Despite its kinship with the polar bear or the brown bear, the panda bear enjoys neither meat nor fish. The only thing it likes to eat is bamboo. Lots and lots of bamboo. At certain times of the year, it can feed on the shoots, which are much more nutritious. At other times, on the other hand, it has no choice but to chew its leaves. And yet it is always full of protein and fat.

How can an animal that only eats plants be so fat? And, above all, how can it be that it stays that way despite the fact that its only food is scarce at certain times of the year?

Both questions, but especially the last one, have been a mystery to scientists for years. However, a study just published in Cell Reports by scientists from the Institute of Zoology of the Chinese Academy of Sciences provides an answer. It seems that the key lies in the bacteria that live in your gut.

What is the diet of the panda bear?

It is estimated that a giant panda bear (Ailuropoda melanoleuca) can spend between 10 and 16 hours a day eating. This is because they generally only eat bamboo and, although it is very abundant in their habitat, they need to eat a lot.

Specifically, it eats about 20 kilos a day, which is almost 14% of the weight of an adult panda. And it eats so much for two reasons.

On the one hand, to be able to supply itself with nutrients. Approximately, 100 grams of bamboo shoot, the panda’s food in high season, contains 0.30 grams of fat, 2.60 grams of protein, 5.2 grams of carbohydrates and 2.2 grams of fiber. Therefore, at 20 kilograms they would be ingesting 60 grams of fat, 520 grams of protein, just over a kilogram of carbohydrates and 440 grams of fiber. But that’s not really what they’re taking in. Here comes the second reason why they eat so much: the panda bear only uses 20% of the bamboo it eats.

This is because they are apparently still better able to digest meat, like their brown and polar cousins. Why they do not do so is well studied. Some studies suggest that they do not taste meat well, others that it is due to an abnormal functioning of the reward systems. It is not clear, but the fact is that they must eat a lot of bamboo to supply themselves with nutrients.

Above all, it is important that they maintain that layer of fat that keeps them safe from the cold and the scratches they get from the bamboo canes. And for that they need to eat a lot. But they can’t always eat as much as they would like. From late spring until summer they can eat shoots, with all the nutritional quality mentioned above, yes, but after that they have to make do with the leaves, which are much less nutritious. And yet they remain strong and with that layer of fat they need so much.

Are there bacteria to optimize bamboo?

The authors of the study just published suspected that the microbiota of the panda bear might be behind the optimization of bamboo during seasonal changes.

It would not be the first time, as changes have been observed at different times in the colonies of bacteria living in the gut of other animals. For example, as explained in a statement, some species of monkeys have a different composition in their microbiota between summer, when they feed on fresh leaves and fruits, and winter, when they have no choice but to eat the bark of trees. Changes have even been observed in the microbiota of some human foraging tribes, as they have to adapt to the foods that predominate at each time of the year.

To find out if the same was true for the panda bear, they collected feces from these animals in spring-summer or autumn-winter and performed a fecal transplant to a group of germ-free laboratory mice. Then, for three weeks, they fed the rodents only bamboo.

After this time, the mice that had received spring-summer feces gained significantly more weight and had a higher proportion of fat in their bodies compared to those that received fall-winter feces. Despite having eaten exactly the same.

All that remained was to find out what was in that microbiota to generate such tangible changes. To do this, they proceeded to analyze the feces and found that in the spring-summer season, when they fed on sprouts, it was richer in a bacterium called Clostridium butyricum. And the metabolism of this bacterium generates a substance called butyrate, which seems to be the key to everything, because it up-regulates the expression of a circadian rhythm gene, called Per2, which increases the synthesis and storage of lipids. In other words, when butyrate is generated, it activates the reading of the genes that carry the instructions for storing the fat that reaches the body.

Thus, the increased fat intake that is achieved with the sprouts will be stored for when the shortage arrives in autumn-winter.

So, yes, it may have been easier for the panda bear to give up vegetarianism. But, in reality, their organism is quite well adapted to base their diet on bamboo. They do their part by spending half their day eating and then the gut bacteria do the rest.

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