What makes you fat?

We take a look at some theories about the causes of obesity.
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02.Diet and exercise


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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