Giant Panda: Why are they still fat even though they have “diet meals” every day?

For humans, a "high-fiber, low-fat" diet is the secret to losing weight. But there is an animal that eats this "diet meal" every day, but still grows fat and round. Who is it?

On January 19, Beijing time, the team of Academician Wei Fuwen from the Institute of Zoology, Chinese Academy of Sciences, published progress in Cell Reports, revealing the interaction mechanism between the diet of wild animals and intestinal microorganisms from the perspective of conservation metagenomics, and uncovering the mystery of how giant pandas can be plump even on a vegetarian diet.

Giant panda: I was not born a vegetarian

In taxonomy, giant pandas are classified as Carnivora, Ursidae. However, after millions of years of fierce competition and natural selection, giant pandas have become more and more picky eaters.

Today, 99% of their diet consists of bamboo.

But at the same time, their teeth and digestive tract still retain the characteristics of carnivores.

Seasonal dynamics of gut microbes help giant pandas maximize nutrient utilization

(Photo provided by the research team)

How do giant pandas digest bamboo with their meat-eating stomachs? As early as 2011, Wei Fuwen's team pioneered the use of metagenomic sequencing technology in the study of the intestinal microbiome of wild animals, proving that the key to giant pandas' ability to digest cellulose and hemicellulose in bamboo lies in the intestinal microbes of the Clostridium family.

In 2017, the team discovered that the Qinling giant panda's habit of "eating bamboo shoots in late spring and early summer, and bamboo leaves in other seasons" can induce the seasonal dynamics of intestinal microorganisms.

In 2021, they further applied multi-omics methods to reveal the interaction mechanism between giant panda intestinal microorganisms and secondary metabolites in bamboo.

"Food affects the composition and function of an animal's intestinal microbes, and in turn, intestinal microbes play a vital role in the animal's dietary adaptation and nutritional metabolism," said Wei Fuwen, one of the co-corresponding authors of the paper and an academician of the Chinese Academy of Sciences . "The special habits of the giant panda make it an ideal model for studying the co-evolution of host dietary habits and intestinal microbes."

Giant pandas at the Chengdu breeding base.

Image source: Tuchong Creative

You can still store meat by eating bamboo shoots

For animals, lipids are actually important nutrients that are essential for growth, development, reproduction and other life activities. Therefore, high-fiber, low-fat bamboo is not an ideal food for most wild animals.

So how do giant pandas rely on eating bamboo every day to maintain the energy and nutrition they need for growth and activities? And how do they grow into "chubby" animals that can weigh up to 180 kilograms?

First of all, it has a strong chewing power. The giant panda is classified as a carnivore. During its long development, its carnassial teeth have lost the ability of carnivores to tear muscles and cut tendons, but have become as wide as the molars at the back, so its tooth roots are harder and more stable. Strong teeth combined with powerful chewing muscles not only give it a round and plump cheek, but also make it suitable for squeezing and grinding bamboo while eating.

Image source: Tuchong Creative

Secondly, although the giant panda has a short intestine, which may make it difficult to digest, there are mucus glands distributed in the oral mucosa, tongue lamina propria, and esophagus. In addition to abundant intestinal glands, there are also many goblet cells. The secretions of these glands can protect the mucous surface of the digestive tract from damage by rough bamboo on the one hand, and on the other hand, facilitate the smooth passage of these rough bamboos, acting as a lubricant.

In addition, the panda's small intestine has a thicker muscle layer, which allows for powerful peristalsis, thus providing the ability to efficiently digest, absorb and transport food. Metagenomic studies have also found that the panda's intestinal flora is rich in substances such as Clostridium and β-glucosidase that can digest plant cellulose, which can also help digest the cellulose in bamboo.

The research team, in collaboration with Professor Wei Hong from the Army Medical University of the Chinese People's Liberation Army, used fecal microbiota transplantation and germ-free mice for the first time to construct mouse models colonized with giant panda leaf-eating Enterobacteriaceae and bamboo shoot-eating Enterobacteriaceae, respectively. Combining multi-omics methods such as metagenomics, metaproteomics, metabolomics and transcriptomics, the researchers finally locked in a key intestinal bacterium - Clostridium butyricum.

"We found that Clostridium butyricum is more abundant in the intestines of giant pandas when bamboo shoots are their main food. This allows giant pandas to digest the crude fiber in bamboo shoots more fully and produce more butyrate," said Huang Guangping, co-first author of the paper and assistant researcher at the Institute of Zoology, Chinese Academy of Sciences. "This matches the phenomenon that giant pandas gain weight significantly during the bamboo shoot feeding period, proving that the bamboo shoot feeding period is a critical period for giant pandas to replenish energy and nutrients."

Related paper information:

https://doi.org/10.1016/j.celrep.2021.110203

Comprehensive sources: China Science Daily, Science Press, Interface News, etc.