As most people are aware, rates of overweight and obesity are increasing all around the world. In 2016, the World Health Organization estimated that 39% of the world’s adults were overweight and 13% were obese, or in other words, more than half (52%) the world’s adult population was overweight or obese. Excess energy from foods and drinks and insufficient physical activity are thought to be the major contributing factors, but as is the case with most things in life, it is of course more complicated than that.
Energy in foods and drinks
Energy is not a nutrient in itself, but it is required by the body for metabolic processes, physiological functions, muscular activity, heat production, growth and synthesis of new tissues. The main sources of energy are carbohydrates, fats, and proteins, and if you choose to drink, alcohol. Most countries require packaged foods and drinks to list their energy content on their labels, and depending on where you live, energy is measured in either Calories or kilojoules:
1 Calorie = 4.2 kilojoules (kJ).
An essential principle of nutrition and metabolism is that change in body weight is associated with a miss-match between the energy content of foods and drinks and the energy expended by the human body for basal functions, absorption of ingested food and physical activity.
Our energy intake from foods and drinks fluctuates depending on numerous individual and environmental factors that present each day. On the other hand, energy expenditure, though largely determined by body weight and composition (i.e., lean body mass and fat mass), is itself affected by both physical activity and food consumption.
Unsurprisingly, measurement of these factors is difficult under free-living conditions, but precise measures of energy expenditure can be obtained in a controlled environment, such as a respiratory chamber – airtight cabins measuring two by three metres in which people are “locked” in for one or more days. While it may sound unpleasant, modern respiratory chambers have a bed, toilet, television, internet and all other basic comforts.
By measuring oxygen consumption and carbon dioxide production within an enclosed environment, the respiratory chamber provides a precise estimation of the energy expended by a person every minute over long periods of time. This method is considered the “gold standard” for the measurement of energy expenditure over 24 hours because of the ability to distinguish the three main components of energy expenditure: 1) resting metabolic rate (basal metabolism); 2) the thermic effect of food, and 3) physical activity.
Alessio Basolo and colleagues recently reviewed the evidence on energy balance and body weight from respiratory chamber experiments.
Resting metabolic rate
Resting metabolic rate, or our basal metabolism, is essentially the energy needed to ensure the functioning of our vital organs when we are at rest. It is the main component of 24-hour energy expenditure, and it can be further classified in the energy expended during sleeping and the energy expended to be awake, excluding physical activity.
The main determinant of our resting metabolic rate is our lean body mass (i.e., the weight of our organs, bones, muscles, blood and skin, and everything else which is not fat, but has mass or weight) accounting for ~70% of energy expenditure, whereas fat mass (i.e., adipose or fat tissue), sex, age, ethnicity, and familial traits explain ~15%.
Thermic effect of food
The thermic effect of food is the increase in energy required to digest, absorb, assimilate and store nutrients after consuming foods or drinks. It accounts for ~10% of the 24-hour energy expenditure when people are in an energy balance state on a typical Western diet, but it shows a large variability among people.
The thermic effect of food is discussed in more detail in this month’s Perspectives.
The energy cost of physical activity, defined as the energy consumed during spontaneous and voluntary exercise, is the most variable component of 24-hour energy expenditure, varying from ~15% in very sedentary people to ~50% in those who are very active. Similar to resting metabolic rate, the energy cost of physical activity depends on your body composition, age, sex, and genetic traits, plus interactions between biochemical, physiological, and brain reward pathways and environmental stimuli.
Energy balance and body weight
Commonly, our understanding of overweight and obesity is based on the simple static model of energy balance, which suggests that a chronic positive energy balance (i.e., when daily energy intake from foods and drinks persistently exceeds energy expenditure) leads to an increase in body weight and development of overweight/obesity due to the storage of surplus energy as body fat (adipose tissue).
In everyday life, such perfect matching between energy intake and energy expenditure is barely achieved, and the adipose tissue serves as a dynamic depot, protecting against unavoidable deviations (e.g., energy surpluses or deficits) from the energy balance equation, and constantly conveying signals to our brains and nervous system, to communicate the amount of energy stored.
Evidence is emerging that circadian rhythms play a major role in energy balance by acting as the “timekeeper” of the human body and it is regulated by central and peripheral clocks. Clock associated biological processes anticipate the daily demands imposed by the environment, being synchronized under ideal physiological conditions.
The central or master clock, located in the brain’s hypothalamus, collects light cues, which are the main stimuli for maintaining 24-hour cycles. It conveys circadian signals to peripheral clocks that are present in several organs (e.g., liver, pancreas, endocrine glands, kidneys and heart). Those output signals are important to synchronize the circadian rhythm with peripheral activities, by regulating the expression of specific genes (mainly CLOCK and BMAL1), which are involved in many physiological functions, such as regulation of blood pressure, glomerular filtration rate in the kidneys and glucose metabolism. Peripheral clocks, in turn, convey signals back to the central clock.
Unsurprisingly, communication networks between circadian clocks are involved in many physiological functions and it leads to integrated rhythmic control of metabolic and behavioural processes such as the sleep cycle, hunger and body temperature. Environmental factors including the macronutrient composition of foods and drinks, meal timing, physical activity and light exposure, are considered external signals which might interfere with the circadian clocks.
Emerging evidence suggests that disruption of this temporal synchronization between circadian clocks and environmental stimuli may lead to the development of obesity and its comorbidities like type 2 diabetes and heart disease. Indeed, several studies have shown that the main components of 24-hour energy expenditure such as resting metabolic rate, the thermic effect of food, and physical activity might be affected by disruption of the circadian rhythm. For example, shift workers tend to struggle more with their weight and are more prone to developing type 2 diabetes.
Recent studies have also suggested that energy intake occurring mainly earlier during the day promotes long-term weight management. This months Diabetes Care explores the issue of meal timing.
If we are to effectively slow the rise in global overweight and obesity rates, we will need to increase our knowledge and understanding of energy balance beyond the simple static model.
- World Health Organisation. Obesity and overweight.
- Basolo and colleagues. Energy Balance and Control of Body Weight – Possible Effects of Meal Timing and Circadian Rhythm Dysregulation. Nutrients. 2021
- Sun and colleagues. Meta-analysis on shift work and risks of specific obesity types. Obes Rev. 2018
- Li and colleagues. A meta-analysis of cohort studies including dose-response relationship between shift work and the risk of diabetes mellitus. Eur J Epidemiol. 2019
Dr Alan Barclay, PhD, is a consultant dietitian and chef with a particular interest in carbohydrates and diabetes. He is author of Reversing Diabetes (Murdoch Books), and co-author of nearly 40 scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).