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Explainer: Diabetes and obesity – the biggest epidemic in human history

Psammomys obesus or the Israeli sand rat provides an insight into how the thrifty gene hypothesis may work. Tino Strauss

In the last few decades, the number of people with diabetes has more than doubled globally, making the combination of type 2 diabetes and obesity (known as diabesity) the largest epidemic the world has yet faced.

The most recent global predictions suggest that currently there are 285 million people with diabetes worldwide; in Australia alone, there’s been a 300% increase in the number of people with diabetes over the last 30 years.

Recognising the gravity of the situation, the United Nations General Assembly unanimously passed a resolution declaring diabetes an international public health issue on December 21, 2006.

This is only the second disease after HIV/AIDS to attain such an important status.

Diabetes is a chronic disease, which, through complications, can impact seriously on the quality of life of individuals and their families through premature illness and death.

Because it affects people who are still in the workforce, it has a major impact on both individual and national productivity.

So, how is it that a relatively uncommon disease in past times now poses a magnitude of global threat to societies akin to climate change?

The thrifty gene hypothesis

To explain this, the late James Neel, a renowned American geneticist proposed the “thrifty gene” hypothesis.

He suggested the gene or genes for type 2 diabetes – formerly known as adult-onset diabetes – conveyed a survival advantage to early hunter-gatherer and agricultural societies.

This is because such societies were subject to periods of nutritional hardship: “feast or famine” scenarios.

Neel proposed the thrifty gene prompted fat deposits in the body during periods when food was abundant, providing the basis for survival during periods of food shortage.

But the modern, sedentary lifestyle with its over nutrition and continuous “feasting” means the gene has become a disadvantage.

It now inclines people to develop obesity and diabetes.

Lessons from the Pacific

The Pacific island of Nauru is now in the news for political reasons. But, in 1975, I was discovering that it had the highest rate of diabetes of any nation in the world.

Nauruans present an extreme case of how type 2 diabetes can rapidly reach epidemic proportions and illustrate the role thrifty genes may play.

The same epidemic is now emerging in other developing countries and indigenous communities such as our own Aboriginal and Torres Strait Islander peoples.

The high diabetes rate in the Nauruans was attributed to the change from a more traditional diet and way of life to a modern one – we used the term “Coca-colonisation” to describe it.

There’s very limited food produced on the island so nearly all the food was imported western products – and most of it had dubious nutritional value.

Despite its appeal, the thrifty gene hypothesis is somewhat controversial because it’s difficult to prove in human populations unless an environment similar to that of the Nauruans is reproduced and the population studied for 30 to 40 years.

However, the theory finds support in an excellent animal model of obesity and type 2 diabetes in the Israeli sand rat.

This desert-dwelling, burrowing rodent is native to desert regions of the Middle East, where it exists on a diet of saltbush.

In their native environment, Psammomys obesus remain healthy with no evidence of diabetes or obesity.

But when they can access a relatively high-energy diet, a substantial proportion of the rodents develop mild to moderate obesity and severe type 2 diabetes within a few months.

Put them back into a desert environment, and within a few months, they lose their obesity and the diabetes is reversed.

A new theory

Another theory now gaining considerable momentum emphasises the importance of events during gestation: the focus here is on the intra-uterine environment.

This theory suggests that the intra-uterine environment, and in particular, maternal diet, influences the risk of developing type 2 diabetes.

The classic study that lends support to this theory relates to the Dutch Winter Hunger.

People who were conceived during the “Hunger Winter” of 1944-45 in the Netherlands have been found to have higher rates of type 2 diabetes and heart disease compared with those born when food was more readily available.

Their diabetes and heart disease have been attributed to the starvation diet experienced by their mothers during pregnancy.

There is growing evidence that exposure of the foetus in the uterus to poor maternal nutrition, tobacco use and alcohol will increase the risk of diabetes, obesity and heart disease in adult life.

This scenario may explain the very high rates we see now, particularly in our indigenous population and other indigenous groups globally, and the spectacular rise of diabetes in countries such as India and China.

This theory highlights the need for a “whole-of-life” approach to the prevention of type 2 diabetes and heart disease.

Tackling diabetes and obesity is likely to be one of the most important challenges for the Australian public health community in the 21st century.

It is a huge challenge for the global community and it is a battle that we can and must win.

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