Your gut produces 90% of your serotonin: what probiotics actually do

Your gut hosts more than 100 trillion microorganisms, produces approximately 90% of your body's serotonin and houses between 70% and 80% of your immune system. It is not a "second brain" as a metaphor — it is an organ with its own nervous system (the enteric) that communicates with your brain in a constant, two-way conversation.

You have probably heard of probiotics. They are in yogurt, in kefir, in supplements with bottles that promise "50 billion of something". But between marketing and confusion, few people know what a probiotic actually does, which strains matter, what the digestive enzymes that sometimes accompany them are for, and when it makes sense to take them.

This guide is going to be straightforward. No "probiotics are good for you". We are going to explain what is happening inside your gut, why microbiota affect your mood and immunity, what the science says about specific strains and what to look for when choosing a supplement.

What gut microbiota are and why it is not just "digestion"

Gut microbiota are the ecosystem of bacteria, viruses, fungi and other microorganisms that live in your digestive tract. It is not a passive guest. It is a functional organ weighing between 1 and 2 kilos that performs functions far beyond digesting food.

When microbiota are balanced (called eubiosis), beneficial bacteria dominate over potentially pathogenic ones and the system works. They help digest certain nutrients your enzymes cannot process alone, produce vitamins (K and several B-group), train and regulate the immune system, and produce metabolites that influence organs as distant as the brain.

When that balance breaks (dysbiosis), the consequences do not stay in the gut. Gut dysbiosis has been documented to be involved in irritable bowel syndrome, inflammatory bowel disease, allergies, obesity, type 2 diabetes and — most surprising for many people — mood disorders such as anxiety and depression.

The gut-brain axis: the connection that changes everything

If there is one concept that has revolutionised microbiota research in the last ten years, it is this: the gut and the brain communicate bidirectionally and constantly. It is not a metaphor. It is anatomy. The vagus nerve (the longest in the autonomic nervous system) physically connects the gut with the brainstem and transmits signals in both directions.

But the vagus nerve is not the only route. Gut-brain communication operates through three simultaneous channels:

Neural pathway (vagus nerve). Neurons of the enteric nervous system (the gut has more than 500 million neurons) detect chemical and mechanical signals and transmit them to the brain. The brain in turn sends instructions back that modulate intestinal motility, secretion and permeability.

Systemic pathway (hormonal and metabolic). Microbiota produce neurotransmitters and their precursors: serotonin, GABA, dopamine, tryptophan. They also generate short-chain fatty acids (acetate, propionate, butyrate) that cross the gut and blood-brain barriers, directly influencing brain function.

Immune pathway. 70-80% of the body's immune tissue is in the gut (gut-associated lymphoid tissue, or GALT). Microbiota modulate the production of pro-inflammatory and anti-inflammatory cytokines. If there is dysbiosis, inflammatory cytokines can be released that travel to the brain and alter cognitive function and mood.

Spain's CSIC (National Research Council) runs an active research programme on this axis, led by Yolanda Sanz at IATA-CSIC. Her team has identified that certain bacterial species such as Christensenella minuta are good serotonin producers and reduce vulnerability to stress and depressive behaviour in animal models. Human research is advancing, but the direction is clear.

8 signs your microbiota may be unbalanced

Dysbiosis does not always present with obvious digestive symptoms. Sometimes the first indicators are extra-intestinal. These are the most common signs:

If you recognise three or more signs, the first step is to review your diet (fibre, variety, ultra-processed foods) and consult your doctor if symptoms persist. A probiotic can help restore balance, but it does not replace diagnosis when there is underlying pathology.

Probiotic strains: which have evidence and what for

Not all probiotics are equal. The difference between an effective supplement and a useless one is in the strains, not in the billions of CFU on the label. Each strain has specific functions. Let's look at the most studied:

Lactobacillus acidophilus: the baseline strain

One of the most studied and used. It produces lactic acid that lowers gut pH, hindering pathogen colonisation. There is evidence for reducing symptoms of lactose intolerance, vaginal Candida infections and antibiotic-associated diarrhoea.

Bifidobacterium lactis: immunity and regularity

Reinforces the intestinal barrier and modulates the immune response. Several trials have shown improvement in bowel regularity (especially in functional constipation) and reduced incidence of respiratory infections in older adults and children.

Lactobacillus rhamnosus: the most versatile

Probably the strain with the most scientific literature. Effective for preventing and treating traveller's diarrhoea, antibiotic-associated diarrhoea and paediatric gastroenteritis. There are also preliminary data on its effect on anxiety (the famous "stress-free mouse" study was done with L. rhamnosus).

Lactobacillus plantarum: barrier and gas

Stands out for its ability to strengthen gut barrier integrity (reduces intestinal permeability or "leaky gut") and for its documented effect in reducing bloating and flatulence in patients with irritable bowel syndrome.

Lactobacillus reuteri: the anti-inflammatory

Produces reuterin, a natural antimicrobial substance. It has been studied in infant colic (with positive results), oral health and modulation of systemic inflammation.

Strain table: main function and evidence

A multi-strain probiotic, with diversity across genera (Lactobacillus + Bifidobacterium + Enterococcus + Lactococcus), offers broader functional coverage than a single-strain product at high dose. Diversity is the key, just as in the microbiota itself.

Digestive enzymes: the complement almost no one knows about

Many bloating and gas issues are not due to microbiota themselves, but to the fact that food reaches the colon half-digested. The body's digestive enzymes sometimes cannot keep up — especially with age, with diets rich in processed foods or under stress (which reduces enzymatic secretion).

Digestive enzymes in a supplement attack this problem at the root:

Amylase (1,200 u): breaks down starches and complex carbohydrates. Less undigested starch = less fermentation in the colon = less gas.

Protease (300 u): breaks down proteins. Poorly digested proteins in the colon produce putrefactive compounds (gas with a particularly unpleasant odour).

Lactase (200 u): breaks down lactose. Especially relevant for people with intolerance — estimated to affect a significant share of European adults, with notable variation by region.

Lipase (10 u): breaks down fats. Improves digestion of fatty meals and reduces post-meal heaviness.

Cellulase (50 u): breaks down plant cellulose. Useful for improving digestion of legumes, cruciferous vegetables and other fibre-rich foods.

The combination of probiotics + digestive enzymes is not coincidental. Enzymes improve mechanical digestion, reducing the fermentation load. Probiotics improve the microbial ecosystem. And prebiotics (FOS) feed the good bacteria. It is a three-pronged approach.

Vittalogy

Probiotics + Enzymes

EUR 19.90

40 billion CFU, 14 probiotic strains, 5 digestive enzymes, FOS (prebiotic), ginger, aloe vera and zinc. Triple formula: probiotic + prebiotic + enzymatic.

60 capsules 1 month 40 billion CFU ISO & GMP

View Probiotics + Enzymes

How to choose a probiotic with criteria

The probiotic market is huge and quite confusing. These are the criteria that really matter:

Strain diversity. Look for formulas with at least 8-10 different strains across multiple genera (Lactobacillus, Bifidobacterium, Enterococcus). Diversity is more important than the raw CFU count.

Sufficient CFU. For general maintenance, 10 to 40 billion CFU per day. Less than 1 billion is a token dose. More than 100 billion is rarely justified outside specific medical protocols.

Prebiotic included. Fructooligosaccharides (FOS) or other prebiotics feed the beneficial bacteria. A probiotic with prebiotic (synbiotic) usually delivers better results than the probiotic alone.

Digestive enzymes. If your main symptoms are bloating, gas and heaviness, a formula that includes enzymes attacks the problem from two angles: better mechanical digestion and a healthier microbial ecosystem.

Support ingredients. Zinc contributes to normal macronutrient metabolism and acid-base balance (EFSA claim). Ginger has prokinetic properties (improves gastric motility). Aloe vera has a soothing effect on the intestinal mucosa.

The Vittalogy formula combines the three pillars (14 probiotic strains, FOS prebiotic and 5 digestive enzymes) with ginger, aloe vera and zinc. 2 capsules per day on an empty stomach.

And if you are looking for a holistic approach, combining probiotics with magnesium (which favours intestinal muscle relaxation) and good rest with Sleep Complex (sleep regulates the HPA axis, which in turn modulates microbiota) creates a synergistic effect. The gut-brain axis works better when you sleep well.

Frequently asked questions about probiotics

Sources and scientific references