May 28, 2009 - Health + Traits

Environment, Not Genes, Key To Increasing Disease Rates

Type 1 diabetes is on the rise in European children, says a new report.

Researchers studied type 1 diabetes data collected between 1989 and 2003 at 20 centers in 17 European countries. Their results, published online yesterday in the Lancet, show that more children, especially younger children, are being diagnosed with the disease each year.   Based on the trends they saw, the scientists calculate that there were 94,000 kids under the age of 15 with type 1 diabetes in Europe in 2005, and that by 2020 that number will soar to 160,000.

While researchers aren’t exactly sure why this is, they do know that it’s not due to changes in the prevalence of susceptibility genes.   Genes just don’t change that quickly.

An almost 70% increase in disease prevalence in one generation must be due to changes in non-genetic factors. Most random genetic changes in a population come and go pretty quickly, especially mutations that reduce fitness.   And if a new mutation does manage to stick, it would take millions of years, not tens of years, to see its effects.   Even for mutations that provide a benefit, like the one that led to the lactose tolerance seen in many people with European ancestry today, it takes a few hundred years to build-up to high enough levels in the population to cause an observable change in a trait.

An increase in disease incidence due to changes in non-genetic factors, whether they are environmental or cultural, has been seen for many diseases.   It’s especially apparent when groups migrate from low- to high-risk countries for a particular condition.   Just this month a study showed that Asian Americans who are more “westernized” have adopted the sunbathing ways of their families’ new homes, which the authors suggest may be the cause of increasing rates of skin cancer in this group.

But the effects of lifestyle changes can also be seen in shifts in disease rates within a population. The prevalence of obesity in United States adults, for example, jumped from 15% in the late 1970’s to nearly 35% today thanks to the trend toward eating more and exercising less.   And because of the increase in obesity, rates of type 2 diabetes are also up.

Many scientists attribute the increase in incidence of several immune system-related disease to what on the surface seems like a good thing about modern lifestyles: fewer infections.   The so-called “hygiene hypothesis” suggests that without the types of infections our species evolved to deal with (many of which are still prevalent in developing nations), our immune systems don’t get the right training.   The lack of challenges to the immune system has been linked to increased rates of allergic diseases like asthma and eczema and autoimmune diseases like Crohn’s and multiple sclerosis.

For some diseases, the reason behind their apparent increases has more to do with increased detection than changes in environment. Up until a few years ago, for example, it was thought that only about one in every 3,000 people in the United States had celiac disease.   But now, thanks to better guidelines on how to diagnose the disease, physicians are finding that about one in every 133 is affected.

On the other hand, some conditions may appear to be increasing because disease awareness is a hammer that makes a lot of people feel like nails.   It has been put forward that restless legs syndrome, for example, is far less prevalent than some estimates suggest and that increases in diagnoses can be traced to “disease mongering” by pharmaceutical companies.

The authors of the Lancet study suggest that the changes in type 1 diabetes rates they are seeing are due to something about modernization.   They point to the fact that the biggest increases were seen in eastern European countries, which have seen the most rapid changes in lifestyle in the last few decades.   But whatever the culprit is, it is obviously not affecting all children.   And that’s where genetic susceptibility comes in.   DNA variations that increase risk may not be changing in prevalence, but type 1 diabetes, like almost every other common disease, is the result of a complex interplay of genes and environment.

(23andMe customers can see how their genes influence their risk of type 1 diabetes in Clinical Reports.)

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