In Depth › Analysis and Opinion

Is your brain making you fat?

We're getting fatter, but it's not all our fault, says physiologist Professor Michael Cowley.

Spend enough time in public places or even watching television and it's clear that people are becoming more overweight. Indeed rates of obesity are rising in both adults and children.

But the average adult's weight increases by less than 500 grams a year — the equivalent of just a few potato chips a day. This demonstrates the remarkable job our bodies do of balancing our food intake and energy output.

So for those of us whose waistlines are expanding — what's going wrong? And are we solely to blame?

^ to top

'Crisis of plenty'

The recent increase in the amount of fat in our bodies is due to dramatic changes in our lifestyle over the past 100 years or so — a change we're not equipped to deal with from an evolutionary perspective.

For many people, physical effort has become optional, and with the increasing availability of energy-rich, tasty foods, in larger and larger portions, we've created a 'crisis of plenty' in our pantries and refrigerators.

So we are pushing ourselves to gain weight on both sides of the energy balance. And our bodies are all in favour of that.

In evolutionary terms, it's good to put on weight. We need energy stores to carry out activities that use lots of energy, like growing and repairing muscle, as well as carrying pregnancies, and raising children — and historically, food supplies weren't always a given.

Our brains developed ways to maintain our fat stores by detecting the levels of a hormone called leptin, which is secreted into the blood by fat cells.

The brain mostly tries to keep this hormone level constant, by making us hungry and burn less energy when our leptin levels drop.

It's an effective system for people whose lives depend on having reserves of energy to survive famine, but for those of us battling obesity in a time of plenty it's got some serious downfalls.

For starters, leptin makes it difficult for most of us to lose weight easily through dieting.

When we diet, leptin levels drop rapidly. The brain notices this decline and triggers strong hunger, and also makes us burn less energy, trying to preserve body fat — effectively counteracting the diet.

On top of the diet dilemma, obesity itself interferes with the brain's regulatory system. It seems that the high levels of leptin from excess body fat makes the brain leptin-resistant, and less able to detect levels of stored fat.

The brain actually gets tricked into thinking we are leaner than we really are — so too much fat seems to reprogram the brain into thinking we're starved.

^ to top

Addicted to eating

Leptin aside, there's another factor working to increase bodyweight for some obese people — compulsive food consumption mirrors drug addiction.

Recent brain imaging studies have shown that addictive drugs activate a set of 'reward' pathways in the brain. These brain systems seem to tell the brain that something is very important, and are usually activated by things that are essential for survival.

Addictive drugs highjack these brain pathways. For some obese people, food also activates these pathways, to them we could say that food has characteristics of an addiction.

The downside of weighing more With increased body weight, and more importantly increased levels of body fat, come increased risks of many diseases. The diseases most commonly associated with obesity are diabetes, and cardiovascular diseases like heart attack and stroke. Unfortunately it seems that having one of these diseases increases the risk of having the others: this is called 'metabolic syndrome'. We do not understand the basis of these diseases occurring together, but there is growing evidence that fat which accumulates around the belly, termed 'visceral fat', or 'abdominal fat' contributes to the increased risks. This may be because abdominal fat secretes different things into the bloodstream than does fat in other parts of the body. These factors seem to activate the immune system, and cause it to secrete infection-fighting chemicals that can also have problematic effects on the whole body, leading to increased disease risk.

While it seems like a flawed system, our body fat regulation evolved under pressure to cope with too little food. In our evolutionary past we haven't really needed to evolve ways to deal with too much fat — it hasn't happened often enough to have been a burden to survival.

But while our lifestyle has changed our brains haven't yet caught up, they're still operating like we're living in times of regular famine.

However, not everyone becomes obese in our current food rich environment, so it's clear that individuals have different tendencies to become overweight or obese.

This suggests the high rates of obesity present today are due to an interaction between our genetic predisposition and our environment.

While a number of individual genes have been shown to cause obesity, it is unlikely that these gene mutations alone are responsible for the current rising rates of obesity.

What's more likely to have a bigger impact on our tendency to gain weight is our recent evolutionary history.

Although all humans share a common ancient ancestry, our recent evolutionary histories differ depending on where our recent ancestors lived.

For example some people come from places where the environments were less productive, or where the food supply was less reliable.

They have evolved a 'thrifty' genotype that enables them to thrive with periodic low food availability: they store food efficiently when they can, to prepare for a possible future famine.

This is a really good adaptation when there is regular famine, but with most of us in the developed world trying to decrease our food consumption what was once an evolutionary advantage now causes increased risks.

Our different genetic histories also show up in the way that some people can become obese and not become unwell, while for others only a moderate increase in weight can bring on serious disease.

^ to top

Resisting obesity

Evolutionary biologist Professor Jared Diamond has pointed out that the relative abundance of food in Europe for many hundreds of years might have created conditions that would have allowed someone to become obese if they were determined to overeat and not exercise.

He suggests that there may have been a selective pressure in Europe to develop resistance to obesity, and obesity-induced diseases.

If this is correct then we might sadly discover that as people from non-European backgrounds become more affluent and more obese, they will have even higher prevalence of diabetes and heart disease.

Our understanding of the pathways in the brain that sense stored fat, stomach contents ,and blood sugar has helped us understand obesity and how it contributes to diabetes and heart disease, and has led to the development of several new drug therapies.

But we cannot wait for magic bullets to help us lose weight.

We have a responsibility to ourselves and our children to develop ways of living that incorporate better eating habits and more active lifestyles, to make vigorous activity part of our day not a special occasion.

Professor Michael Cowley is a physiologist at Monash University. He conducts research into how our brains control metabolism, and develops therapies for obesity and related diseases. He was the winner of the 2009 Science Minister's Prize for Life Scientist of the Year.

^ to top