells are alive and functioning, and we are therefore expending energy to power all of their processes. Active transport (the transport of molecules across membranes), anaerobic respiration (the liberation of energy from glucose in the absence of oxygen), mechanical work (contraction of smooth, skeletal, and cardiac muscle cells), anabolic processes (the construction of new carbohydrate, protein, and lipid molecules), heat production, and even bioluminescence — the very act of living requires energy, and a lot of it.
Based upon data from a study sample of 433 women and 344 men of ages 20 to 96, the average woman requires around 5,700 kilojoules (1,350 Calories) of energy per day just to function, and the average man requires around 7,350 kilojoules (1,750 Calories). That is before working, or eating, or even moving. This energy, which is used only in performing our critical cellular functions, is known as basal metabolic rate, and is measured in a highly controlled fasting and resting state. Individual rates vary considerably, but more than two thirds of the population (one standard deviation) expend between around 4,800–6,600 kilojoules (1,150–1,550 Calories) and 6,250–8,500 kilojoules (1,500–2,000 Calories) for women and men respectively, per day. Our physical size, primarily, accounts for the difference between the sexes, with the liver, brain, and muscles contributing to the majority of that demand.
The high degree of variability between individuals of the same sex is explained by genetics. However, across the spectrum, the greatest predictor of basal metabolic rate is not our sex, nor our age, nor even our height. It is our weight! Yes, contrary to the popular justification for overweight and obesity — “some people have a slow metabolism” — greater weight is strongly associated (r = 0.75) with a higher metabolic rate. Quite simply, the overweight and obese have more active metabolisms than the healthy population. The common justification does not stand up to the evidence. And the reason is simple: overweight and obese individuals have greater muscle mass and adipose (fat) tissue, both of which are metabolically active, albeit minimally for the latter. So whilst a moderate amount of body fat demands relatively little energy, a large amount does not, and fat is the only component of our bodies for which there is no limit.
Outside of the laboratory, two of the most accurate methods of predicting our basal metabolic rate are the Mifflin-St. Jeor and Katch-McArdle equations. The former is widely accepted as being the most reliable and accurate formula for both obese and non-obese individuals when body fat is not known. The latter may be used when body fat is known. This equation is simpler because it accounts for the fact that basal metabolic rate is primarily (approximately 63%) a function of fat-free mass, and is largely independent of height, age, and sex once fat mass is discounted.
Mifflin-St. Jeor equation
Basal metabolic rate should generally establish a floor for our dietary energy input. If diet is restricted too severely, it can lead to excessive fatigue and cognitive inhibition, and it can further cause an unnecessarily large loss of fat-free mass, mostly in the form of muscle, which is then counterproductive in terms of facilitating subsequent fat loss.
Our total daily energy expenditure further includes the thermic effect of feeding, also known as diet-induced thermogenesis (the energy required to facilitate digestion) and activity energy expenditure, which is comprised of non-exercise activity thermogenesis (the energy used in general day-to-day movement) and exercise activity thermogenesis (the energy used for dedicated exercise, such as sport).
Total daily energy expenditure
The most commonly quoted average contributions to total daily energy expenditure are 60–70% for basal metabolic rate, 10–15% for diet-induced thermogenesis, and 15–30% for activity energy expenditure. However, these values differ enormously between individuals, owing to large variations in genetics, body composition, diet, and lifestyle.
Since basal metabolic rate and diet-induced thermogenesis represent the great majority of energy that most people expend each day, and since the former is largely predetermined, various commentators over the years have dismissed the role of activity and exercise in weight loss. However, non-exercise activity thermogenesis alone can vary by as much as 8350 kilojoules (2000 Calories) per day between individuals of a similar size, and it is now being recognised that non-exercise activity might play a large role in the prevention of overweight and obesity. We are therefore being encouraged to increase our incidental exercise by walking more frequently, taking the stairs, running our errands, using a standing workstation, sitting on a Swiss ball, or even just fidgeting.
Exercise activity can potentially have the greatest effect on total daily energy expenditure, and hence weight loss, since there is no theoretical limit to the amount of exercise we can perform. We are limited only by our physical fitness. A study of cyclists in the Tour de France found their mean (average) total daily energy expenditure to be 25,400 kilojoules (6,050 Calories), with the highest expenditure recorded being 32,700 kilojoules (7,800 Calories)! Similarly, a study of cyclists in a six-day professional tour found their mean to be 27,400 kilojoules (6,550 Calories), with exercise activity accounting for 62% of their total daily energy expenditure. To put that figure in context, the cyclists would have had to have eaten the equivalent of twenty-one MacDonald’s Cheeseburgers per day just to avoid losing weight!
Ideologues too often defend overweight and obesity, conflating notions of liberty and self-expression with desirability and wellbeing, in so doing stimulating a cultural sentiment of acceptance and inevitability. And supplement and pharmaceuticals companies forever try to sell us an easy pill to swallow — that we are slaves to our genes and victims of our circumstances. Their profits depend on our believing it, and for so many of us it is a tempting fantasy.
But let us be honest with ourselves. Yes, energy intake is imperfectly associated with rates of overweight and obesity, and yes, the traditional “energy in minus energy out” equation is simplistic, failing to account for how our bodies respond to an energy surplus or deficit. However, data demonstrate patently that worldwide rates of overweight and obesity follow energy intake, and similarly that the rise in rates of obesity can be explained almost entirely by a corresponding rise in average energy intake over the same time. Human beings have not changed significantly, but our diets and lifestyles have changed drastically.
We should be very clear: overweight and obesity are self-inflicted lifestyle illnesses, the consequence of poor choices and behaviours. Genetic factors are significant, but they do not determine overweight and obesity. They instead determine our response to overfeeding. Our condition is entirely within our reach and control, but we must first accept responsibility for it.