“The single greatest predictor of a healthy gut is the diversity of plants in your diet.” — Dr. Will Bulsiewicz

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Who’s the Author

Dr. Will Bulsiewicz is a gastroenterologist and gut health expert.

In his book Fiber Fueled , Dr. Will Bulsiewicz shows us science-backed ways to improving health through gut microbiome optimization.

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Core Idea

Our gut microbiome – the trillions of bacteria living in our intestines – plays a crucial role in nearly every aspect of health.

The best way to nourish them?

Eating a wide variety of plant-based, fiber-rich foods.

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Happy Gut Microbiome = Healthy You

Your gut is home to 39 trillion microbes — mostly bacteria — that act like a control center for your entire body.

What do these bacteria do?

They affect our digestion, metabolism, immune system, hormones, and even mood and mental health.

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Key Element 1: Short-Chain Fatty Acids (SCFAs)

When gut bacteria ferment fiber, they produce SCFAs like butyrate.

SCFAs are crucial for our overall health:

• Gut health: Protects gut lining, prevents leaky gut
• Brain: Boots mood, reduces anxiety
• Weight: Lowers appetite, increases fat burning
• Immunity: Lowers inflammation, autoimmune risk
• Hormones: Improves insulin sensitivity, blood sugar control

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Key Element 2: Serotonin

90% of serotonin (the happy hormone) is made in your gut — not your brain.

Gut bacteria like Bifidobacteria and Lactobacillus help produce serotonin.

They ferment fiber into tryptophan — the building block of serotonin.

When your serotonin level is low, it leads to

• Depression
• Anxiety
• Sugar cravings
• Lower sleep quality

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“Your gut microbiome is the control center for your entire body. Feed it well, and it will return the favor for life.”

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Fiber is the #1 Superfood

Most health problems stem from lack of fiber.

For example, bloating, constipation, fatigue, skin issues, and even anxiety.

Why?

Fiber feeds your gut bacteria — especially prebiotic fiber found in plants.

The more types of fiber you eat, the more diverse your gut microbiome becomes.

The more diverse your gut bacteria, the healthier you are.

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The Plant Diversity Rule 🌈

More plant diversity = healthier gut.

Dr. Will Bulsiewicz suggests eating 30+ different plants per week.

This includes fruits, veggies, whole grains, beans, nuts, seeds, herbs, and spices.

Why?

• Each plant contains unique types of fiber and polyphenols that feed different strains of gut bacteria.
• Diversity > Quantity.

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A 2018 American Gut Project study found that:

• People who ate 30+ plants per week had more diverse gut bacteria.
• Those who ate 10 or fewer plants had less diversity — even if they ate mostly organic or avoided junk food.

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Why Animal Products Hurt Your Gut

High-fat, low-fiber diets (like keto or carnivore) harm your gut bacteria by starving the good bacteria and feeding harmful bacteria.

Animal products contain zero fiber and can increase inflammation when eaten in excess.

Low-carb diets starve your gut microbes. Fiber is not optional — it’s essential.

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What to Eat (The Fiber Fueled Diet)

• ✅ Whole Grains (quinoa, oats, brown rice)
• ✅ Legumes (beans, lentils, chickpeas)
• ✅ Fruits & Vegetables (berries, broccoli, leafy greens)
• ✅ Nuts & Seeds (flaxseeds, chia seeds)
• ✅ Fermented Foods (sauerkraut, miso, kimchi)
• ✅ Sprouted Beans 🌱 (he loves them!)

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Big Takeaway

👉 The single best thing you can do for your health:

Eat more plants and more fiber — with as much variety as possible.

We’ve made health too complicated with our extensive lists of foods to avoid, complex percentages of fats-to-protein-to-carb ratios, elimination diets, calorie counting, even weighing our food…

It just doesn’t need to be this complicated. Diversity of plants. That’s it. That’s all you have to remember.