Research Review: Is obesity contagious? | Precision Nutrition

Research Review:
Is obesity contagious?

By Helen Kollias, Ph.D.


We’ve all heard this phrase over and over again:

The obesity epidemic.

It has become cliché.

It’s on the news, on the net and in the papers. Heck, I’ve seen it in pretty much any scientific research article about weight loss, weight gain, diabetes, heart disease, high blood pressure and anything else that could be tied into “‘the obesity epidemic.”

But aren’t epidemics caused by infectious things like viruses (HIV) or bacteria (bubonic plague)? And don’t they need to spread?

Epidemics don’t necessarily need to be infectious, but this week’s review shows that obesity is contagious and in fact, spreads through a population much like the flu.

Is obesity really an epidemic?

Thinking of obesity as an epidemic that can spread through a population from person to person is an interesting counterpoint to two other prevailing theories of obesity:

  1. Obesity is caused by Big Food who makes addictive food or saturates our free will in sugary cereal and bacon cheeseburger ads.
  2. Obesity is caused by genetic factors.

Probably obesity is multi-factoral — there are likely many reasons for obesity. But thinking about obesity as something that we can “catch”… well, that’s a pretty interesting — and challenging — concept.

Facebook made me fat!?

Last year I reviewed an article about how your social network influences how much you weigh and this is a continuation of that idea.

In another study (1), researchers found that if you have an obese friend, you’re 57% more likely to be obese (funny thing is, having an obese spouse only increases your risk by of being obese by 37% — I guess it’s true that we never listen to our partners!).

The researchers didn’t stop there. They played the Kevin Bacon “six degrees of separation” game, but with degrees of separation from people that were obese.

One degree of separation (your friend) makes sense. You and your friend hang out together, you do stuff together and probably have similar likes and dislikes that could lead to gaining weight.

How about two degrees of separation? Your friend’s friend (two degrees of separation) being obese increases your chance of being obese by 20%. Remember these are people you may have never met.

Even people three degrees of separation from you could increase your chance of being obese (by 10%). Hmm… that is a little harder to figure out.

Fourth degree and up didn’t have any sort of impact on obesity risks. Phew.

Research question

This week’s article asks:

Is obesity contagious?

Alison L Hill, David G Rand, Martin A Nowak, Nicholas A Christakis. Infectious Disease Modeling of Social Contagion in Networks. PLoS Computational Biology, 04 Nov 2010 DOI: 10.1371/journal.pcbi.1000968


In this study the researchers used a mathematical model. The model is similar to what scientists use for analyzing different infectious diseases, like the flu. Models like this let scientists examine how quickly a disease could spread, how infectious it is, and when it might peak in a population.

However, unlike, say, a flu-based infection model, this model includes the possibility that you can become obese spontaneously, which can’t happen with the flu.

The model broke down the spread of obesity into three parts:

  1. Transmission of infection: how quickly obesity spreads from person to person
  2. Spontaneous infection: how quickly obesity occurs from non-person to person causes, like having a greasy burger joint move in next door or watching more TV
  3. Recovery: how quickly people become non-obese
Obesity contatigion fig 1
Figure 1 – Various ways of becoming obese (schematic of the susceptible-infected-susceptible (SIS) model).

By looking at these three parts, researchers could guess three things:

  1. your chances of becoming obese in a year;
  2. how much your friends increase your risk; and
  3. when the obesity epidemic might reach its height.


All this math was based on real people from a huge multigenerational study, called the Framingham Heart Study, which started in 1948 (though this study only looked at 1971 and onward).

The FHS measured volunteers every 4 years. Since 1948, there have been over 7,500 people from 3 generations in the study.


After a lot of math, the researchers figure that the obesity rate will continue to climb until 2050, when 42% of Americans will be obese (BMI>30 and does not include overweight).

That was the best case scenario.

What does that mean? Well, today 34% of the US adult population is obese. Most experts figured this was the peak for obesity or at the very least the plateau, since the percentage has been stable for the last 5 years.

If you graph obesity rates and what the study predicts, you get a nice graph (Figure 2) that looks very much like a graph of infectious disease. The graph plots:

  • measured obesity prevalence according to NHANES (National Health and Nutrition Examination Survey) and FHS data
  • the susceptible-infected-susceptible (SIS) computer model.
obesity contagion fig 2
Figure 2 – Comparison between actual rates of obesity and the math model they used in this study

Obesity is cumulative

These projections of future rates of obesity are based on data from the Framingham study, which finds that recently, obesity has gone up by 2% a year.

This rate goes up an additional 0.5% with each obese social contact you have. For example, if you have 4 obese friends, then your rate goes up to 4% (2% + 0.5% X 4).

What’s even more interesting is that this effect now seems to be cumulative in a way it wasn’t a few decades ago. In 1970, having friends that were obese that didn’t matter as much as it does today.

Weird thing is, the rate of going the other way (becoming non-obese) hasn’t changed. In 1970, your chance of becoming non-obese if you were obese was 4%, and that hasn’t changed.


This is a good news, bad news kind of story.

Bad news:

  • A non-obese person has a 2 percent chance of becoming obese every year.
  • Each obese person with whom you have social contact increases your chance of becoming obese by 0.5%. Thus, having 5 obese social contacts more than doubles your risk of becoming obese.
  • The obesity rate will likely be above 42% in 40 years.

The good news is that even though obesity will continue to increase it will be slower than the last 20 years.

Bottom line

Even though obesity can be spread in a way similar to the flu the most important risk factor in becoming obese is non-social — aka your own nutrition and activity.

You are still responsible for you.

But having social contacts with fitter people makes it a little easier to stay leaner.

Eat, move, and live… better.

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Click here to view the information sources referenced in this article.