“Fasting is for people who need to lose weight.” Unfortunately, that is the primary association we hold in our thoughts about fasting because of our tendency to simplify complex topics into a takeaway message. The presence of the Unholy Trinity within us has its foot on the brakes, and our thinking tends to move no further from that point. Today, I am going to walk us through how to think about fasting when you are thin. However, what I have to say applies even if you are not thin – you will soon see why.

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Contents

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What fasting is, and NOT [top]

Scientifically speaking, fasting is the abstinence from caloric intake for a duration of time. This is why you are fasting whenever you are asleep.

Fasting on any one day does not mean you will go into a caloric deficit because you may well eat more than your caloric requirement during the time of feeding later that day! Reiterating, “fasting” refers to a restriction of feeding times and says nothing about caloric restriction.

So, you might fast and lose fat. Or, you might fast and gain both muscle and fat mass. Fasting is a powerful protocol that sits alongside other protocols you implement in your lifestyle. Much more about fasting in a minute – first, let us quickly tie down fat and muscle with logical reasoning.

Gaining and Losing Fat [top]

Excess calories that you absorb through food and drink that do not get utilised are stored as fuel for future use. The human body’s capacity to store it as carbohydrates is limited to only 500g of carbohydrate for a fully developed adult male and less for others. So, all other fuel is stored as fat when the limited capacity carbohydrate stores are full. Whether you are active or sedentary, whether you fast or not, if you consume excess calories, you will store additional fat. That is guaranteed.

Fat stores are very low-maintenance. The physiological processes to keep them alive are not energy hungry. When you gain fat, you are not really increasing your basal metabolic rate (BMR), your baseline energy requirement just to stay alive. Think of them as large warehouses for drums of fuel that do not need much maintenance.

The story with muscle is quite different.

Gaining and Losing Muscle [top]

You gain or maintain mass in a specific muscle when you expose your body to an effort required to use that specific muscle and subsequently provide it with fuel and building blocks (protein). That very same muscle loses mass quickly if you do not use it for several days. Why? Unlike fat, which can sit in your body for years without being used and is in storage units that are designed to be low maintenance, muscle is comparatively very energy hungry just to keep alive. So, to keep a pound of muscle alive requires a lot more energy than keeping a pound of body fat alive, and so your BMR does increase with every gram of muscle you gain and decreases when you lose it. It is inherent in your biology to take away resources from parts of your body that are not being used to reallocate them to other parts. The resource-hungry nature of muscle tissues means that your body will take away the resources required to maintain them if you do not use them. As a result, your muscle mass goes down and you look less muscular.

The Miserable Asymmetry [top]

Not just muscle, most things that are not used wither away if they are not required for basic survival. If you stop using a part of your brain, e.g. a language you learned briefly many years ago, you will lose a large part of that ability. Unfortunately, the same is not true with fat. It is there specifically for the purpose of fueling you to survive during periods of low or no calorie intake. When the fat in your fat stores is not used, your body does not get rid of it, like it does for your muscles or other organs and faculties.  Your biology has evolved to do this over millennia. If you gain fat and do not use it, it will stay there until you die.

What does “thin” mean? [top]

The word “thin” is not a scientific term. Instead of calling someone “thin,” scientific discussions use terms such as underweight (based on BMI), low body fat, or ectomorphic (a body type with a slender build).

In my earlier conversation on Fitness for Thin Folk, I talked about how we, for our conversations, could understand what “thin” means, and you could remind yourself about it here.

Fasting for Fat Loss [top]

A lower body fat for most people would mean better health. Although fasting does not guarantee fat loss, you can use fasting to help lower your body fat if that is your goal. Here is how. If your fasting window is such that what you eat during the feeding window leads to fewer calories being absorbed from the food than the calories you burn in the entire day, then you will get there in due course. Typically, with a daily fasting window that is long, there is a tendency to also consume fewer calories in the narrow feeding window. A tendency, not a causality. There is no guarantee that you will lose fat if you fast.

Consider these 2 simple examples: You could fast for just a few hours, but eat very little during an active day and lose some fat on that day. Or, you could fast for 22 hours and have a large meal from which you absorb many more calories than you burn that day. You fasted for 22 hours, but you will gain fat. Ask your friends who fast during the month of Ramadhaan about the many fasting Muslims who gain fat during that month!

For any 24-hour duration, you are never going to have an exact match between the calories absorbed from food versus the calories you burn. So, every day you are either gaining or losing some or a lot of excess fuel. If your carb tank is full, the excess calories will be stored as fat. If your carb tank is empty, your calorie deficit will be met by fat being burned. Keeping everything else equal for some days, if you are in calorie balance daily, your carb tank (500g carb limit) and fat stores fill up and empty daily in a rhythmic pattern. I described this here 9 years ago!

You can lose fat and gain muscle by using the fuel required for building muscle that comes from the fat stores. If you notice you are not losing fat, while gaining muscle, it is because you are not getting into a calorie deficit; you are simply providing the required fuel from food and drink to build the muscle. To a very good first approximation, it is simple arithmetic applied to human physiology. [top]

A happy asymmetry that I covered in an earlier conversation is that when you fast, you can lose fat but not lose muscle because nature has not designed muscle to be a source of fuel. I reiterate, you will lose muscle if you do not use it, not because you are fasting. So, if you are not using your muscles because you are fasting, you must by purunoid that if that disuse continues beyond a few days, you will lose that muscle, but it is from disuse, not because of fasting. [top]

Why Fast? [top]

So, if fasting does not lead to muscle loss and does not guarantee fat loss, why fast?

Fasting, both for shorter durations and prolonged periods, offers a range of health benefits that extend beyond mere fat loss possibilities.

Shorter fasting intervals (typically 12–24 hours), such as daily time-restricted eating, can lead to improvements in insulin sensitivity, reductions in blood pressure, and favourable changes in lipid profiles, specifically decreases in triglycerides and LDL cholesterol. These periods are sufficient to induce a metabolic switch from glucose to fatty acid and ketone utilisation, which enhances metabolic flexibility and may promote cellular stress resistance, autophagy, and reduced inflammation. [top]

Prolonged fasting (4–21 days) further amplifies these effects, driving improvements in overall metabolic health and cardiovascular risk factors, a significant reduction in inflammation, and enhanced cellular stress resistance. Cellular adaptations during prolonged fasts include the enhancement of autophagy, DNA repair, antioxidant defences, and mitochondrial biogenesis, all of which contribute to reduced oxidative stress. These mechanisms collectively may offer protection against chronic diseases such as type 2 diabetes, cardiovascular disease, neurodegenerative disorders, and certain cancers.

Fasting also positively modulates the gut microbiota, increasing friendly gut microbes that reduce inflammation and support the immune system. The body’s ability to recycle proteins and maintain amino acid availability for tissue repair, even during extended fasts, also highlights its adaptive intelligence. [top]

Beyond the physiological, fasting contributes to improved mental and emotional well-being, increased stress resistance, and the potential for extension of Healthspan. The practice can foster greater mental and emotional strength and offers benefits across physical, mental, emotional, and spiritual dimensions. During prolonged fasts, for example, a 5-day water-only fast, following the initial 48-72 hours period, most individuals report no further feeling of hunger and describe their brain working “beautifully”, which makes days four and five their most mentally productive. Fasting can also lead to a reduction or elimination of cravings for certain foods like sweets and provides a valuable, “costless” opportunity for personal growth by consciously overcoming primitive hunger signals. It can also support efforts to reduce dependence on substances such as caffeine, alcohol, and nicotine. [top]

Why Fast if you are Thin? [top]

While much advice on fitness targets overweight individuals, it is important to consider what “thin folk” should aim for to achieve a happy and long Healthspan, as being thin does not automatically guarantee health or fitness.

Having ascertained that Fasting is a very beneficial practice without considering the fat gain or loss, it will be a beneficial practice for thin individuals. Fat gain or loss is something that keeps changing over time. Fasting is something you can do for a defined duration you set. For example, 16 hours, 36 hours and so on. Having set the duration, you can do it on a regular basis, for example, daily, or at regular intervals, e.g. a prolonged fast once a year. And with any combination of fasting duration and frequency, you can decide separately whether you would like to gain or lose fat over the slightly longer term.

So, if you are a thin individual who would like to capture the anti-ageing benefits of fasting but is worried about losing more fat, what should you do?

Gaining fat when fasting: If you are thin and want to gain fat while having a fasting protocol, it is quite simple. First, you cannot forget that sleep is the most important health protocol that adds to or takes away from other protocols. Second, you must exercise, whether you are fat or thin, young or old, or even pregnant, unless you have been advised by a wise physician who understands your conditions to avoid exercise for a period. Consume more healthy calories during your feeding window. This means consuming calorie-dense foods as part of a diverse plant-based diet that is also dense in micronutrients. A cheap and healthy example is peanuts. A more expensive and healthier example is walnuts. An expensive but less healthy example is cashews. [top]

So, how often could a thin person fast and for how long? What else should they consider?

Fasting Durations and Frequencies for Thin Individuals [top]

• 14-18 Hours (overnight): A thin person can do this every day, build muscle, maintain fat, and enjoy healthy food.
• Up to 36 Hours (1.5 days): A thin person, especially a Hindu interested in syncing with Ekadashi, could do it twice a month.
• Up to 72 Hours (3 days): A thin person interested in this duration could do it once a month.
• Up to 120 Hours (5 days): This is considered a “sweet spot” for maximum stimulation of the body’s systems to reset and rejuvenate, happening between day 4 and day 5. A thin person ought to be fine doing it twice a year, with a recommendation for at least once annually.
• Beyond 5 Days (e.g., atthai): For those seeking greater benefits, an atthai (around 8.75 continuous days without food, and water only during daylight hours) once a year is recommended for thin individuals.

As good health is our goal, anyone doing a combination of these different durations of fasting should plan their fasting calendar appropriately to ensure that both their macronutrient intake (for body mass and composition) and their micronutrient intake (to support essential physiological functions) are appropriate for their lifestyle and their health goals. If you feel that your body fat has dropped too much, just eat your way back up – but, please do it with healthy food. Remember, it is not just about the macronutrients for fuel and building blocks; the micronutrients are what help your systems function appropriately, or not!

Maintaining physical activity, including strength training, during a fast is imperative as it aids in metabolic adaptation. Remember, fasting does not cause muscle loss; not doing strength training causes muscle loss, whether you are fasting or not!

Important Considerations for Thin People [top]

Being Thin Does Not Mean You Are Fit or Healthy – Just because you are thin does not automatically mean you are fit or in good health. You may still face health challenges and need to look beyond weight alone to assess your overall well-being.

Muscle Loss During Fasting Is Often Misunderstood – If you are worried that fasting will cause muscle loss, know that this fear is unfounded. Your body is designed to use fat as fuel during fasting and will not irrationally break down muscle tissue that is essential for movement. If you do regular strength training and also fast and then notice a drop in bulkiness, it is likely due to fat loss — especially (dangerous) visceral fat — not muscle loss.

Preserve Your Muscle Through Strength Training. Fasting is not the real risk to your muscle — the real risk to muscle is your failure to stimulate it. To maintain your muscular strength, it is essential to continue strength training during your fast. You can adjust your workouts in terms of time, volume, and intensity, but do not skip them because you are fasting.

Focus on Nutrition During Eating Windows. If you are thin and tend to have a low appetite, you need to be especially mindful of the quality and variety of foods you eat. Make sure you are getting enough micronutrients — such as vitamins, minerals, and phytochemicals — as well as phytonutrients. The goal is to nourish your body without gaining fat at the expense of muscle or falling into nutrient deficiencies. And if you do want to also gain fat, then go back up and read this again.

Prioritise Your Sleep. Good sleep hygiene is crucial – getting this wrong is a dealbreaker for good health. Pay attention to the quality, quantity, consistency, and continuity of your sleep. It affects your overall health and can significantly influence how well your fasting routine works.

Fasting F++ [top]

I spoke about Fasting F++ earlier. It is a metaparadigm that combines your various lifestyle protocols with fasting to enhance your health and wealth, aiming for a significant Return on Time Invested (ROTI). It is designed to be highly customisable to you, moving away from a one-size-fits-all approach, and encourages you to experiment based on scientific understanding. Recognising that you are thin and have a certain set of goals, you can design and experiment with various Fasting F++ protocols.

Safety Guidelines [top]

Underweight/Malnourished: If you are severely underweight or malnourished, you should not use a Time Optimized Feeding (TOF) Ratio more demanding than 12:12 (12 hours fasting, 12 hours eating). Prioritise other lifestyle changes to improve your health before attempting more challenging fasting protocols.

Pregnancy: If you are trying to conceive and are underweight, adjust your TOF so your fasting window is no longer than 14 hours in a 24-hour period (a 14:10 TOF Ratio). Check that your weight gain is within the band of what is normal. Do not become underweight relative to that, and do not gain more weight than is healthy. Even a 4kg baby at birth does not justify a 30kg gain in weight.

Protect Your Capital: Always remember the motto “Safety First” when designing and implementing Fasting F++ protocols. Avoid “get-rich-quick hacks” that could eat up your internal wealth.

Hunger vs. True Need: Feeling hungry is not a valid reason to abort a fast, as hunger pangs are often transient brain signals that disappear within minutes. However, if your intuition strongly suggests a need to abort for safety, do so. I said “intuition” and “safety”, not the inability to gain mastery over your hunger pangs.

Medication: If you are on medication, be aware that fasting can increase its potency. Consult with a healthcare professional experienced in fasting if you are on medication, especially for conditions like diabetes. Just be aware that your physician is likely to be quite clueless about fasting as a free tool for amazing health.

Tracking Progress [top]

Measuring and maintaining data related to processes (e.g., fasting/feeding timestamps, workout details) and outcomes (e.g., body mass, body composition, blood markers, mental/emotional state) is important for optimising personal Fasting F++ protocols.

Take-Home Message [top]

Fasting is a powerful health tool far beyond weight loss, offering benefits like enhanced insulin sensitivity, reduced inflammation, improved lipid profiles, and increased mental clarity, even for thin individuals. For those who are thin, fasting can be safely practised by combining it with strength training to preserve muscle, prioritising nutrient-dense foods during eating windows to avoid deficiencies, and ensuring proper sleep hygiene. By tailoring fasting durations and frequencies to individual needs, thin people can optimise metabolic health, support cellular repair, and promote a longer, healthier Healthspan without the misconception that fasting leads to muscle loss or is only for weight management.

Parting Message [top]

It is not just a play on words – fasting will slow down your ageing!

It is your smallest of habits that define who you are today and who you will be tomorrow. If everything you do is consistent with your core beliefs and desires, then a long and healthy life of joy is pretty much guaranteed to be yours. If you would like to be guided in detail, you know how to reach me, and if you found this useful, please do share it with others.

Puru

Dr Purnendu Nath spends his waking hours focusing on helping individuals and organizations reach their goals, to make the world a better place. He speaks, writes and advises on topics such as finance, investment management, discipline, education, self-improvement, exercise, nutrition, health and fitness, leadership and parenting.