You’ve probably heard about your microbiome—the trillions of bacteria living in your gut. If you’re like most people, you’ve seen ads for probiotic supplements promising to “support gut health.”

Here’s what those ads don’t tell you: your gut bacteria don’t need more bacteria. They need food. Specifically, they need fiber.

Meet Your Microbiome

Your gut contains approximately 100 trillion microorganisms. That’s more bacterial cells than you have human cells in your entire body. You are, quite literally, more bacteria than human.

These bacteria aren’t passengers. They’re active participants in your health:

• They produce vitamins (including vitamin K and some B vitamins)
• They regulate your immune system
• They influence your mood and mental health through the gut-brain axis
• They affect inflammation throughout your body
• They even influence your weight and metabolism

But here’s the crucial part: these bacteria need to eat. And they don’t eat protein or fat. They eat fiber.

Prebiotics vs Probiotics

The supplement industry loves selling probiotics—bacteria you can consume to “replenish” your gut.

But here’s the thing: if you’re not feeding the bacteria you already have, adding more won’t help. It’s like restocking an aquarium with fish while forgetting to feed them.

What your gut bacteria actually need are prebiotics—the food that feeds beneficial bacteria. And prebiotics are, you guessed it, fiber.

Specifically, they need:

• Inulin (found in chicory root, Jerusalem artichokes, garlic, onions)
• Resistant starch (found in cooked and cooled potatoes, rice, and beans)
• Pectin (found in apples, berries, citrus fruits)
• Beta-glucan (found in oats and barley)

Notice what all these have in common? They’re all forms of fiber, and they’re all found in whole plant foods.

The Fiber Void

The average American eats about 15g of fiber per day. The recommendation is 25-35g. But our ancestors ate 50-100g.

That’s a huge gap. And your microbiome feels it.

When bacteria don’t get fiber, several things happen:

1. Beneficial bacteria die off: The good guys literally starve
2. Harmful bacteria proliferate: Bad bacteria that feed on other sources (like the mucus lining of your gut) take over
3. Diversity decreases: A diverse microbiome is a healthy microbiome. Low fiber decimates that diversity
4. The gut barrier weakens: This can lead to “leaky gut” and systemic inflammation

The Meat and Dairy Problem

Animal products contain zero fiber. They don’t feed your beneficial gut bacteria at all.

But it gets worse. Studies show that diets high in animal products actually promote bacteria that contribute to inflammation and disease. These bacteria produce metabolites like TMAO (trimethylamine N-oxide), which has been linked to cardiovascular disease.

Meanwhile, plant-based diets promote entirely different bacterial populations—ones associated with reduced inflammation and better health outcomes.

It’s Not Just About Digestion

When we talk about fiber and gut health, people often think we’re just talking about “regularity.” That’s selling it way short.

Your gut bacteria produce short-chain fatty acids (SCFAs) when they ferment fiber. These SCFAs:

• Reduce inflammation throughout your body
• Regulate your immune system
• Influence insulin sensitivity
• Affect your brain function and mood
• May reduce cancer risk

SCFAs are so important that researchers are studying them as potential treatments for autoimmune diseases, metabolic syndrome, and even mental health disorders.

And you can’t get them from a supplement or a probiotic. You get them by eating fiber and letting your gut bacteria do their job.

The Fibertarian Advantage

When you follow the fibertarian principle—only eating foods with natural fiber—you automatically:

• Feed your beneficial gut bacteria every meal
• Promote bacterial diversity
• Support SCFA production
• Maintain a healthy gut barrier
• Reduce systemic inflammation

You’re not just eating for yourself. You’re eating for the trillions of organisms that keep you healthy.

Practical Application

Want to optimize your microbiome? Here’s what works:

• Eat diverse plant foods: Different fibers feed different bacteria. Variety is key
• Include fermented foods: Sauerkraut, kimchi, and fermented vegetables provide both fiber and beneficial bacteria
• Eat resistant starch: Cook potatoes, rice, or pasta, then cool them. The starch becomes resistant (i.e., fiber-like)
• Don’t skimp on legumes: Beans and lentils are microbiome superstars
• Eat the whole fruit: Don’t juice it. The fiber is the point

The Bottom Line

Probiotic supplements might help in specific clinical situations. But for most people, they’re an expensive Band-Aid that doesn’t address the root problem.

Your microbiome doesn’t need more bacteria. It needs food. And that food is fiber.

Every time you choose a fiber-rich whole plant food over a fiber-free processed or animal product, you’re voting for a healthier, more diverse, more resilient microbiome.

Your gut bacteria are literally waiting for you to feed them.

What are you waiting for?