Cube-shaped poo

Do you like pizza? If so, I’ve got good and bad news for you.

The good news is that the 2019 Ig Noble prize winner in the category of medicine is Silvano Gallus, who received the award for…

… collecting evidence that pizza might protect against illness and death…

The bad news, for most of you, is that this applies…

…if the pizza is made and eaten in Italy.

Obviously, it’s a bit surprising that pizza can be considered a health food. But if you accept that, it’s also a bit surprising that it has to be Italian pizza. So, what’s going on?

The Ig Nobel prizes are a satirical version of the Nobel prizes. Here’s the Wikipedia description:

The Ig Nobel Prize (/ˌɪɡnˈbɛl/ IG-noh-BEL) is a satiric prize awarded annually since 1991 to celebrate ten unusual or trivial achievements in scientific research, its stated aim being to “honor achievements that first make people laugh, and then make them think.” The name of the award is a pun on the Nobel Prize, which it parodies, and the word ignoble.

As such, the prize is awarded for genuine scientific research, but for areas of research that are largely incidental to human progress and understanding of the universe. For example, this year’s prize in the field of physics went to a group of scientists for…

…studying how, and why, wombats make cube-shaped poo.

It’s in this context that Silvano Gallus won his award. But although the Ig Noble award says something about the irrelevance of the subject matter, it’s not intended as a criticism of the quality of the underlying research. Gallus’s work with various co-authors (all Italian) was published as an academic paper ‘Does Pizza Protect Against Cancer‘ in the International Journal of Cancer. This wouldn’t happen if the work didn’t have scientific merit.

Despite this, there are reasons to be cautious about the conclusions of the study. The research is based on a type of statistical experimental design known as a case-control study. This works as follows. Suppose, for argument’s sake, you’re interested in testing the effect of pizzas on the prevention of certain types of disease. You first identify a group of patients having the disease and ask them about their pizza-eating habits. You then also find a group of people who don’t have the disease and ask them about their pizza-eating habits. You then check whether the pizza habits are different in the two groups.

Actually, it’s a little more complicated than that. It might be that age or gender or something else is also different in the two groups, so you also need to correct for these effects as well. But the principle is essentially just to see whether the tendency to eat pizza is greater in the control group – if so, you conclude that pizza is beneficial for the prevention of the specified disease. And on this basis, for a number of different cancer-types, Silvano Gallus and his co-authors found the proportion of people eating pizzas occasionally or regularly to be higher in the control group than in the case group.

Case-control studies are widely used in medical and epidemiological studies because they are quick and easy to implement. The more rigorous ‘randomised control study’ would work as follows:

  1. You recruit a number of people for the study, none of whom have the disease of interest;
  2. You randomise them into two groups. One of the groups will be required to eat pizza on a regular basis; the other will not be allowed to eat pizza;
  3. You follow the 2 groups over a number of years and identify whether the rate of disease turns out to be lower in the pizza-eating group rather than the non-pizza-eating group;
  4. Again, you may want to correct for other differences in the 2 groups (though the need for this is largely eliminated by the randomisation process).

Clearly, for both logistic and time reasons, a randomised control study is completely unrealistic for studying the effects of pizza on disease prevention. However, in terms of reliability of results, case control studies are generally inferior to randomised control studies because of the potential for bias.

In case control studies the selection of the control group is extremely important, and it might be very easy to fall into the trap of inadvertently selecting people with an unusually high rate of eating pizzas. (If, for example, you surveyed people while standing outside a pizzeria). It’s also easy – by accident or design – for the researcher to get the answer they might want when asking a question. For example: “you eat a lot of pizza, don’t you?” might get a different response from “would you describe yourself as a regular pizza eater?”. Moreover, people simply might not have an accurate interpretation of their long-term eating habits. But most importantly, you are asking people with, for example, cancer of the colon whether they are regular pizza eaters. Quite plausibly this type of disease has quite a big effect on diet, and one can well imagine that pizzas are not advised by doctors. So although the pizza-eating question is probably intended to relate to the period prior to getting the disease, it’s possible that people with the disease are no longer tending to eat pizza, and respond accordingly.

Finally, even if biases are eliminated by careful execution of the study, there’s the possibility that the result is anyway misleading. It may be that although pizzas seem to give disease protection, it’s not the pizza itself that’s providing the protection, but something else that is associated with pizza eating. For example, regular pizza eating might just be an indicator of someone who simply has regular meals, which may be the genuine source of disease protection. There’s also the possibility that while the rates of pizza eating are lower among the individuals with the specified diseases, they are much higher among individuals with other diseases (heart problems, for example). This could have been identified in a randomised control study, but flies completely under the radar in a case-control study.

So, case-control studies are a bit of a minefield, with various potential sources of misleading results, and I would remain cautious about the life-saving effects of eating pizza.

And finally… like all statistical analysis, any conclusions made on the basis of sample results are only relevant to the wider population from which that sample was drawn. And since this study was based on Italians eating Italian pizzas, the authors conclude…

Extension of the apparently favorable effect of pizza on cancer risk in Italy to other types of diets and populations is therefore not warranted.

So, fill your boots at Domino’s Pizzas, but don’t rely on the fact that this will do much in the way of disease prevention.

 

 

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