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The Microbes That Pay Your Energy Bill

• Dr. William Wallace • Episode 59

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Your gut microbes don’t just digest food, they can power you. In this episode, we uncover a hidden energy stream: short-chain fatty acids produced when microbes ferment plant fibers, potentially supplying anywhere from 2% to 10% of your daily calories. A new Cell study quantifies this microbial contribution with a unique level of precision, revealing how dietary choices drives the yield. We look at the mechanisms behind this energy exchange, , and show why increasing fiber intake is one of the most potent, underappreciated tools for improving metabolic health immune function, disease resistance, etc. We can now say it also contributes to energy flux.

00:00 Introduction: The Hidden Fuel Source

00:13 The Role of Gut Microbes in Energy Production

01:16 How Gut Microbes Transform Fiber into Energy

02:49 Measuring Microbial Energy Contribution

04:51 Impact of Diet on Microbial Energy Harvest

06:32 Significance of Microbial Fermentation

07:37 Implications for Human Health and Diet

09:19 Conclusion: Feeding Your Microbial Partners

PMID: 40744013

Speaker 1:

Some people can eat less and still seem to have more energy. They aren't secretly snacking or running on caffeine. Their bodies are tapping into a hidden fuel source, one that most of us overlook entirely. This energy doesn't come from their food so much as from trillions of microscopic tenants in their gut. These microbes transform the toughest parts of plants fibers that you can't digest into compounds your body can absorb and burn, and a new study in Cell has finally measured with unprecedented precision just how much energy they're giving you. The results might make you rethink what's really powering your day. For the sake of setting the scene, imagine if part of your grocery bill was quietly being paid by bacteria not in a science fiction sense, but right now inside you. In people eating fiber-rich diets, gut microbes can cover up to one-tenth of daily calorie needs. But how Surely, you're saying to yourself right now. How can this be if fiber is not digested and absorbed? Well, the answer is that our microbiota can supply us energy by turning what you can't digest fibers, resistant starches and other complex carbohydrates into compounds that you can. This is energy harvested in the shadows of your metabolism. It happens in the large intestine and anoxic meaning low oxygen environment, where trillions of microbes ferment plant fibers, releasing short chain fatty acids acetate, propionate, butyrate, along with smaller amounts of lactate, formate and sucinate From the nutrients that fuel microbial growth. That being fibers, most of the carbon from that food doesn't vanish. It's reborn as these acids and it's now thought that almost all are absorbed back into the body. Even dietary protein and the host's own mucin can be fermented, though there are side players here, accounting for only about one-fifth or 20% of your gut bacteria's energy needs. The main act and primary source of energy for your gut microbiota is carbohydrate fermentation, and while the total amount of this microbial energy supply in Western diets is modest about 2-5% of daily expenditure it's possible that it can triple with fiber-rich eating. Change the diet and you change the yield. Change the microbes and you alter the mix of acids. But how do you measure something this invisible? You can't just put a calorie counter in the colon.

Speaker 1:

The researchers behind a new cell study built a systems-level framework part lab experiment, part metabolic accounting to do exactly that. First, they recreated gut fermentation outside the body. They took 22 of the most common gut bacterial species and grew them under controlled, oxygen-free conditions, no matter the growth medium or pH. These microbes showed remarkable consistency. Over 90% of the carbon from carbohydrates ended up as fermentation acids. They also found that this efficiency barely budged under different conditions. Growth rates could vary, but the per-cell rate of turning carbs into acids stayed the same. That's because of the main energy cost for these bacteria. Making new biomass, in other words cell replication and growth, is the primary energy cost for our gut bacteria, and that doesn't change much. Next came the integration step. They took these per-cell fermentation rates and merged them with human data on microbiome composition, digestion and diet. They even ran two completely different calculations, one starting from the amount of bacterial biomass lost in feces and another starting from the amount of fiber and resistant starch that escapes digestion in the small intestine. Both gave the same answer.

Speaker 1:

In typical Western diet, gut microbes produce about 450 millimoles of fermentation acids per day. Almost all of that over 98%, is absorbed by us. The host Protein and mucin can be fermented too, but again, at most they contribute one-fifth of the total. Carbohydrates are the clear fuel source, and when they scaled the model across diets, the pattern was clear. What you eat, not which microbes you have, sets the total energy capture. For Western diets this works out to just 2-5% of your daily calories, but under fiber-rich diets that fraction can triple. In the Hadza of Tanzania, who eat seasonal tuber-heavy diets, it can hit 10% or more. And in lab mice, where resistant carbohydrate intake is high, microbial fermentation can supply over 21% of total energy needs. That number is important, not because we have to care so much about how much energy a rodent's microbiota supplies, but because it tells us that experimental models using rodents may exaggerate some systemic effects compared to humans. So in that way it's important information. To make sure their calculations weren't an artifact of one method, the researchers cross-checked everything. Estimating microbial energy harvest from the amount of fiber that reached the colon gave the same answer as estimating it from bacterial biomass that's lost in feces. Even a theoretical calculation based purely on the ATP required to grow new bacterial cells came out nearly identical. This consistency matters because it means we can trust the number in the case of this particular study, and the number tells us that more than 90% of the carbon in microbiota-accessible carbohydrates ends up in fermentation products and that most of that is absorbed.

Speaker 1:

These compounds, short-chain fatty acids, aren't just calories. They're chemical signalers. Butyrate powers colon cells, fueling ATP production to maintain the gut lining. Acetate can travel to the liver, where it influences glucose production. Together, these acids shape immune signaling, gut pH and even the brain-gut communication. When diets are stripped of fiber, microbes don't become less efficient, but they have less fuel. The proportion of carbohydrate carbon converted into acids stays high, yet the total amount of these acids produced falls. That means less microbial energy returned to the host and fewer of the signaling molecules that power colon cells, influence liver glucose production and help regulate inflammation. Over time, this reduction in total output could shift metabolic balance and raise risks tied to chronic inflammation and impaired glucose control, and that's why the study's findings do more than quantify a hidden calorie source. They make dietary fiber an even more urgent public health priority.

Speaker 1:

In humans, microbial fermentation can cover anywhere from 1.7 to 12% of daily energy needs, depending on the diet. In laboratory mice it's over 21%, why the gap Mice eat far more resistant carbohydrates and their chow is often autoclaved, making it even less digestible to the host and more available to microbes. For them, this microbial partnership is a cornerstone of energy balance, but for us it's more of a supplementary income system. Again, this difference matters. It means we can't assume every effect seen in a mouse will be as strong as in a human and, like any good investigation, this one has blind spots.

Speaker 1:

The analysis froze microbiome composition in place. It didn't model how communities change over time. Cross-feeding between microbes where one species waste becomes another's fuel wasn't fully mapped, and the focus was on the big-ticket metabolites like acetate, propionate and butyrate. Smaller, less abundant compounds like hydrogen, sulfide and trimethylamine were left out, even though they can impact health. Even so, this is the most precise accounting to date of the biggest single metabolic exchange between humans and their microbiota. It gives us hard numbers for something we've long suspected.

Speaker 1:

When you feed your microbes, they feed you back.

Speaker 1:

Remember that this energy partnership runs on supply. The more microbial accessible carbohydrates you send downstream from legumes, whole grains, fibers, vegetables and tubers, the more short-chain fatty acids your microbes can return to you. Even modest increases in fiber can move you from the lower end of the range 2% of daily energy toward the higher fiber benchmarks of 5-10%. Second, it's not just about calories. These acids are signaling molecules that influence metabolism, immunity and gut health in ways we're only beginning to quantify. They help maintain the gut barrier, modulate inflammation and fine-tune energy homeostasis. And finally, diet is the lever that matters most. You can't buy a different microbiome at the store, but you can feed the one you have in ways that increase the benefits it provides. When we think about diet, we tend to count what goes into our mouths and what gets burned by our muscles. But there's a third player in that equation, one that's been with us for millions of years, quietly harvesting energy from the scraps we can't digest. Feed them well, and they'll keep paying part of your energy bill Until next time. Stay healthy.

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Dr. William Wallace

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