Gone are the days when skipping meals was nothing but a metabolism-wrecking fad. In a recent article, I talk about the hands of time and how simple lifestyle strategies can reverse our biological age clock. One of these strategies is fasting: a handy cleansing and protection ritual against metabolic evils.
Obesity, type 2 diabetes, high cholesterol and high blood pressure are just a few of the conditions that can be helped by diets featuring bouts of intermittent or alternate-day fasting, according to a plethora of studies.
Fasting can not only help fight disease but also shift our perspective to meet our day-to-day nutritional needs and improve long-term survival more intentionally.
Science of Intermittent Fasting
A basic concept to understand when discussing fasting or fasting-mimicking diets is the “low-energy state,” or energy deficit, which constitutes a mild stressor that ignites a host of pro-longevity effects. Suddenly, the body thinks that food is scarce and turns on the survival instinct software associated with a lean, nimble and enhanced body—while sharpening cognition—to stay alive.
When we fast, we first create a calorie deficit by having one or two out of the three customary daily meals, and we induce another deficit in either glucose (from limiting carbs) or amino acids (from reducing protein) when we refeed—in the context of certain fasting regimens. In time-restricted feeding (TRF) experiments, the fast is usually followed in either a protein-sparing fashion or a low-carb setting, depending on the study’s aims.
On a molecular level, the low-energy fasting state activates a suite of epigenetic regulators known as sirtuins that act as the body’s survival software plugins.
These enzymes help with DNA repair and stabilizing gene expression to exhibit rejuvenating effects—which means that they send signals to activate some genes and inactivate others harmoniously to extend the lifespan.
What the Experts Have to Say
Before the work on intermittent fasting, caloric restriction was perhaps the most studied form of a low-energy state. Studies on caloric restriction showed that it could increase lab rat lifespans by more than 20%. Restricting caloric intake by 30% has also been shown to extend the lifespan of worms, flies, mice, dogs and monkeys.
While a lot of literature on fasting comes from research performed in model organisms, a few dozens of human studies support that low-energy states cause beneficial metabolic shifts and positive effects on biomarkers of longevity—especially the mTOR and AMPK pathways, which I discuss at length in one of my latest articles.
For example, a 2021 study reported that fasting from dawn to sunset for four weeks improved blood pressure, reduced BMI, decreased waist circumference, and boosted the activity of DNA repair proteins.
Before that, in a 2015 Newcastle University study, an ultra-low calorie, low-carb diet also helped reverse type 2 diabetes for the first time in human models. The eleven patients involved in the study consumed only 600 calories a day and significantly improved their diabetes one week after starting the protocol.
One of the reasons researchers found for the diabetes reversal was that the low-energy state, induced by a period of caloric restriction, removed fat that clogged the pancreas, allowing normal insulin secretion to be restored.
By now, periods of fasting are understood to benefit people who are overweight as it helps them shed excess weight, become more insulin sensitive and maintain normal blood sugar levels. In turn, these changes also lower cardiovascular disease risk.
Other diseases like type 1 diabetes, multiple sclerosis, and even several types of cancer seem to attenuate when therapy is combined with fasting or a fasting-mimicking diet.
Molecular Mechanisms of Fasting
In a fasted state, several healing processes begin to occur. Mechanistically, a fast is a mild stress that causes changes in the expression of longevity genes. Many genes that repair damage to DNA or mitochondria are suddenly turned on and upregulated.
In particular, a genetic “survival response circuit,” involving the sirtuin genes that regulate ageing, gets triggered by low-energy states. The ignition key, or gene switch known as pyrazinamidase/nicotinamidase 1 (PNC1), which revs up the activity of sirtuin enzymes in the cells, was identified in yeast. Humans share the same gene, only under a different name: nicotinamide phosphoribosyltransferase (NAMPT).
What links PNC1 and NAMPT to longevity control is the fuel (nicotinamide adenine dinucleotide or NAD) that these make for the sirtuins to do their job of de-ageing the body. And the more NAD is present in our system, the more protection the sirtuins provide.
The practice of fasting also leads to vast changes at a cellular level. It sets in motion a metabolic program called autophagy, which removes waste material from all cells. This process of removing damaged and dysfunctional proteins that build up inside cells over time may prevent or slow the onset of several diseases, including cancer and Alzheimer’s disease.
Autophagy also appears to play a key role in diabetes; beta-cell autophagy (BA) is important for normal beta-cell function and insulin production.
At the same time, intermittent fasting helps to turn on an important mechanism called apoptosis to rid the body of cells that have become senescent, which is a state in which cells can no longer divide. In older people, senescent cells (also called zombie cells) add excess inflammation and even promote tumour formation. Refeeding completes this regeneration process by replacing the lost cells.
Beyond cellular rejuvenation, intermittent fasting also imparts good body composition, which by extension, positively impacts health markers such as blood pressure, resting heart rate, cholesterol, triglycerides and blood sugar.
What Happens When You Fast
During a 72-hour water fast, your body reaches peak cellular cleanup by the end. After you refeed, your immune system starts rebuilding with new cells. Here’s a day-by-day breakdown of what’s happening inside your body:
0–24 hours
- You burn through stored carbs (glycogen) in your liver and muscles.
- Insulin drops. Blood sugar stabilizes.
- Body starts switching from glucose to fat for fuel.
- Early autophagy (cell cleanup) begins, but it’s light at this stage.
24–48 hours
- Glycogen is basically gone. Fat burning ramps up. Ketones start showing up in your blood.
- Autophagy (cellular “spring cleaning”) kicks in harder — damaged proteins, weak mitochondria, and old cell parts get broken down and recycled.
- Inflammation markers drop.
- Human growth hormone spikes (helps protect muscle).
- Immune cells start clearing out old/dysfunctional ones.
48–72 hours
- Ketosis is strong — fat is your main fuel now.
- Autophagy peaks. Body aggressively clears junk cells and damaged proteins.
- Immune system “resets” — old white blood cells die off, stem cells prep to make fresh ones once you eat again.
- Insulin sensitivity improves.
- Cellular repair and stress-resistance genes upregulate (longevity-linked pathways like AMPK, sirtuins).
When you stop feeding the body, something subtle happens upstairs too. The mind, usually tangled in noise, starts to unclench. The gut and brain are wired together — literally. They trade signals through the vagus nerve, through hormones, through the chemistry of hunger and calm. When the gut isn’t busy digesting, it’s not flooding the body with the usual chatter of enzymes, insulin spikes, and cortisol ripples. The whole system quiets down. You can feel it — that slow hush that settles somewhere behind your ribs.
During a fast, your energy stops bouncing between meals and digestion. It steadies. The brain shifts fuel from glucose to ketones — a cleaner, steadier burn that doesn’t come with the highs and crashes. People describe it as mental clarity, but it’s more like mental silence. Fewer tabs open. No static. The same way your stomach gets a break, your thoughts do too.
There’s also something deeper — when you’re not constantly processing food, you’re not constantly processing the world. The act of eating is stimulation. It’s data. Every taste, every bite, another signal to sort through. Without it, your system turns inward. You start metabolizing emotions instead of calories. The body repairs; the mind resets. It’s a kind of stillness that feels earned, not imposed — the reward for doing nothing but letting your biology do its own housekeeping.
How to Fast and Tips
The peace that comes from fasting isn’t handed to you — you earn it. The first stretch is rough. Your body fights back. Hunger isn’t just physical; it’s a kind of panic, an alarm built into your wiring. Your brain screams for dopamine, for comfort, for the familiar rhythm of chewing and swallowing. That’s the glass ceiling — the edge where most people stop. Push through it, though, and something flips. The noise quiets. The craving dulls. You realize the hunger won’t kill you; it’s just a signal fading in the background.
There’s a strange pleasure in that surrender. You stop chasing relief and start feeling everything sharper. The body starts running on its own clean power source, and you can feel it. It’s not the buzz of caffeine or the rush after sugar. It’s steady, primal energy, stripped of the usual spikes and crashes. It feels ancient, like your cells remember this state from another time.
Evolutionarily, we’re genetically programmed to go hungry for a long stretch of time when food is scarce. Today, with our overabundant food supply, very few people are prepared to endure an extended period of time without food. This stems from a fear of lacking sustenance and because we perceive fasting as much more draconian or socially extreme than it is.
That’s why fasting feels different from any other kind of discipline. It’s not performative. It’s private, internal. You cross through discomfort and find a quiet kind of euphoria on the other side. A natural high earned through restraint, not reward. And once you taste that clarity — that deep, effortless stillness — you start to understand why our ancestors fasted not just to survive, but to connect.
Fasting has many advantages, one of which is reclaiming time, translating into increased productivity. All this time associated with our fixation on food—preparing food, eating food, cleaning up, eliminating and shopping for food—we can save and allocate to more productive tasks or things that give us joy.
During and post-fasting, you’ll feel like the normal you; only food won’t be on your list of priorities as much. It gives people who identify with having a type A personality a more flexible schedule, and others will find that their food addictions or cravings will be largely diminished, if not completely suppressed.
Experimenting With Fasting
The many different fasting regimens under the intermittent fasting umbrella, such as 13:11 (fast for 13 hours, eat for 11), 16:8, 18:6, and 20:4, lessen the burden of a complete fast while still promoting the positive health effects of a low-energy state. All you have to do is time it to fit into your schedule.
Dipping your toes into fasting might look like doing this diet for as little as two non-consecutive days per week, and this is enough to reap at least some benefits.
Alternatively, if you can tolerate fasting well, you may also choose to do short fasts periodically throughout the year to promote powerful cellular cleansing, such as once a quarter for five to seven days. I once tried a one-off three-day fast and felt reborn.
One sign that it might be time to end a longer monk-like fast is vomiting yellow or green bile, which can indicate an electrolyte imbalance. If this happens, keep hydrated with small sips of water and re-introduce liquid foods for a couple of hours before adding solid foods back in.
People who have the knowledge and personal experience of practising some form of caloric restriction, especially in the form of a low-carb, modified Atkins, Ornish or ketogenic diet, usually adapt more quickly to a fasted state.
A good introduction to ketogenesis is the book by Jimmy Moore and Eric Westman, Keto Clarity. Two other great books to pick up are Lifespan by Dr David Sinclair and The Longevity Diet by Dr Valter Longo. I also recommend familiarizing yourself with the work of Dr D’Agostino, who is an expert in all things ketogenic and fasting.
Transition to Fasting
The best preparation is to eat smaller meals in the run-up and cut out snacking and any high-calorie drinks. You might also consider tapering off carbs a few days leading up to your fast or starting an intermittent fasting protocol.
If you’ve never been in a state of nutritional ketosis before, it is normal to experience headaches while the body learns to adapt to a low-energy state. The headaches are caused by dehydration as cutting down food also reduces water intake, so it’s important to hydrate with water and electrolytes before, during, and after a period of fasting.
During the fasting window, you might find yourself starving, and you can scratch that itch by having a handful of nuts or maintaining a high fluid intake with lots of tea. It also helps to find a distraction, such as going for a long walk a few hours before breaking the fast.
Fasting and Hunger
I wrote in a 2017 article that various hormonal inputs govern the regulation of hunger in our bodies. Fasting changes the levels of two such hormones known as ghrelin and leptin. High or normal ghrelin increases appetite, while high or normal leptin (derived from fat cells) dampens it; keeping these in balance is recommended to make fasting easier to maintain.
High-carb refeed meals often cause rebound ghrelin secretion. So avoid processed and unhealthy foods after fasting, and prepare healthy, fresh foods for your first meal once the fasting period is over. Preparing a protein-rich meal can help increase satiety when ghrelin is high. Ghrelin is also sensitive to low sodium, so taking these electrolytes could make a difference.
When it comes to leptin, hunger rises when it drops, and severely low leptin has been cited as causing weight loss plateaus. Repeat prolonged fasting will lead to a loss in body fat and, by extension, leptin production occurring in fat cells will begin to slow down—which might abnormally increase appetite and cause overeating.
This suggests that for most people, but especially leaner individuals, a moderate approach to intermittent fasting, such as 16:8, may be preferable to reap the benefits of fasting without negatively affecting hunger hormones or causing uncontrolled eating.
I also find it very helpful to distinguish between hunger from negative emotions or boredom and from our body’s cues. Over time, we have lost the connection to physical sensations indicating true hunger. So bringing awareness to what drives us to eat is key.
Intermittent Fasting Recipes
It’s sometimes nice to have a few tried and true, wholesome, fast-friendly drinks that we can utilize when fasting gets tough, when we travel while fasting, or to break the fast. So I present to you four synergistic keto coffee and tea recipes that you can combine with intermittent fasting to ease into fasting, turn off cravings, and increase focus and energy.
They are best consumed near the end of the daily fasting period or during the active eating window. The recipes are easily modifiable and can be tweaked to include more protein or fat for different energetic needs. If your main goal with fasting is to lose weight, you might want to limit the fat content in these recipes.
Original Dave Asprey Keto Coffee
- Brew 1 cup (8 oz) of coffee using filtered water just off the boil. Prefer quality coffee beans or freshly ground to instant coffee (to avoid toxins formed during the production of mass-market coffee). To make good coffee, use a Pour Over or a French Press. The latter is a remarkably simple way of making coffee that’s just as good, if not better, than coffee made with a fancy (more expensive) coffee maker.
- Add in 1-2 tablespoons of MCT oil or coconut oil. Unlike coconut oil, MCT oil is purified to contain six times more fatty acids that cause the desired mental and physical performance effects from keto coffee. MCT oil is strong, so start with 1 tsp and work your way up over several days. Bulletproof Upgraded Brain Octane is a really good brand for MCT oil.
- Add in 1-2 tablespoons of grass-fed, unsalted butter or ghee. The butter you pick must come from grass-fed cows. Grass-fed butter is higher than grain-fed butter in omega-3 fatty acids, CLA, beta-carotene, vitamin A, vitamin K, D, E, and antioxidants than regular butter or margarine. Grass-fed butter is also high in butyrate, a short-chain fatty acid that can both prevent and decrease inflammation. It may also protect against mental illness, improve body composition, increase metabolism, and improve gut health. Kerrygold is an excellent grass-fed butter brand, and it’s stocked in most supermarkets.
- Mix it all in a high-powered blender (that can be safely used to blend hot liquids – Nutribullet isn’t!) for 20-30 seconds until it is frothy, like a foamy latte.
Collagen Boost Keto Coffee
- Boil water. Prepare a cup of coffee using the French Press with 3 heaping scoops of freshly ground coffee.
- Put the rest of the hot water in a blender. Add 2 tablespoons of unsalted grass-fed butter, 1 tablespoon of MCT or coconut oil, and 2 tablespoons of collagen protein (Collagen Hydrolysate from The Great Lakes is excellent).
- Mix it all in a high-powered blender (that can be safely used to blend hot liquids – not the Nutribullet!) for 20-30 seconds until it has a latte consistency.
Breakfast Power Keto Coffee
- Boil water. Prepare a cup of coffee using the French Press with 3 heaping scoops of freshly ground coffee.
- In a high-powered blender designed for hot liquids, add in the base of the keto coffee, i.e., 2 tablespoons of grass-fed butter or, as a variation, 8 oz of full-fat whipping cream, 1 tablespoon of MCT oil or coconut oil and 8 oz of sugar-free almond milk. Mix it all together.
- Add 25 g of sugar-free hydrolysed whey protein like Isoflex (or your favourite plant protein powder – mine is ON Gold Standard Plant-Based) and mix again until there is a layer of foam on top.
Pu-Erh Keto Tea
- Boil about 1 litre of water.
- Combine 2 tablespoons of a Chinese tea with weight loss and anti-ageing properties called “Pu-Erh”, 1 tsp of Kabuse Sencha green tea or any kind of green tea, and 1 tsp of Yerba Mate in a steeper. You can also use a turmeric and ginger tea or any other tea you like but try to have a mix of black and green tea.
- Brew the tea for 1 to 3 minutes, depending on the depth of flavour desired.
- Pour the tea into a high-powered blender designed for hot liquids.
- Start the next brewing with the rest of the water (same tea leaves as they can be infused multiple times).
- Add 1 tbsp of butter and 1 tbsp of MCT or coconut oil into the blender with the already-made tea.
- Pour in the second infusion. Stir it up, and enjoy!
Final Thoughts
Fasting may not seem for you at first, but it is often just a matter of finding which form of intermittent fasting works best with your schedule. If you’ve never been keto-adapted before, the first ten days of fasting might be uncomfortable, but the body eventually gets used to it, and fasting becomes a breeze.
The most important takeaway from this article and the entire fasting research literature is that we should aim to eat less often to simulate stressors like food unavailability that engage the body’s natural healing abilities.
Sources
- Fasting and time-restricted feeding in healthy lifespan
- Meal frequency and timing in health and disease
- Fasting: molecular mechanisms and clinical applications
- Effects of intermittent fasting on health markers in humans
- Effect of 6-mo. calorie restriction on biomarkers of longevity
- Fasting changes the body’s hunger response
- Dietary carbohydrate restriction as the first approach in diabetes
- Dietary restriction extends lifespan in dogs
- Intermittent fasting in metabolic syndrome
- Alternate day fasting in healthy, non-obese humans
- Alternate-day fasting and body composition
- Time-restricted feeding in humans with prediabetes
- A 24-hour fast reduces inflammasome activation in humans
- Fasting and diabetes
- Intermittent energy restriction and multiple sclerosis
- A fasting-mimicking diet in patients with breast cancer
- PNC1 govern lifespan extension by calorie restriction
- Unlocking the secrets of longevity genes
- NAMPT in longevity control and metabolic adaptation
- NAD+ and sirtuins in aging
