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Metabolism Matters

Lots to digest on metabolism and obesity

Diet conceptThe causes of obesity are complex and have not been fully uncovered, despite scientific breakthroughs. The relationship between metabolism and body weight is an area of great interest as well as controversy for researchers, healthcare providers and patients alike. Here, we provide an overview of the close interlink between metabolism and body weight.

Linking Metabolism, Energy and Weight

Energy is a fundamental requirement of all living things. Humans obtain energy through the intake of food (energy intake).

Metabolism refers to the chemical processes whereby energy is converted for use by the human body (energy expenditure or energy output).

The body is in “energy balance” when the energy intake is equal to the energy output. To put it simply, an individual’s body weight or Body Mass Index (BMI) is a reflection of his state of energy balance.

A consistent imbalance of energy results in a change in body weight. If too little energy in consumed to balance the energy expended, a state of negative balance occurs and the body loses weight through the loss of fat tissues. Conversely, taking in more energy than is expended will lead to a positive balance. A positive balance that is large enough and/or continues for a long enough time will lead to weight gain. Therefore, obesity can be viewed as the end result of long-term positive energy balance.

pink dumbbellsAccounting for Energy Expenditure

While energy intake can be easily defined based on the amount and type of food intake, energy expenditure is more complex.

  • Basal metabolic rate: Most of the energy consumed (about 65% to 70% of the total) is used to maintain resting or basal metabolic rate. This is the minimum level of energy expended by the body to sustain life and is used to maintain vital organ functions, e.g. heartbeat, breathing, brain, liver and kidney function.
  • Response to food intake: About 10% to 15% of energy intake is spent in response to food intake – to digest, metabolise and store ingested nutrients.
  • Physical activity: The rest of the energy requirements are for physical activity. This is the most variable component of daily energy expenditure and can vary based on the intensity and patterns of physical activity. Energy is also needed for growth and in response to cold, but their contributions are relatively minor in adults.

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Debunking a Metabolic Myth

It is a popular belief that obesity could stem from a lower metabolic rate and total energy requirements. This idea was suggested by earlier studies that reported energy requirements in obese individuals that were lower than the estimated values for the given weight. However, the self-reported energy intake and expenditure were largely inaccurate.

 

Later studies using careful measurement tools revealed a different story. The total energy expenditure of an obese individual was actually greater than his lean counterpart. Taking into account a person’s weight and the body composition, especially the proportion of lean mass, metabolic rates between lean and obese individuals were actually no different!

In other words, we can probably rule out a low “metabolism” or rates of energy expenditure as the primary culprit of obesity, although certain medical conditions such as very low thyroid hormone levels (severe hypothyroidism) are associated with hypometabolism and excess weight.

Regulating Body Weight

Although the concept of energy balance is fundamentally true, the body has a weight that it naturally gravitates to and healthy individuals are able to maintain their body weight despite large fluctuations in daily energy intake. This is due to the fact that the body burns calories more slowly than normal after weight is lost, and faster than normal when weight is gained. In other words, it is harder to lose or gain weight than to maintain the same weight.

Given such highly evolved adaptive responses, how can a person gain excessive weight? Why is obesity on the rise globally? These enigmas have several explanations.

A pair of female feet standing on a bathroom scale with green apFirst, our understanding of the relationship between body weight and metabolism is incomplete and current scientific data is limited. For example, most studies examining the link between metabolism and obesity are either based on measurements at a single point of time or short-term measurements, which would not account for the long-term effect of subtle variation in energy balance.

Moreover, an individual’s metabolic rate and adaptive response is the collective effect of his/her body composition (proportion of lean mass to fat mass), age, gender, and physical activity, which changes daily as well as over the long term. It also varies greatly from person to person. For example, overfeeding studies have found that despite an overall increase in metabolic rate, weight gain was highly variable between individuals. Similarly, when overfeeding was stopped, certain individuals found it harder to lose weight than others.

This difference has been partly ascribed to genetic factors. This is supported by studies on identical twins. When overfed, identical twins showed highly similar metabolic responses. Identical twins exposed to vastly different environments growing up turned out to have similar body weights.

Clearly, we need to focus on more than just energy metabolism in explaining and managing obesity. Similarly, heritability is not equal to eventuality. It is important to also look at how behaviour and environment affect changes in energy balance over time, and learn more about the body’s ability to regulate its energy stores.

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Dr Tan Hong Chang is a Consultant in the Department of Endocrinology at Singapore General Hospital.
Posted by ezyhealth on Mar 5 2015. Filed under Health. You can follow any responses to this entry through the RSS 2.0. Both comments and pings are currently closed.

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