The Human Metabolism, The Obesity Epidemic and Strategies to Move Forward

‘The Obesity Epidemic’ is a catch-all term that summarises the drastic increase in the incidence of overweight and obesity, in both adult and child populations in the Western world over the past 3 decades. And the statistics tell their own story: -

  • In 2011-12 68.2% of Australians over the age of 18 were classified as overweight or obese (35.3% Overweight, 27.5% Obese)
  • Continuing with the trajectory of the past 30 years would result in 86% of Americans becoming overweight or obese by 2030 (Apovian, 2010)
  • Prevalence of overweight and obesity has increased in Australia, from 56.3 in 1995
  • Since 1980, the obesity rate for school-aged children has tripled

Measures of overweight and obesity are generally measured by ‘Body Mass Index’ (BMI). Developed in 1860’s, BMI is calculated by dividing bodyweight (kg) by height (m) and multiplying itself. A BMI >25 but <30 is deemed overweight, and a BMI of >30 deemed to be obese. Whilst the BMI is recognised a great tool for assessing weight, it doesn't actually determine body fatness. Thus, people such as athletes and bodybuilders will have a high BMI due to the large amount of lean body mass they carry, not because of body fatness.

Obesity represents one of the most serious global health issues we face today, with the World Health Organisation declaring it the ‘greatest health threat facing the West’. Mortality rates increase as BMI’s above 25 increase. This is due to the direct link between BMI and susceptibility to many degenerative diseases, the most important being Type II Diabetes (Speakman, 2004). An important point to make here is that the effects of obesity on health are entirely reversible if the person in question loses weight. Hence the fact that obesity is labelled a ‘preventable’ disease.

“Obesity is a problem of imbalance between energy intake and expenditure” (Speakman, 2004)

The figure below, courtesy of James Krieger neatly summarises the scenario of the body storing and releasing energy over a 24 hour period. If a person stores more fat (shown in green) than they burn (shown in blue) over a 24 hour period then they will have have gained fat over the course of a day. If this trend continues over many months and years this will eventually express itself as increased bodyweight. However, it isn't usually the case that weight gain occurs in such a linear fashion. We tend not to slowly accumulate very small amounts of fat each day, but rather be weight stable for protracted periods, interspersed with periods of gross imbalance during which a lot of weight gain can occur. This phenomenon can be observed during the holiday season in the United States (from Thanksgiving in November through to New Year) for example, where weight gain is considerably higher during this period (Yanovski et al, 2000).


So how does this imbalance between energy intake and energy expenditure manifest itself as obesity? 

Well it has to do with it’s interactions with the body’s metabolism and the resulting effects this has on the way our body functions. Human Physiology complies with the first law of thermodynamics, which states that energy can be transformed from one form to another but cannot be created or destroyed (Hall et al, 2012). Energy used by the body is derived from the macronutrients found in food (protein, carbohydrates, fat, and alcohol). Absorbed carbohydrates, proteins and fats are transformed in the body into substrates that can be used immediately for energy or they may be stored as mainly as adipose (fat) tissue. The body is continually going through a process of storing and releasing energy. The amount of storage and the amount released will vary throughout the day (e.g. storing fat after eating a meal, burning fat when asleep at night).

So, if obesity is caused by a long-term gross imbalance between energy intake and energy expenditure, what is causing this imbalance? 

As you can see in the diagram below, there are both environmental and genetic components to obesity (Speakman, 2004). Becoming obese is neither solely the result of your behaviour, nor is it solely genetic; your genetics have a large impact on your behaviour: -

“Our genes predispose us to obesity, but genes can only be expressed in environments, and it is the gene-environment interaction that is most important” (Speakman, 2004)


So how does the body burn energy? 

Well, the human metabolism is comprised of the following components: -

  • Basal Metabolic Rate (BMR) - Refers to there rate at which the body expends energy to sustain basic life processes. Having a higher amount of lean body mass will result in a higher BMR than for the same amount of mass as fat - this is because lean tissue has a higher energy requirement that fat tissue.
  • Energy Expenditure of physical activity and exercise (EEPA) - The most variable of the components that make up the human metabolism and is the only component that is easily altered.
  • Thermic Effect of Food (TEF) - Refers to the increase in energy expenditure associated with the body’s processing of food (digestion, absorption, transportation etc)
  • Thermoregulation - Refers to the alterations in metabolism necessary to maintain the body’s core temperature

The human metabolism is very adaptive, in that it will increase or decrease based on feedback mechanisms from within the body. Increasing energy intake will result in an up-regulation of your metabolism, through increased activity levels, an increased thermic effect of food, an increased basal metabolic rate. The body increases its metabolism in a bid to maintain homeostasis. It is actively trying to prevent weight gain. 

Then why does obesity occur at all if the body tries so hard to defend against it using these feedback mechanisms?

Unfortunately the scenario we see with the obese population is that the feedback mechanism is broken. This is most notable in the case of the hormone Leptin and the resulting Leptin resistance that occurs as a result. Leptin is a hormone released by adipose tissue that interacts with the brain to reduce hunger. Levels of leptin correlate with the amount of body fat someone has: the higher the body fat, the more leptin that is produced. This mechanism appears to work well in normal weight individuals. However, in obese individuals the high leptin levels do not induce the appropriate expected response i.e. a reduction in food intake and an increase in energy expenditure. If these responses were present we would expect weight loss and a correction of the obese state. It appears therefore that obese humans are resistant to the effects of leptin. It has been theorised that this may be because of a transportation defect in getting Leptin to the brain. Or it has been suggested that it could be a defect in Leptin receptors in the brain.

Ok, so we can see that obesity can cause a damage to the internal feedback mechanism within the body, but what other factors contribute to obesity? 

Genetics/Evolution: According to obesity researcher Stephan Guyenet, we’ve evolved to be driven toward calorie dense, non-toxic food in an energy scarce ancestral environment. However, we now live in a society where most of the food is at a level of reward/palatability that our species has never encountered before. We're surrounded by it, and everywhere we turn, someone is jockeying for our attention, trying to get us to purchase their food.  

The following food properties favour eating: -

-Calorie Dense





-Absence of bitterness

These preferences are innate. Back when humans were hunter-gathers, it makes sense that we would favour foods that are highly calorie dense and highly palatable, given the sporadic nature of eating. Humans could have gone days or even weeks without much food, meaning they had to capitalise when they were able to eat. Our genes haven't changed greatly since those times, yet our environment couldn't be more different. Today we have an abundance of highly palatable, calorie dense, convenient food everywhere we turn.

It is clear that different individuals have a certain genetic propensity to store excessive caloric intake as fat (Spiegelman & Flier, 2001). In a classic Study, Bouchard et al (1990) overfed pairs of identical twins by precisely calibrated amounts. Different sets of twins showed remarkable differences in the degree to which these calories were stored as fat, but the tendency towards increase fat mass within each set of twins was remarkably similar. Results from studies on twins show that genetic factors explain 50-90% of variance in BMI (Maes, et al, 1997).

The ‘Obesegenic’ Environment: “It is already quite clear that the modern food environment has changed, and promotes increased energy intake and sedentary behaviour” (Zheng et al, 2009).

“The combined responses that control energy intake and energy expenditure to maintain energy homeostasis have conferred a survival advantage during human evolution. Now, food availability has increased in many countries and advances in technology and transportation have reduced the need for physical activity in daily life. These two factors pose a great challenge for bodyweight regulation and are probably the main reasons that account for the increasing prevalence of obesity” (Jequier and Tappy, 1999)

The environment we now live in is often described as being ‘obesigenic’ due to the way the built environment inhibits active lifestyles (Hall et al, 2014). This environment is one that discourages, rather than encourages physical activity, as evidenced by increased car use and ownership in all western countries, and increasingly so in countries like China who are expected to see large increases in obesity in the not too distant future. However, Swinburn et al (2009) argue that it is increased energy intake, rather than decreased physical activity levels that explain the weight gain in the US population over the past 30 years.

Increased Energy Intake: The pattern over the last few decades has been “more processed, affordable and effectively marketed food than ever before” leading to what Swinburn et al (2011) calls the “passive overconsumption” of calorie dense, highly processed foods. At total of 25% of all energy consumed by the US population comes from nutrient-poor food groups such as sweets and desserts (Apovian, 2010).


Social Influences: Hall et al (2014) describe the way social pressure can cause inertia. For example, overweight individuals might be motivated to lose weight because of the health risks, yet, seeing a growing number of others with similar weight, they may slowly increase their sense of acceptable weight and find less motivation for weight loss.

Psychological Factors: We ingest food to meet energy and nutrient demands of living, but food is also rewarding and therefore meets reward needs as well (Speakman et al, 2011). 

“humans have a hedonistic mind-set, inclined to pursue happiness. Foods rich in sugars and fats offer potent rewards, which promote eating even without energetic requirements for food” (Keijer, J et al, 2014)

Comparing brain activity changes elicited by visual food stimuli in obese subjects before and after losing 10% bodyweight through dieting revealed the anatomy of what Zheng et al (2009) call a ‘hungry brain’. After a 10% weight loss, looking at pictures of food evoked much larger changes in brain activity, despite the fact the subjects were still classified as obese with lots of excess adipose tissue. Being mindful when eating is another important factor - if you are not focused on your food you will tend to eat more of it. People also eat more in front of the laptop or TV when distracted from the food they are eating.

De Castro (2011) states that the Obesity Epidemic is a “complex phenomenon”  that “defies simple description”. I hope I have made it abundantly clear that there are a large number of factors and processes involved that not only act individually but also interact to create a complex web of factors involved in the obesity epidemic.

So far in this article I have highlighted the alarming statistics regarding overweight and obesity, I have discussed the possible causes of obesity and how these interact with the human metabolism. Which leaves the question of how we might go about trying to improve the situation. Whilst a lot needs to be done at a governmental level regarding things such as food provided in school canteens, potential pricing strategies and taxation on junk food, making sports and activities cheaper to take part in, there is also a great deal that individuals can do to improve their own situation straight away. These include: -

  1. Increase energy expenditure in non-exercise forms (more walking, standing desks etc): Non-Exercise Energy Expenditure (NEAT) makes up all physical activity outside of structured exercise. This has the potential to impact daily energy expenditure in a big way. Strategies that try to increase walking, taking stairs, implementing standing desks in workplaces (and home) could have massive implications on the obesity epidemic and health. Standing for an afternoon has been found to increase energy expenditure by an extra 170 calories. So over the course of a week you could burn an extra 1,000 calories just by standing for a few hours each day instead of sitting. Standing after eating a meal has also been found to reduce the blood-sugar response of that meal. On top of this, standing desks have been associated with a decrease in lower-back pain, which is one of the most common complaints of office workers who sit all day. A study by Levine et al (2005) found that obese individuals in their study sat for an average of 2 hours longer than lean individuals recruited in their study.
  2. Build some basic resistance training into your routine: Whilst increasing NEAT is absolutely vital, I believe that for long term success in losing and maintaining weight loss, resistance training should form a part of your weekly exercise regime. Now, this doesn’t necessarily mean going a gym and taking part in hardcore bodybuilding. It could be as simple as performing a circuit of bodyweight exercises every day at home. Exercises like squats, lunges, step-ups, planks, push-ups and dips can be performed with relative ease, with no real space requirements or the need for any equipment. The importance of resistance training lies in its ability to increase or at least maintain lean muscle mass whilst losing weight. This is important as the more lean mass a person has, the higher their BMR will be, due to the higher metabolic cost of muscle compared to fat.
  3. Prioritise sleep: Getting enough sleep is absolutely vital for health and well-being. However, sleep also plays a crucial role in weight loss and improving body composition. A few adverse consequences of sleep deprivation include: Increased cortisol production; reduced testosterone production; increased appetite; increased insulin resistance; poorer nutrition partitioning; decreased cognitive functioning and overall decreased well-being. “Lack of sufficient sleep may compromise the efficacy of typical dietary interventions for weight loss” (Nedeltcheva, 2010).
  4. Eat mindfully (not in front of a TV etc): As discussed previously, we tend to passively over-consume food when distracted. Therefore eating mindfully, chewing food properly, eating slowly and switching off all possible distractions will aid in eating more mindfully, potentially reducing ‘passive overconsumption’.
  5. Prepare meals in advance: “people are significantly more adherent to prepared meal plans than self-selected meal plans” (Lee, 2001). Regardless of how good our intentions may be, we aren’t very good at decision-making. This is largely due to what is know as ‘decision making fatigue’. ‘Executive function’ is your ability to intentionally override what your instincts tell you to do. Not giving in to hunger requires executive function. Executive functioning is an intensive process that fatigues with use. This is generally why people experience will-power failure at night. Over the day they have accumulated ‘decision fatigue’ which results in night time binging. You can overcome decision fatigue by awareness. The mind is plastic - it can be trained. The more often you over-ride your emotional system, the better you become at it. You can also plan your day so that you limit the number of decisions you have to make. 
  6. Make your diet high in protein, fibre and whole foods: Consuming between 1.6-2g per kg of bodyweight is a good general recommendation for individuals engaged in some form of resistance training. Not only will having adequate amounts of protein in your diet help with building muscle, it is the most satiating of the macronutrients, meaning eating a meal high in protein will keep you feeling fuller for longer. Fibre too is extremely satiating, as it adds a lot of bulk to food and delays the transit time through the gut. On top of this, protein has a higher thermic effect, meaning the body expends more calories digesting, absorbing and transporting meals that contain a lot of protein and fibre. Good sources of fibre come from green vegetables, fruit (especially the skin of fruit), and wholegrain sources. Good sources of protein come from meat of all varieties, fish, eggs and dairy.
  7. Don’t use food as a reward: Using food as a reward teaches you to consume that food when experiencing emotional stress. A cheat meal basically creates your own comfort food. Guess what happens when the dieting gets hard? You turn to the comfort food. Many people with a weekly cheat meal or day find themselves thinking about it the whole week. Whereas if you never eat a food, you simply forget about it and you never miss it anymore. It disappears from your mind’s internal menu, just like words disappear from your active vocabulary when you stop using them. Most processed foods simply stop looking like something edible altogether (Henselmans, 2016)
  8. Find a routine that works for you & stick to it: The body craves routine, as evidenced by it’s circadian rhythm. The most obvious rhythm is your sleep-wake cycle. Try to go to bed and wake up at similar times every day. Similarly, try to have a good routine with meals by eating the same number of meals every day, at similar times every day (within 2 hours). Try to schedule training sessions and block that time in your diary so that things don’t get in the way of getting training done. The more of a routine you can get into, the more it becomes habitual, and the less conscious thought you have to give these things. They just become part of everyday life, like brushing your teeth!
  9. Enlist the help of others: Undergoing lifestyle change is tough, and so doing it alone probably isn't a wise move. Therefore try to enlist the help of other people who can support you during your journey. This could be friends and family, co-workers or a fitness professional, such as a personal trainer. Have other people on board will not only provide moral support, but it will also provide you with some accountability, which is extremely important when trying to make changes to your diet, lifestyle and training. 


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