Gut: The Inside Story of Our Body’s Most Underrated Organ

by Giulia Enders

Explorers keen to discover new continents today must turn to the microscopic world within us.

Of our entire microbiome—that is, all the micro-organisms that teem on the inside and outside of our bodies—99 percent are found in the gut. Not because there are so few elsewhere, but because there are simply so inconceivably many in the gut.

Skewed proportions of the different bacteria in our gut have been detected in those suffering from obesity, malnutrition, nervous diseases, depression, and chronic digestive problems. In other words, when something is wrong with our microbiome, something goes wrong with us.

The vast majority of our immune system (about 80 percent) is located in the gut.

If we decided to say “Hi” to each of our gut bacteria individually, we might just manage it in around 3 million years.

However, we cannot receive blood from donors whose blood cells have a different blood-group marker on their surface. It would immediately remind our immune system of bacteria,

If it weren’t for this combat readiness—learned through training by our gut bacteria—there would be no blood groups and any donor could give blood to any recipient.

For newborn babies, who do not yet have many bacteria in their guts, this is indeed the case. They can theoretically receive transfusions of blood from any group without any incompatibility effects.

As soon as babies begin to develop a rudimentary immune system and gut flora, they can only tolerate blood from their own group.

The majority of the microbes in our gut protect us simply by occupying spaces that would otherwise be free for harmful bacteria to colonize.

We receive some help creating this collection—mainly from our mother. No matter how many sloppy kisses we give to the car window, if we are allowed to kiss and cuddle with our mother regularly, we will be protected by her microbes.

Children with insufficient Bifidobacteria in their gut in their first year have an increased risk of obesity in later life compared with infants with large populations.

We are only now beginning to learn the impact the gut-based bacterial community can have on an adult human. In this respect, scientists know more about bees than about human beings. For bees, having more diverse gut bacteria has been a more successful evolutionary strategy. They were only able to evolve from their carnivorous wasp ancestors because they picked up new kinds of gut microbes that were able to extract energy from plant pollen. That allowed bees to become vegetarians.

Beneficial bacteria provide bees with an insurance policy in times of food scarcity: they have no trouble digesting unfamiliar nectar from far-flung fields.

Times of crisis highlight the advantage of hosting a good microbial army. Bees with well-equipped gut flora can deal with parasit...

There is one species of Archaea often found in our gut that thrives on the waste products of other gut bacteria and can glow.

Taken together, our gut bacteria have 150 times more genes than a human being. This massive collection of genes is called a biome.

We know that babies contain more active genes for digesting breast milk than adults do. The guts of obese people are often found to contain more bacterial genes involved in breaking down carbohydrates. Older people have fewer bacterial genes for dealing with stress. In Tokyo, gut bacteria can help digest seaweed, and in Toronto, they probably can’t. Our gut bacteria paint a rough portrait of who we are: young, chubby, or Asian, perhaps.

Researchers identified these enterotypes distributed among Asians, Americans, and Europeans, irrespective of age or gender.

No Africans?

Skin, hair, and nail problems are not the only effects of biotin deficiency. It can also cause depression, lethargy, susceptibility to infections, neurological disorders, and increased cholesterol levels in the blood.

While not resulting in serious neurological or memory disorders, a less severe vitamin B1 deficiency can cause irritability, frequent headaches, and lack of concentration.

The gut flora might include too many bacteria that program a person for chubbiness. These chubbiness-inducing bacteria are efficient at breaking down carbohydrates. If the number of chubby bacteria gets out of hand, we have a problem. Skinny mice excrete a certain quantity of indigestible calories while their overweight peers excrete significantly fewer.

The chubby bacteria in the latter group extract every last smidgen of energy from the same amount of food and cheerfully feed it to Mr. or Ms. Mouse. For humans, that can mean some people pile on the pounds even though they eat no more than others. It could be that their gut flora is extracting more energy from the food they eat.

Bacterial-signaling substances can also latch onto other organs and affect metabolism from there. In rodents and humans, they dock onto the liver or the fatty tissue itself and encourage the deposition of more fat. They also have an interesting effect on the thyroid gland. Bacterial infections hinder its function, causing it to produce fewer thyroid hormones, slowing the rate at which the body burns fat.

Unlike acute infections, which cause weight loss or even emaciation, subclinical infection causes weight gain.

Bacteria are not the only possible cause of subclinical infections—hormone imbalances, too much estrogen, lack of vitamin D, or too much gluten-rich food have...

Not our brains but our guts are the home of gangs of bacteria that crave hamburgers after three days on a diet.

So the theory is this: our bacteria reward us when we send them a decent delivery of food. It feels pleasant and whets our appetite for the next meal.

Several studies have shown our satiety signal transmitters increase considerably when we eat the food our bacteria prefer.

In obese people, the gene that codes for satiety can be defective, and such people simply do not get that full feeling after eating.

According to the selfish brain theory, the brain does not receive enough of the energy eaten as food and so decides that it is still hungry. But it is not only our body tissue and our gray matter that depend on the food we eat—our microbes also need to be fed.

we can more easily tinker with bacteria than with our brain or our genes—and that is what makes microbes so fascinating.

The nutrition we receive from our bacteria is not only important for fighting the flab, it also affects the levels of fats such as cholesterol in our blood. This realization could be quite highly charged as obesity and high cholesterol levels are closely connected with the greatest health issues of our time: hypertension, arteriosclerosis, and diabetes.

The connection between bacteria and cholesterol was first discovered in the 1970s.

American scientists studying Maasai warriors in Africa had been surprised to find the levels of cholesterol in their blood were low, despite a diet consisting almost entirely of meat and milk.

The scientists suspected a mysterious substance in the milk they drank might be causing their cholesterol levels to remain low.

This didn’t prove this.

but no one considered the fact that certain bacteria are required to curdle milk. That would have explained the results of their Coffee-mate® experiment. Bacteria that have already settled in the gut can continue to live there even when milk is replaced with plant-based creamer enriched with cholesterol.

These studies of the Maasai would not live up to modern scientific expectations. The test groups were too small, and the Maasai spend about thirteen hours a day walking and one month a year fasting.

The masaai fast for one month out of the year!

The most likely candidates we know of today are BSH genes. BSH stands for bile salt hydrolase. Bacteria with these genes can convert bile salts. But what do bile salts have to do with cholesterol? The answer is in the name. Cholesterol comes from the Greek words chole (bile) and stereos (solid). Cholesterol was first discovered in gall stones. Bile, which is stored in the gall bladder, is the body’s transport medium for fats and cholesterol.

BSH allows bacteria to alter bile to make it work less efficiently. The cholesterol and fat dissolved in bile can then no longer be absorbed by the body and they end up, to put it bluntly, down the toilet. This mechanism is useful for bacteria because it weakens the effect of bile, which can attack their cell membranes.

Bacteria also have a few other mechanisms for dealing with cholesterol: they can absorb it directly and incorporate it in their cell walls, they can convert it into a new substance, or they can manipulate organs that produce cholesterol.

Most cholesterol is produced in the liver and the gut, where tiny messenger substances manufactured by the bacteria can partly control those processes.

Too much cholesterol is not such a good thing, but neither is too little. If it weren’t for cholesterol, we would have unstable cells and no sex hormones or vitamin D.

Studies have shown a connection between too little cholesterol and memory problems, depression, and aggressive behavior.

To summarize: Bacteria help to feed us, make some foods more digestible, and produce their own substances. Some scientists now support the theory that our gut microbiota can be considered an organ.

Salmonella bacteria are part of the normal gut flora of reptiles.

Barry Marshall

In some parts of the island, there was an astonishingly high incidence of Parkinson’s-like symptoms among the population. Those affected suffered from trembling hands, facial paralysis, and motor problems. Researchers realized that the symptoms were most common in areas where people’s diets included cycad seeds. These seeds contain neurotoxins—substances that damage the nerves.

H. pylori can manipulate our protective barriers, irritate and destroy our cells, and manufacture toxins and damage our entire body by doing so.

Pro bios means “for life.” Probiotics are edible living bacteria that can make us healthier.