The gut microbiota has been a popular area of research over the past few years and few aspects of this research have gained quite as much attention as the role of the gut microbiota in obesity.

One of the studies that started this was published in Nature in 2006. This study has been rather influential as Google Scholar states that, as of today, this study has now been cited 5,842 times.

https://www.ncbi.nlm.nih.gov/pubmed/17183312

The reason for its importance is that it was the first study to show that transplanting the microbiota from obese mice into lean mice made the mice fatter than mice getting transplants from lean mice.

Some previous studies had reported differences in the composition of the gut microbiota between obese and lean individuals, but this was the first study to provide evidence that this might be causal.

The key graph in the paper for this is Figure 3c (yellow highlights are mine).

In germ-free mice transplanted with gut microbiota from obese mice, body fat increased by 47% compared to 27% getting a microbiota from lean mice.

While this looks like a big difference, it is worth noting that this is not body fat percentage. This is the percentage increase in fat in relation to the initial body fat. This is difficult to interpret without knowing how much fat the mice started with.

The study does not state the starting body fat of the mice, but it does tell us in the results how much fat the mice gained in grams.

As we know the percentage increase in body fat and the total weight of that increase, we can calculate the amount of fat each group of mice had before the faecal transplant.

Mice getting a faecal transplant from obese mice gained 1.3 grams of fat, an increase of 47% relative to their starting fat. Mice getting a faecal transplant from lean mice gained 0.86 grams of fat, an increase of 27% relative to their starting fat. If we calculate their starting fat: Lean transplant recipients: 1.3/0.47 = 2.76 Mice getting obese mice transplants had 2.76 grams of fat at the start. Obese transplant recipients: 0.86/0.27 = 3.19 Mice getting lean mice transplants had 3.19 grams of fat at the start. If we add those up… 2.76 + 1.3 = 4.05 3.19 + 0.86 = 4.07 The final body fat of both groups was almost the same at about 4.1 grams

If we graph the total weight of the body fat in the two groups of mice:

(This graph probably wouldn’t get you a paper in Nature).

As you can see, while the mice getting a faecal transplant from the obese mice gained a little more fat, they ended up with the same body fat at the end of the study as the mice getting faecal transplants from lean mice. As mice at this age would typically weigh about 25 grams, at 4 grams of body fat both groups of mice would be considered lean and therefore neither group of mice were obese after the transplantation.

This suggests that the results may not be quite as exciting as they first seemed.

Percentage increases in fat do have valid uses in science but they can also be used to make minimal differences look larger and to make them statistically significant.

It is always worth reading graphs carefully.