Factors Affecting Energy Metabolism: Genetics, Sex, and Aging
Continuing with citations from my article in Obesity Reviews on an aeteological framework for assessing obesity, we now turn to the some of the factors that can affect metabolic rate: Genetics Because heritable factors appear to be responsible for 45–75% of the inter‐individual variation in body mass index (BMI), the potential impact of genetic determinants of metabolic rate upon the predisposition to obesity must be considered. While numerous somatic and mitochondrial genes with potential effects on metabolic rate have been identified, their contribution to human obesity has yet to be defined Likewise, although there is preliminary evidence for intrauterine and perinatal programming of genes involved in energy metabolism, their role in human obesity remain unclear. What is apparent is that the genetic predisposition to obesity (including both energy intake and metabolism) is not explainable on the basis of a small number of common mutations exerting substantial effects on the individual tendency to weight gain. Thus, a great deal of work is still required before investigation into the multitude of genetic determinants of body weight can potentially impact clinical management. Currently, a careful clinical assessment of family history of obesity and related risk factors remains the best measure of genetic risk for obesity. Sex There is a clear effect of gender [sic] on metabolic requirements, whereby, for the same BMI, women consistently display lower metabolic rates (approximately 20% less) than men, largely accounted for by differences in fat‐free mass (FFM). Aging Aging is an important determinant of a decline in metabolic rate, with an estimated reduction of around 150 kcal per decade of adult life. Factors that result in the age‐related decline in energy requirements include changes in neuroendocrine factors (e.g. sympathetic activity, thyroid function, etc.) as well as a reduction in skeletal muscle quantity and quality (resulting from reduced physical activity, reduced protein intake and other less‐well‐understood factors). Additional factors that can affect metabolic rate will be discussed in subsequent posts. @DrSharma Edmonton, AB
A Low Metabolic Rate Will Predispose You To Obesity
Continuing with citations from my article in Obesity Reviews on an aeteological framework for assessing obesity, we now turn to the importance of metabolic rate: Any assessment of obesity should begin with an estimate of energy requirement – specifically recognizing that any decrease in metabolic rate, without a corresponding decrease in energy intake and/or increase in activity will result in weight gain. Thus, in anyone presenting with weight gain, without any notable change in energy intake or activity levels, it is safe to assume that the only explanation can be a reduction in energy metabolism. As a rule of thumb: the lower the total energy requirements, the greater the risk of obesity (simply stated: over‐eating is less likely for someone who needs 4000 kcal d−1 than for someone who needs 1500 kcal d−1). In sedentary individuals, resting metabolic rate is responsible for dissipating the vast majority of daily ingested calories (60–75%) and is therefore a key determinant of energy expenditure. Thus, even a small, sustained percentage reduction in resting metabolic rate, without a compensatory adjustment of energy intake or activity, can account for a large cumulative caloric excess over time (e.g. an unbalanced 3% reduction in resting metabolic rate in an individual with a total energy expenditure of 1800 calories can lead to a caloric excess of 32.4 kcal d−1, which can translate into 972 kcal excess per month). Numerous factors can determine and/or affect metabolic rate. These include genetic and epigenetic factors, gender, aging, neuroendocrine function, sarcopenia, metabolically active fat, certain medications and prior weight loss. Commentary: of course the numeric relationship between caloric intake and weight gain is not as straightforward as many people may think. This is because changes in caloric balance will in turn change caloric expenditure – remember, we are dealing here with physiology, not physics! Thus, a 20 kcal daily excess will only lead to weight gain until the higher body weight uses up the extra 20 kcal to maintain itself, at which point the 20 kcal are no longer in excess of demands and a new caloric balance is found (weight-gain plateau – the reverse happens with caloric restriction). Thus, to continue gaining weight, one has to continue increasing caloric intake to ensure that they stay above actual requirements. This self-limiting nature on the effect of a change in caloric intake (increase or decrease) on weight gain is often forgotten when people make simplistic assumptions that small increases in caloric intake… Read More »
There Is More To Obesity Than Just Energy In and Energy Out
Continuing with citations from my article in Obesity Reviews on an aeteological framework for assess obesity, I have the following quote to offer: In the same manner in which a complete understanding of oedema requires the assessment of the complex physiological systems affecting fluid and sodium homeostasis, understanding obesity requires a comprehensive appreciation of the multitude of factors affecting energy intake and expenditure. Energy expenditure can be further subdivided into non‐activity (= resting metabolic rate + dietary‐induced thermogenesis) and activity thermogenesis (= non‐exercise + exercise activity thermogenesis). For simplicity’s sake, these three elements can be termed diet, metabolism and activity. A change in any one of these elements, if not balanced by corrective changes in the others, will result in a net change in energy balance, which, if positive, will result in caloric ‘retention’ and weight gain. In subsequent posts, I will discuss the many factors that can affect energy metabolism, food intake, and physical activity and how changes to each (if not balance by corrective changes in the others) can lead to weight gain and often pose barriers to obesity management. Stay tuned…. @DrSharma Edmonton, AB
Obesity Can Be Conceptualized As “Caloric Retention”
Several years ago, my colleague Raj Padwal and I published a paper in Obesity Reviews, where we outline a rational approach to an aetiological assessment of obesity. As many readers may not have seen this paper, I will repost several of the key elements we discussed in it. Although some of our thinking has evolved since then, I believe the overall reasoning remain as relevant today, as when we first wrote the paper back in 2010: Obesity is characterized by the accumulation of excess body fat and can be conceptualized as the physical manifestation of chronic energy excess. Using the analogy of oedema, which is the consequence of positive fluid balance or fluid retention, obesity can be seen as the consequence of positive energy balance or caloric retention. Just as the positive fluid balance of oedema can result from a host of underlying aetiologies including cardiac, hepatic, renal, endocrine, infectious, venous, lymphatic or drug‐related causes, obesity can result from a wide range of aetiologies that promote positive energy balance. As with oedema, assessment and management of obesity requires an exploration of the root causes and underlying pathologies. To extend the obesity–oedema analogy, addressing all forms of obesity simply with caloric restriction and exercise (‘eat less and move more’) would be akin to addressing all forms of oedema simply with fluid restriction and diuretics. As this narrowly focused approach is not considered standard‐of‐care in managing patients with oedema, why should it be considered as the preferred method of treating obesity? The classical treatment of obesity, based on increased physical activity and decreased calorie intake, has not been successful. Approximately two‐thirds of the people who lose weight will regain it within 1 year, and almost all of them within 5 years. In our opinion, the lack of efficiency in these therapeutic approaches is likely due to an incomplete understanding of the precise aetiology or aetiologies of obesity and, consequently a failure to address the root causes of energy imbalance. In this paper, we present a theoretical diagnostic paradigm that provides an aetiological framework for the systematic assessment of obesity and discuss how this framework can enhance our ability to diagnose and manage obesity in clinical practice. The framework considers socio‐cultural, physiological, biomedical, psychological and iatrogenic factors that can determine energy input, metabolism and expenditure. Comment: In hindsight, I would note that apart from failure to address the underlying pathology and drivers of… Read More »
Sleep Restriction Leads To Less Fat-Loss
Regular readers will be well aware of the increasing data supporting the importance of adequate restorative sleep on metabolism and weight management. Now, a study by Wang Xuewen and colleagues, published in SLEEP, shows just how detrimental sleep deprivation can be during a weight-loss diet. Their study included thirty-six 35-55 years oldadults with overweight or obesity, who were randomized to an 8-week caloric restriction (CR) regimen alone (n=15) or combined with sleep restriction (CR+SR) (n=21). All participants were instructed to restrict daily calorie intake to 95% of their measured resting metabolic rate. Participants in the CR+SR group were also instructed to reduce time in bed on 5 nights and to sleep ad libitum on the other 2 nights each week. The CR+SR group reduced sleep by about 60 minutes per day during sleep restriction days, and increased sleep by 60 minutes per day during ad libitum sleep days, resulting in a sleep reduction of about 170 minutes per week.Although both groups lost a similar amount of weight during the study ~3 Kg). However, the proportion of total mass lost as fat was significantly greater in the CR group (80% vs. 16%). In line with this substantial difference in fat reduction, resting respiratory quotient was significantly reduced only in the CR group. Importantly, these effects of sleep deprivation on fat loss were observed despite the fact that subjects were allowed to sleep as much as they wanted on the non-restricted days. This suggests that the negative effects of sleep deprivation during weight loss are not made up by “make-up” sleep. Although overall, the amount of weight lost in this study is modest, it clearly fits with the notion that adequate sleep (in this case, during weight loss), can be an important part of weight management. Clearly, the role of sleep in energy homeostasis will remain an interesting field of research, as we continue learning more about how sleep (or rather lack of it) affects metabolism. @DrSharma Edmonton, AB
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