TIME 4 FAT LOSS SERIES – Part 1 – Why do we get fatter?

2 April 2019

Time 4 Fat Loss series – Part 1

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This is the first in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.


Part 1 – Why do we get fatter?

The short answer is energy imbalance. If the energy or calories from the food and drink we consume equals the energy we expend, our body fat stores remain unchanged, which is referred to as a state of ‘energy balance’.  However, when our energy intake is greater than our energy expenditure, this creates an energy imbalance in which the excess energy will be stored as fat. Just one pound of body fat contains approximately 3,500 Kcals of unused energy, which is enough to fuel a 100 kg person to run 35km!

Interestingly, most obese people do not overeat by much, but an excess energy intake of only 70kcals a day, which is equivalent to a single biscuit, can result in a weight gain of 5 stone (32kg) over a period of 10 years. To put this in perspective, over time this small imbalance can turn a slim 25 year old into an obese 35 year old (1).

While energy imbalance is generally recognised as the fundamental cause of an increase in body fat, various factors can disrupt our ability to maintain a state of energy balance.

Presented here is an explanation of some of the most common:


How our genes can make us fatter

Obesity-related genes may affect how we metabolise food and/or store fat. Other genes may control appetite, making us less able to sense when we are full. While some may make us more responsive to the taste, smell or sight of food, or perhaps affect our sense of taste, giving us preferences for high fat foods, or putting us off healthy foods. Certain genes may even make us less likely to engage in physical activity (2).

Our genes may also affect our behaviour, making us inclined towards lifestyle choices that increase our risk of gaining weight: For example, people with variations in certain ‘obesity genes’ tend to eat more meals and snacks, consume more calories per day and often choose the same types of high fat, sugary foods (3).


How much is our body fat determined by our genes?

It is difficult to state exactly how much our body fat levels are influenced by our genetics because of the interaction of our genes, lifestyle and environment. Traditionally it was believed that approximately 25% of the variation among people in body fat levels is determined by genetic factors, with environmental factors accounting for 30% (4).

More recent research suggests that up to 80% of the risk of becoming obese is attributable to genetics factors. For example, the risk of an overweight toddler becoming obese is low if both parents are of a normal weight. However, regardless of their current weight, if a child under the age of 10 has one obese parent or both, the child has more than twice the normal risk of becoming an obese adult  (4). It is important to be mindful that this does not mean that environmental factors such as poor diet and a sedentary lifestyle are not the cause of weight gain. An individual’s, or even an entire family’s, obesity may be the result of lifestyle rather than a genetic predisposition to gaining fat.


The good news

The good news is that even if our genes predispose us to gaining fat, we can reduce the risk of this by changing our eating patterns and adopting other healthy lifestyle habits, such as regular physical activity (3). For example, a study found that people who carried a particular obesity-promoting gene had a 23% higher risk of obesity than those who did not. However, physically active adults who carried the obesity-promoting gene had a 30% lower risk of obesity than inactive adults who carried the gene (5). On the other hand, even people without a genetic predisposition can become obese due to poor lifestyle choices. So while our genes may determine who is likely to put weight on, lifestyle will determine how much.


Environmental influences

Although our genes may have remained stable over the last few decades, our environment has changed considerably, affecting what we eat and how active we are.

Our environment has increasingly promoted the consumption of energy-dense food with the Office for National Statistics (ONS) reporting a 34% increase in the number of takeaways in the UK in just 8 years, despite the government’s anti-obesity push.  At the same time, increasing reliance on technology has decreased our levels of physical activity (6). The population is now approximately 20% less active than in the 1960s. Surveys have shown that nearly a quarter of adults report being physically inactive (doing less than 30 minutes physical activity in a week) (7). If current trends continue, this figure will rise to 35% by 2030.

Less manual occupations, labour saving devices such as escalators, lifts, motorised transport, and inactive leisure activities like computer games, and media devices have all led to a reduction in physical activity levels. The result of these environmental changes is that it is now easier for people to overeat, and more difficult to be physically active (8).

Considering the fact much of the population now live in an obesity producing environment, i.e., one that is stressful, sedentary, and with easy access to relatively inexpensive, high calorie food, it’s no surprise that we are getting fatter.


Hormone problems

Hormones are chemical messengers that regulate numerous processes within the body.  Abnormal levels of certain hormones or a disruption in the body’s ability to control and react to them can cause an increase in body fat in a number of ways:

For example, the underproduction of the hormone thyroxine can reduce an individual’s resting metabolic rate (the amount of energy the body expends at rest) by as much as 30-50% (9). This means that if a person normally expends 1,500 kcal per day at rest, a reduction in thyroxine levels could reduce this to 1000 to 750 kcal per day, which, if they continue to eat the same amount and do not increase their energy expenditure, will lead to weight gain

Cushing’s syndrome is rare condition which affects around 1 in 50,000 people. It is caused by high levels of the steroid hormone cortisol, which may develop as a side effect of long-term steroid treatment or as a result of the overproduction of cortisol by the adrenal glands (10). This leads to an increase in body fat, particularly on the face, upper back, and abdomen.

Polycystic ovary syndrome (PCOS) is a common condition that affects the functioning of a woman’s ovaries. It’s thought to be hormone-related, involving excessive levels of insulin and testosterone. Women with PCOS typically gain fat on the abdomen, which cause an increase in insulin production, which in turn can lead to further weight gain (10).

A variety of other hormones can also affect our body fat levels, such as insulin and sex and growth hormones. These influence our appetite, metabolism and body fat distribution (11). It appears that some people who are obese tend to have levels of these hormones or their body reacts to them in a way that encourages the accumulation of body fat.

However, it is important to remember that conditions such as these are relatively rare with approximately only one case of obesity in every 1,000 being caused by such a hormonal problem (12).


Leptin and ghrelin

In recent years we have begun to understand more about the role of the hormones leptin and ghrelin in the development of obesity. Leptin is a hormone secreted by fat cells which helps to decrease our appetite after we have eaten and promote energy expenditure (4).

Ghrelin is the hunger hormone. It is produced by the stomach to send a message that we need to eat. An imbalance in these hormones can cause an individual to gain weight. For example, low leptin levels have been shown to be a precursor to obesity. Some obese people actually have high levels of leptin, but their bodies are less sensitive to its effects, which results in an excessive food intake leading to an increase in body fat (13).


Psychological issues

Although they may not be immediately obvious, psychological issues including stress, anxiety and depressed mood can all affect our levels of body fat. Many people eat to relieve negative feelings such as depression, anxiety or anger, which is referred to as ‘comfort eating’ or ‘emotional eating’.  A similar condition is ‘stress eating’, which refers specifically to eating induced by stress (14).


Stress can lead to weight gain in a number of ways. These include  

  • Food cravings
  • Poor decision making
  • Loss of sleep
  • Changes in eating
  • Excessive consumption of alcohol
  • Use of drugs (15).

Many obese people have perpetual cycle of mood disturbance, overeating, and weight gain. When they feel stressed and unhappy, they turn to food as a coping mechanism, which although may temporarily improve their mood, the weight gain that results may cause further depression and anxiety. This may reactivate the cycle, leading to a continuous pattern of eating to cope with emotions (16).


Poor sleep

Sleep is an essential, yet often overlooked component of health and well-being. Although a small number of people can get by with only 3 hours per night, most adults need approximately 8 hours (17). There is evidence to suggest that people who sleep less than seven hours per night are more likely to be overweight than those who sleep for nine hours or more. The exact reason for this unclear, but one theory is that sleep-deprived people have reduced levels of leptin, the satiety hormone, and higher levels of ghrelin, the hunger-stimulating hormone (10).

Consequently, when we are tired we are more likely to consume high-calorie snacks to maintain our energy levels throughout the day and we are less likely to engage in physical activity.



While various factors can influence an individual’s level of body fat, it is important not to forget that the fundamental cause of weight gain is the consumption of a greater amount of energy than we expend. Therefore, the solution is to create a negative energy balance, i.e., expend more energy than we consume. If our energy intake from food and drink is less than the energy we expend through physical activity, our body will draw on its own fat stores for fuel. We’ll be looking at the most efficient and effective ways of doing this later in this series


CLICK HERE TO READ: Part 2 – How much fat should we have?