What makes you fat?

We take a look at some theories about the causes of obesity.
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01 .Introduction


CHOICE looks at the science of overweight and obesity and provides an overview of some of the latest theories about what makes people gain weight.

At its simplest, we gain weight and get fat if we suffer an energy imbalance – that is, if the energy we eat exceeds the energy we burn off through metabolic processes and physical activity. But it's clearly more complex than that, and some people gain weight far more readily than others with an apparently similar lifestyle.

It's generally accepted that while some genes may predispose people to obesity, obesity is not inevitable. Rather, it's an interaction between genetics, lifestyle and environment. Some of the more recent avenues of research into obesity include looking at systemic inflammation, the role of our gut bacteria and environmental contaminants. Here we look at some of this research, and how it relates to the modern obesity crisis.

For more information about weight loss, see Diet and exercise.

Systemic inflammation

Research over the last two decades has shown that obesity and other metabolic diseases such as diabetes and cardiovascular disease, are linked with a chronic low-grade systemic inflammation. This inflammation, also called 'metaflammation' because of its relationship to metabolic processes, has various physiological effects, including changes to glucose and fat metabolism which lead to insulin resistance and weight gain.

Inflammation can be detected by measuring inflammation-related chemicals in the blood. Researchers have found that a diet high in saturated and trans fats, sugar and high glycaemic index foods, and modern processed or modified foods, is associated with this inflammation. Fruit and vegetables, fish and fish oil, nuts, olive oil and herbs and spices seem to have an anti-inflammatory effect. Drinks including tea, wine and beer appear to have anti-inflammatory effects as well, though too much alcohol has been associated with increased inflammation.

Foods with high energy density and/or low satiety – which feature prominently among so-called pro-inflammatory foods – can result in weight gain and, ultimately, obesity ¬– which itself has a pro-inflammatory effect. Weight loss, however, has an anti-inflammatory effect. Your overall diet, particularly energy balance, is therefore important, and can contribute to inflammation.

Physical activity can have an anti-inflammatory effect, although the dose makes the poison: too much (in terms of duration and intensity) or too little (independent of weight) exercise can be inflammatory. An increasing body of research shows that the amount of sitting we do, irrespective of other activity, causes inflammation.

EDCs and other environmental contaminants

Obesogens, which are stored in fatty tissue, are thought to act via an inflammatory effect, but may also exert their effects on the genetics of a developing foetus, consequently affecting future generations.

Chemicals known to have an endocrine (hormonal system) disrupting effect, called endocrine-disrupting chemicals or EDCs, may also be linked with obesity. 

These include certain phthalates, which are found in plastics, cosmetics and fragrances.

  • Dichlorodiphenyldichloroethylene (DDE), which is a chemical formed from the breakdown of the once widely used dichlorodiphenyltrichloroethane (DDT) organochlorine pesticide
  • Polychlorinated biphenyl (PCB), a persistent organic pollutant now banned but once used on various industrial and manufacturing applications; 
  • Dioxins, industrial and combustion-related chemicals
  • Possibly bisphenol A
And it’s not just in humans – studies have found that low-level environmental contamination of these so-called “obesogens” may be linked with weight gain in other mammals.

Obesogens, which are stored in fatty tissue, are thought to act via an inflammatory effect, but may also exert their effects on the genetics of a developing foetus, consequently affecting future generations.

The gut bacteria of the microbiome

Most people are aware that we have lots of bacteria living in our gut – for example, the so-called good bacteria and bad bacteria, which may get out of balance if we’re sick or take certain medications. We’re born with them, though the types and numbers are modified over our lifetime. Collectively called the microbiome – and considered by some to be an ‘organ’ in its own right – the bacteria in and on us comprise about two kilograms of our body weight and these bacteria outnumber human cells 10 to one. The importance of the microbiome is gaining more and more attention, and different assortments of bacteria have been associated with intestinal and bowel conditions such as ulcerative colitis and irritable bowel syndrome, and also immune disorders such as allergies and type 1 diabetes.

But it’s becoming increasingly apparent that the microbiome is linked with metabolism and metabolic diseases, including type 2 diabetes and obesity. It’s known that obese people and lean people have different bacterial profiles, though whether this is a cause or effect of obesity is unknown. For example, when sterile (bacteria-free) lean mice exposed to non-sterile obese mice, they get fat, despite there being no change to their energy intake – suggesting it’s the bacteria that causes obesity. The use of antibiotics in human and veterinary medicine has also been linked with obesity.

There are several suggestions as to how the microbiome may cause obesity. Certain bacteria, the ones more predominant in obese people, are better at extracting energy from food for the host, leading to weight gain. It’s also been suggested that the microbiome is linked with obesity through inflammation (see above). Other theories relate to chemical or hormonal communications between the host and the microbiome, or the regulation by the microbiome of host genes related to energy expenditure and storage. Most likely it’s a combination of several effects.

Small trials in which microbe-rich faecal matter (that’s poo to you) is transplanted from healthy individuals into the lower intestine of people with colitis have been successful in treating the disease, leading some to suggest similar treatment for obesity and type 2 diabetes. However, there are far too many unknowns at this point, such as which bacteria are beneficial and which pathogenic, and whether artificially changing someone’s microbiome may cause other problems. More likely scenarios are the development of microbial supplements of helpful bacteria, perhaps in combination with antibiotics targeting undesirable bacteria, or else drugs that mimic their actions.


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While it would be great to blame environmental chemicals, our forbears and undesirable bacteria for all our weight gain woes, our lifestyle choices also play an important role in weight control. Here we look at how diet, exercise, sleep and stress can affect our weight.

What we eat

A popular myth, often used to promote low-carbohydrate diets, is that advice to follow a low-fat diet has failed, resulting in population weight gain despite eating less fat. Fat, it’s argued, has been unfairly demonised, and carbohydrates are to blame. But while it’s true that the percentage of total kilojoules, or energy, we receive from fat has decreased, we’re eating more kilojoules than ever, and the absolute amount of fat has in fact increased in the last few decades.

However, choosing some low-fat or reduced-fat processed foods – such as ice-cream, pastry, biscuits, or flavoured yoghurt – may result in weight gain because the fat may be replaced with sugar to make it tastier. So these often (though not always) contain as many kilojoules as regular-fat products – but because they’re “low fat”, people think they can eat more of them. These foods may also be less satisfying than their regular-fat counterparts. This doesn’t mean low-fat products should be avoided – many low-fat dairy products such as milk and cheese contain as much or more protein and calcium as their regular-fat counterparts, but less fat (much of which is saturated) and fewer kilojoules.

It’s easy to blame high fat and/or sugar junk food such as ice-cream, biscuits, chocolate, pizza, soft drink and chips for weight gain, but eating excessive amounts of energy-dense healthy foods –muesli, dried fruit, nuts, seeds and dairy foods – instead may also cause weight gain. And meals that appear to be healthy, low-kilojoule options, such as salad, might conceal high-fat dressing, croutons, hard cheese and fatty meats. Fruit juice may also be a problem, with many people not realising how much energy it contains: measure for measure, juice contains more nutrients than fizzy drinks or beer, but the energy content is similar.

Kilojoule for kilojoule, the time of day when you eat doesn’t seem to make much difference. But it may affect how much and what you eat, which could lead to weight gain. Studies show that people who eat a good breakfast within a couple of hours of waking are less likely to be overweight that those who don’t – and, conversely, that breakfast skippers are more likely to be overweight, probably because they eat high-fat or high-sugar foods later in the day. If you eat well earlier in the day, you may have more energy to be more active during your waking hours, ultimately burning more energy.

Other studies have found that many overweight people overeat at night-time, and that night-time meals are often fattier than other meals. This is especially true for those who eat lightly during the day, when they over-compensate for the lack of food. And many people gorge on junk food later in the day when they’re tired, stressed or overwhelmed. But any weight gain would be due to excess kilojoules rather than the time of day.

How we move

Physical activity is important for weight control, but about 60% of people don't get enough, according to the Australian Bureau of Statistics. Cardio exercise, such as walking or jogging, burns energy when you’re doing it, but resistance training or weight lifting results in increased muscle mass, which boosts your metabolism so you’re burning more energy even when you’re not exercising. Yet the modern workplace, personal motorised transport and labour-saving devices all mean we don't need to use our muscles much anymore for everyday activities.

Old-order Amish people, who live much like our pre-industrial revolution forebears did, walk an average of about 16,000 steps a day (as measured by a pedometer) as opposed to the average Australian adult's approximately 9000 steps and the average American's approximately 5000. Rates of obesity among these Amish are correspondingly lower, despite having a diet high in saturated fat and refined sugar.

Our leisure activities – computers, gaming and television – are also becoming increasingly more sedentary. Time spent viewing television seems to be associated with obesity in children, though not necessarily because of lack of physical activity. Rather, it seems to be because of the snack food eaten while watching, or because of foods shown on TV that entice us to eat after watching TV – possibly by increasing levels of the appetite-stimulating hormone, ghrelin.

Too much stress

Many of us know the feeling of wanting to eat something when we feel stressed, and preferably something high in fat and/or sugar – it's a form of emotional eating. While it may make you feel a little better, there's actually a chemical cascade going on within us that makes us want to do this – with the unfortunate effect of potentially inducing weight gain.

When you suffer from chronic stress, your body increases its production of cortisol. Higher levels can be useful because they prepare the body for action by increasing blood sugar and insulin release, providing an energy burst. However, prolonged, abnormally high levels of cortisol are a problem, because it increases appetite and also causes the elevated blood sugar to be stored as fat that is more likely to be deposited in the abdomen. Stress also increases the release of a neurotransmitter called neuropeptide Y, which has a double-whammy weight gain effect of increasing appetite and conserving energy. At the same time, leptin, which helps control the effects of neuropeptide Y, is decreased.

Not enough, or too much sleep

Consistently sleeping too little (less than six hours) or too long (more than eight hours) has been shown to increase the risk of obesity, as well as other diseases such as diabetes and cardiovascular disease. Levels of the hunger-stimulating hormone ghrelin tend to be higher in sleep-deprived people, while the appetite control hormone leptin is lower than in people who sleep the average amount. This means people who sleep less tend to eat more, and especially carbohydrate-rich or fatty foods – and they more than compensate for the extra energy required for being awake longer. At the same time, people who are hungry tend to sleep less – so going on a food-deprivation diet could be counter-productive.

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