Fasting: the Science, the Myths, and What Actually Works

And they found no difference between the calorie-restricted monkeys and the monkeys who could eat as much as they wanted in terms of lifespan. Both groups lived about as long. So now the scientists were kind of in a pickle. They were like, well, one study showed that reducing calories is helpful, and the other study shows that it did absolutely nothing. So where do we stand? What is going on?

Hello angels, and welcome back to the Glucose Goddess Show. I’m still Jessie Inchauspé, French biochemist, really obsessed with helping you understand your health. And today we’re going to talk about fasting.

Unless you’ve been living under a rock for the past few years, you’ve probably heard about intermittent fasting — it’s become all the rage. Many people have tried it, but not many really know the science I’m about to talk about. Today we’re going into the science of fasting: what actually happens when we fast? Does it really extend lifespan? Does it really help with cancer prevention, metabolism, or weight loss? What are the main drawbacks, if any? And if you like fasting, some very important things you need to know — otherwise you might not be helping yourself.

So let’s dive in. First, fasting and lifespan. This idea that fasting extends our lifespan actually comes from a long time ago. It all started with a rat study called “Effects of Intermittent Feeding Upon Growth and Lifespan in Rats,” done on 28 male rats in 1982. The scientists found that the average lifespan of the rats increased by up to 80% — they lived almost twice as long when put on alternate-day fasting: one day eating, the next day fasting. This was the beginning of the belief that fasting or reducing calories could extend lifespan. It was an exciting result — but here’s the problem: it was never fully replicated.

A meta-analysis (a study reviewing multiple studies) looked at data from 1934 to 2012 — “Dietary Restriction in Rats and Mice: A Meta-analysis and Review of the Evidence for Genotype-dependent Effects on Lifespan” — and found less impressive results: caloric restriction increased lifespan in rats by 14–45% (not 80%), and in mice by up to 27%. So, not conclusive — reducing calories or fasting doesn’t always make rats or mice live longer.

Scientists then thought, what about monkeys? Since monkeys are genetically close to humans, maybe this would tell us if the same applies to us. Two massive studies began.

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Now, back to fasting and lifespan. The scientists used a type of monkey that is 93% genetically identical to humans. The first big study was at the University of Wisconsin, starting in 1989 and lasting 20 years on 76 monkeys. It showed a positive effect on lifespan when calories were restricted — the monkeys who ate less lived longer. But here’s the catch: another study, at the NIH starting in 1987, lasted 23 years on 34 monkeys and was published in Nature. It found no difference in lifespan between calorie-restricted monkeys and those who ate freely — both groups lived about as long.

So which one is right? Here’s what scientists noticed: in the Wisconsin study (where calorie restriction worked), the monkeys’ diet was highly processed and high in sugar; in the NIH study (where it didn’t work), the diet was much healthier. That led to a new hypothesis: maybe if your diet is already healthy, reducing calories doesn’t add much benefit — but if your diet is unhealthy (high in processed carbs and sugars), eating less helps you live longer. So it’s not just about fasting or calorie restriction — it depends on what you eat the rest of the time. If you’re looking to extend lifespan, fasting may not be the solution; we don’t have strong data proving it works.

But we do have strong data showing that regular exercise can increase lifespan by up to seven years — boring but true. Many studies show that runners and cyclists live 7–10 years longer than sedentary people, with fewer metabolic diseases.

So exercise is one of the best things you can do for your health — much more reliable than fasting for lifespan. Now, what else do we know about fasting? Lots of cool things. Many studies show that intermittent fasting — eating within a limited window each day (like 6 or 12 hours) — can have various benefits. A huge 2019 review called “Effects of Intermittent Fasting on Health, Aging, and Disease” went through all the evidence.

The first big benefit: lower glucose and insulin levels. Every time you eat carbs (starches like bread, pasta, rice, potatoes, oats, or sugars like juice or desserts), your blood glucose rises, and your body releases insulin to store it. Too much insulin over time causes problems like type 2 diabetes. So, the less you spike glucose, the less insulin you need — which helps your health. Eating fewer carbs or eating less often both help reduce insulin levels. That’s why daily caloric restriction or intermittent fasting (like fasting 24 hours, three times a week) can reverse high insulin in people with type 2 diabetes or prediabetes. It makes sense: eating less of what caused the problem helps fix it.

But fasting isn’t the only way — you can also eat differently. My glucose hacks (link in description) can help you reduce glucose spikes without fasting by changing how and when you eat.

Studies also show that even without changing what or how much you eat, just condensing your meals into fewer, shorter eating windows (for example, three meals instead of six) reduces insulin and helps prediabetes.

Step one: avoid snacking and shorten your eating window.

Step two: change what you eat with my hacks. Another key benefit of fasting is improved metabolic flexibility — your body’s ability to switch between burning glucose and fat for fuel. If you need to snack every two hours or get shaky when you don’t eat, your body isn’t good at burning fat. Fasting trains your body to switch to fat-burning by depleting glucose stores, forcing it to use fat and produce ketones — which are also great fuel for your brain. You can also improve metabolic flexibility by simply avoiding big glucose spikes. So fasting or glucose hacks both work.

A third major benefit of fasting is autophagy — your body’s “self-cleaning” mechanism. When you haven’t eaten for a while, your body starts recycling old, damaged cells to reuse parts, like doing a big spring cleaning. “Auto” means self, “phagy” means eating — self-eating. This process removes old, damaged cells and promotes renewal, keeping you healthier. Autophagy happens all the time, but fasting increases it.

Studies show that autophagy suppresses tumor initiation — it can reduce cancer risk by cleaning out damaged cells that might otherwise become cancerous. There’s a 2023 Nature paper, “Autophagy and Autophagy-related Pathways in Cancer,” explaining this. Other research shows fasting can improve cancer treatment outcomes (“Cyclic Fasting-Mimicking Diet and Cancer Treatment: Preclinical and Clinical Evidence”).

Autophagy also occurs in the brain — one study (“The Effect of Caloric Restriction on Working Memory in Healthy Non-Obese Adults”) found that two years of caloric restriction improved memory, possibly thanks to increased autophagy cleaning up brain cells.

Another 2023 mice study on Alzheimer’s disease found intermittent fasting improved memory and reduced cognitive decline. Of course, it’s in mice, so take with caution — but fascinating nonetheless.

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Now, fasting has a good reputation, but it’s not a miracle solution. It has benefits, but also drawbacks — and you need to know them. The first drawback: muscle loss. Muscles are incredibly important. During prolonged fasting, your body breaks down muscle to get protein, because it needs protein every day. If you’re not eating enough, it pulls protein from muscle tissue.

So, when fasting, it’s critical to eat enough protein during eating periods to maintain muscle. Otherwise, fasting will cause fat loss and muscle loss — not good. Remember this sentence: the fewer calories you consume, the more those calories should come from protein. Muscle mass is key for longevity.

A 10-year study of 4,500 adults aged 50+ (“Associations of Muscle Mass and Strength With All-Cause Mortality Among US Older Adults”) found that low muscle mass increased mortality risk by 40–50%. Strength, even more than size, predicted lifespan. So if you’re fasting — skipping breakfast, doing 16:8, alternate-day fasting, whatever — you must eat enough protein when you do eat. Aim for about one gram of protein per pound of body weight daily (for example, 150 g for someone 150 lbs). That’s much more than old guidelines but necessary to thrive. One egg is about 7 g of protein, so it takes planning.

If you need help calculating your protein needs, there’s a link in the description to my free protein guide — it’ll calculate your target and show you which foods provide how much protein.

Every meal should include protein; make it a habit. Another study found that people doing alternate-day fasting lost more muscle mass than people who simply ate 25% fewer calories daily — long fasts lead to more muscle breakdown. Most people already eat too many starches and too little protein; adding fasting can worsen that. Better to fix what you eat (increase protein, reduce sugar) than to fast blindly and lose muscle.

Another key point: how you break your fast. After fasting (even overnight), your digestive system is empty and sensitive — the first thing you eat goes straight into your bloodstream. So, never break your fast with something sweet like fruit juice or a smoothie. That’ll cause a huge glucose spike. Always break your fast with protein and fiber. Your first meal of the day should be savory — built around protein. You can get free savory breakfast recipes in the description of this episode. Whether your “breakfast” is at 7 a.m. or 2 p.m., the same rule applies: the first meal of your day should be savory. Ideally, start with veggies (a little salad or broccoli) — this creates a protective mesh in your gut and prevents glucose spikes.

In short: avoid sugars and starches on an empty stomach. Don’t eat sweets at the beginning of a meal — have them at the end as dessert. Never snack on sugar between meals. Sugar should only be dessert — not breakfast, not a snack. Lastly, for females of reproductive age, fasting is a stressor on the body. If you already have other stressors — caffeine, intense workouts, saunas, cold plunges, emotional stress — fasting may be too much. Never fast if it feels difficult. Fasting when relaxed (like on vacation) can be fine, but if you’re stressed, don’t add more.

I once met a young woman with PCOS who was fasting 16 hours a day, doing boot camps, cutting all carbs, drinking coffee, doing saunas and cold plunges — and she had lost her period. Too many stressors. Fasting is a stress; do it only if it feels easy. To recap: fasting is not a magic pill. If you fast 16 hours a day but eat junk the rest of the time, it won’t help — fasting doesn’t erase a bad diet. Yes, fasting can lower insulin, improve metabolic health, increase flexibility, and boost autophagy, but if it costs you your muscle mass, it’s not worth it. Be educated. If you fast, eat enough protein (remember: the fewer calories, the higher the protein proportion), and break your fast with savory, not sweet.

Also, fasting benefits depend on your starting point: if you’re already healthy, fasting may add little; if you have metabolic issues like type 2 diabetes, it could help — but always check with your doctor.

Finally, a 2021 umbrella review of multiple clinical trials showed that methods like alternate-day fasting and time-restricted eating improve metabolic health, especially in people with obesity. The less healthy you are, the more you benefit; the healthier you are, the less difference it makes.

So what can everyone learn? Avoid snacking; condense your meals into real meals; have dessert instead of snacks; and remember: breakfast is literally “break fast,” so treat it wisely.

Free resources in the episode description: my 10 glucose hacks (to lower insulin without fasting) and my protein guide to calculate how much you need and which foods to eat.

Let me know what you think — I loved doing this episode.

I hope you enjoyed the science, and I’ll see you next time.