Death match: Big Macs vs. Unprotected Sex

Greetings, fellow nerds.

My Spanish teacher in Costa Rica (Carlos P. - the P. stands for a word which happens to be the foulest word in the Portuguese language, which was entertaining to the Brazilian immigration officials - that's entirely another story though) asked me why Americans are fanatical about staying AIDS-free, but don't give a pair of dingo's kidneys if they die of being too fat. Carlos, the more I think about it, the more it sounds like you're on to something, and I'm going to follow up your comment with a numerical analysis: today we're going to weigh the risk of HIV contracted from having unprotected sex against the increased risk of dying from obesity by eating one Big Mac™.

HIV risk from unprotected sex: what are the numbers?

First off, let's quantify the HIV risk of having sex with an American chosen at random. The CDC estimates the percentage of HIV-positive US residents to be 0.4% (as of 2003). That's already a low number, but to asses the risk of catching HIV from unprotected sex we have to multiply by the transmission rate: that is, given that you have unprotected sex with somebody who's HIV-positive, what are your chances of getting it?

What is this transmission rate? Take a guess. Now decrease it by a factor of 10. If your guess was like mine, you'd still be way above the truth. I don't know if it's general squeamishness or over-zealous sex ed teachers, but the risk of HIV transmission between otherwise-healthy people is between 5 and 50 cases per 10,000 acts, depending on exactly what kind of (more-than-just-oral) unprotected sex you're having (Wikipedia link + original article). That means if you and your (randomly-selected vis. HIV status) American partner are healthy, you have between a 2 in a million and a 20 in a million chance of contracting AIDS from unprotected sex.

Let's put that into perspective. If you live to be 80, you'll have lived 29 200 days. What are the chances (everything else being equal) that today your number's up? That would be 1 / 29200 = 34 in a million, almost twice the HIV risk associated with receptive anal intercourse from a randomly-chosen American male.

My heart goes out to people who have contracted HIV from unprotected sex. I'm sure they have been demonized for being so careless as to indulge in this (so-called) risky behavior. Let's find out just how risky their behavior really was, in terms of Big Mac™ eating.

What's the Lethal Dosage of Big Macs™?

First of all, I don't want to single out Big Macs™, McDonald's™, or even just the fast food industry as unique bringers of ill-health. The Big Mac™ is however a nearly-ubiquitous unit of culinary over-indulgence; let it here symbolize any overly-calorific meal.

There's a good chance that fast food-related factors other than too many calories cause health problems. Films such as "Super Size Me" suggest that high concentrations of fast food can kill - suppose Morgan Spurlock had spent 100 days on the McDonald's™-food-only diet and found that to be lethal. Then we could estimate the risk of eating a Big Mac™ to be 1 in 300 (for 300 meals). That's a risk of more than 3000 per million meals, or between 150 and 1500 times the risk of contracting HIV from unprotected sex. When spread out over many years, the lethality of Big Macs™ can't be that high, so let's get a low-ball estimate of the risk on Big Mac™ poses by its calories alone.

Weight Gain from a Big Mac™ Meal

Surprisingly, if you eat a Big Mac™ and nothing else as a meal, you get about the right number of calories. Assuming that you should be eating 1800 calories a day, the Big Mac™'s 600-calorie payload doesn't sound so bad. It's the side dishes which add the real risk. Full meals at fast food restaurants can have as many as 1825 calories - 1025 too many for an 1800 calorie-a-day diet. For now, let's assume the typical fast food meal has 800 superfluous calories which will be carried around by the diner essentially forever. (I'm not sure if eating too much has a net positive or negative effect on one's metabolism: if you're heavy you might decide to drive instead of walk so much that it counteracts the need to fuel a bigger body.) That 800 extra calories per fast food meal translates to just under a quarter pound of extra body weight; let's see what that does for your health.

Mortality Increase per Big Mac™

I could give you a laundry list of symptoms you could get by being overweight, but instead I'll just boil it down to a number: how much does your mortality rate increase if you have that extra quarter pound on your paunch? According to this JAMA article, in 2000, 111 909 excess deaths were caused by obesity among the 23.3% of Americans who were obese or extremely obese (defined as having a BMI above 30). In 2000 there were 281,421,906 Americans total, which means that 0.17% of obese people died that year from being obese. That's just for the year though - since HIV can let you live for 15 years or more, the risk of dying from obesity in the same span of time you'd expect as from an HIV infection is 2.5%, or 1 in 40.

The last factor to consider is how likely it is for an extra quarter pound to push you into the danger zone. (Aside: in reality, there probably isn't any sharp divide between safe and risky BMIs, but we can still get a feel for the effect size of being obese by this discretization.) From the same study, 33.8% of Americans have a BMI from 25 to 30. Assuming an even distribution of BMIs in the 25-30 range and an average height of 5'10", about 0.8% of people with a BMI between 25 and 30 would be tipped into the BMI > 30 class from that one extra fast food meal. If we assume the risk of already-obese people is at least as great as the risk to overweight people, that means that for 47.1% of the American population, eating one Big Mac™ has a .8% chance of increasing your mortality risk over the next 15 years by 2.5%. Multiplying probabilities, the total risk of that Big Mac™ killing you within 15 years (again, assuming you're a randomly-chosen American) is at least 94 in a million, or between about 5 and 50 times the HIV risk of unprotected sex! That's an average too; if you're overweight, Big Macs™ are at least twice as deadly.

Caveats

There are a few weak points in my argument; let me list them. Here are some factors which may make HIV more dangerous than I let on.

1. People who have more unprotected sex with multiple partners tend to have partners with higher HIV risk too.
2. Other STDs can increase the rate of transmission of HIV by increasing the volume of fluids exchanged.

Additionally, I didn't factor in that being underweight can be risky too; in other words, Big Macs™ could be a benefit to skinny people.

However, my final analysis also didn't take into account the fact that fast food is nutritionally poor, an additional danger I have not accounted for. Moreover, I only counted the obesity risk over 15 years, while the risk in fact continues for as long as you are obese.

Conclusion: Big Macs™ are More Deadly than Unprotected Sex in America.

It would be incorrect to state that every Big Mac™ consumed poses more risk of death than every act of unprotected sex. It also would be wrong to claim that the HIV/AIDS epidemic has been totally eclipsed by a wave of obesity; factor #1 under Caveats is too big to ignore in an epidemiological context. However, given my reasonable assumptions, I find that on average Big Macs™ are 5 to 50 times more deadly than unprotected sex with a randomly-chosen American. Carlos P., your intuition was right.

Here's the take-home message:

1. HIV is harder to contract than you might think.
2. Too much food can kill you.
3. We should worry more about our diets, and perhaps less about disease.
4. If you're like me, you probably worry too much about the wrong things.
   

To address #4, I've started a wiki to keep track of the risks around us. Help me complete and update it; then maybe we can keep a sense of perspective when it comes to banal topics like Big Macs™ and ominous-sounding plagues like HIV/AIDS.