Three Days of Water Fasting: Science, Metabolism, and Practical Guidance

Emily Clarke

Welcome back to the Hormone Restoration and Thyroid Management Podcast Series—I'm Emily Clarke, joined by Dr. Richard Nkwenti. Today, we’re really diving into the nitty gritty: what actually happens to your body during a 72-hour, water-only fast. So Richard, I know the phrase “switching from burning sugar to burning fat” gets tossed around a lot, but the actual timeline of these metabolic stages is pretty fascinating.

RICHARD NKWENTI

Absolutely, Emily. What people underestimate is that there’s this metabolic cascade, if you will. In the very first 24 hours, your body is mainly using stored glucose. After that—let’s say around the 14-hour mark—digestion wraps up, and your insulin drops. This drop is actually your metabolism’s green light: “Hey, time to prep for burning stored fat.” Then you hit around 18 hours—give or take—and that’s when your body flicks on autophagy, this, uh, elegant cellular cleanup process. Picture Pac-Man, but for damaged cell parts. This process keeps ramping up for the rest of the fast.

Emily Clarke

Yeah, and I—oh, this brings me back actually to my first water fast in grad school. I wasn’t doing it for weight loss, just pure scientific curiosity about autophagy. I remember the first day was all about hunger and, honestly, feeling a bit moody! But then by the second day, things shifted, and suddenly, I could almost feel that deep inner energy switch. My brain got this sharpness, almost like... I don’t know, like someone had cleaned the windows inside my skull. Was that me actually entering ketosis, do you think?

RICHARD NKWENTI

Pretty much. Day two, after maybe 24 to 36 hours, your body exhausts its glycogen and begins breaking down stored fat for fuel. Those fatty acids get sent to the liver, which transforms them into ketones—a clean, anti-inflammatory energy source for the brain and muscles. That brings a lot of people mental clarity, especially as BDNF, this “miracle-grow” for your brain, starts rising around the 24-hour mark too. It’s nature’s way of sharpening your senses when food is scarce.

Emily Clarke

So then—we’re up to day three. This is billed as the magical day, right? Not just fat burning, but specifically visceral fat. The so-called dangerous stuff that builds up around our organs.

RICHARD NKWENTI

Exactly. With extended fasting—by day three—the body mobilizes energy not just from subcutaneous, but from visceral depots. The liver actively breaks down visceral fat, creating more ketones, and inflammation tends to drop even further. And, studies suggest your insulin is now at its lowest, which is crucial for targeting that metabolically active, “inflammatory” visceral fat. That’s when you’re truly in the thick of healing and deep metabolic change.

Emily Clarke

And what about autophagy at this stage? I remember reading these nuclear stem cell studies—by the 72-hour point, the body starts ramping up immune regeneration, clearing out these “senescent” immune cells and giving you a reboot, almost?

RICHARD NKWENTI

Yes, that’s right, Emily. Data shows that by the 3-day mark, you’re not only in full autophagic mode but your body initiates stem cell regeneration, especially in the guts and immune system. To use a nature analogy, it’s like pruning a tree—the deadwood is cleared, allowing new growth. But again, it’s the cumulative effect of all these overlapping metabolic shifts—insulin drops, inflammation drops, stem cells are produced, and fat is being burnt, particularly from places you want it gone most.

Emily Clarke

So, let’s move to the science—what does the clinical data actually show us about these three-day fasts? There’s a lot of hype, but can we really expect to burn pure visceral fat and get other major health boosts in just a few days?

RICHARD NKWENTI

It’s a good question. The evidence from both intermittent and, more dramatically, from prolonged fasting trials does show consistent, moderate weight loss—sometimes from 2% to even 10% over several days to weeks. But here’s the part people often miss: a significant portion of that is lean mass, not just fat. In some studies, two thirds of weight lost was lean tissue, one third was fat. That’s not trivial, especially over repeated fasts. You’ve got to be careful.

Emily Clarke

That’s such an important caveat. There’s this narrative online that fasting only burns fat, but losing muscle mass is absolutely a risk, isn’t it? Although, you do often hear that growth hormone protects muscle during fasting—so, I’m a little fuzzy there, Richard.

RICHARD NKWENTI

It’s true—fasting does trigger a surge in human growth hormone, especially after the 24-hour point. That hormone does help protect lean muscle, but it doesn’t completely negate the risk. In clinical trials, we’ve seen decreases in blood pressure, improvements in insulin sensitivity—especially in people without diabetes. Effects on cholesterol and lipids, though, are mixed. Some trials show reduced LDL and triglycerides; some, no major effect. And those benefits? They often fade a few months after refeeding, even if weight loss is kept off.

Emily Clarke

So, it’s not a miracle cure, and sustainability is a real issue. I’m guessing—like most “biohacks”—there’s also a high dropout rate and potentially a lot of adverse events?

RICHARD NKWENTI

Absolutely, Emily. On the risk side, reports include metabolic acidosis, headaches, insomnia—sometimes real hunger and even rebound eating. I’ll mention a patient of mine, actually. He started a structured fasting protocol hoping to improve blood pressure and insulin sensitivity. And it worked, for a time. But because he was on antihypertensive medication, we had to monitor him so closely—adjusting dosages, watching for hypotension. It’s a bit like walking a tightrope: powerful results, but only safe if you have the right support.

Emily Clarke

Which makes sense, right? Because in those reviews, the major take-home seems to be that fasting can offer clinically significant weight loss and improvements in risk markers, but you can also run into problems, especially when you have underlying health issues or you’re unsupervised.

RICHARD NKWENTI

Yeah, and building from what we discussed on previous episodes—like how thyroid or estrogen dominance can complicate metabolism—fasting itself interacts with all those systems too. Plus, the psychological side; in some studies, depression and binge eating symptoms drop slightly, but there’s a risk for restrictive or disordered eating. It’s not one-size-fits-all. Tailoring the protocol, and knowing when to stop, is essential.

Emily Clarke

Right, let’s wrap up with the “how-to” and “should you?” Because, look—I think you made a perfect analogy earlier, comparing fasting to using a chainsaw: powerful, transformative, but risky if you haven’t read the user manual. So what’s in the actual manual for a three-day water fast, Richard?

RICHARD NKWENTI

First: electrolytes. If I could shout that from the rooftops, I would. Hydration with water is vital, but you want unflavoured electrolytes—sodium, potassium, magnesium—otherwise, dizziness or heart palpitations are common. Second: keep your body moving, but gently. Walking and sunlight are good, vigorous exercise is not. Third: track your progress. Apps can help, or just keep a journal and tune into how you feel hour by hour.

Emily Clarke

And when it’s time to break the fast, “low and slow" is really the rule, right? I remember after my first attempt, I practically inhaled a huge meal and immediately regretted it, so bone broth or a soft food like scrambled eggs, maybe a quarter of an avocado—ease things in! Big boluses of carbs or heavy animal protein are a recipe for disaster after three days.

RICHARD NKWENTI

Exactly, and here’s where caution becomes critical. Who should not attempt this kind of fast? Pregnant or breastfeeding women, the underweight, anyone with a history of eating disorders, and folks on certain medications—especially for diabetes and blood pressure. If you’re chronically ill, please only try this with medical guidance. The risks just outweigh the rewards without proper support.

Emily Clarke

And—quick gut check before people get swept up in the excitement: why do you actually want to do a fast? Are you hoping to solve a chronic issue, or just chasing a quick fix? Do you have support, or are you going it alone? I feel like self-reflection is as important as the protocol itself.

RICHARD NKWENTI

Beautifully put, Emily. Fasting is just one tool. Like we’ve stressed all series, real results come from a holistic approach: hormones, sleep, stress, testing, and personalization. No shortcuts, no magic. The power is in knowing your body and asking the right questions—about readiness, safety, and sustainability.

Emily Clarke

I think that’s a lovely place to leave it for today. Thanks as always, Richard. And of course, thanks to all of you for tuning in—don’t forget to send us your questions for future episodes. We’re only scratching the surface of what’s possible with targeted, safe hormone and metabolic management.

RICHARD NKWENTI

Thank you, Emily. And thank you to all our listeners—keep learning, keep questioning, and remember, true health comes from understanding, not shortcuts. Until next episode, goodbye!

Emily Clarke

Cheers, Richard! Take care everyone. See you next time.