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Does Intermittent Fasting Work? — Transcript

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0:00In the 2007 World T20 Championship, the
0:04group match between India and Pakistan
0:06ended in a tie. And back then, it was
0:08decided by a bowl-off, where three
0:11bowlers get to bowl one ball each, and
0:14whichever team hits the stumps more
0:16often won the match. Now, India won that
0:19match, and the then captain, MS Dhoni,
0:22used a rather smart psychological trick
0:25to make it happen. If you are wondering
0:27what a 20-year-old cricket match has to
0:29do with intermittent fasting, keep
0:31watching. Suppose I give two people
0:33exactly the same food and the same
0:36number of calories. One eats it between
0:3812:00 noon and 8:00 p.m. at night. The
0:40other person spreads it across the day.
0:42Has the first person discovered a
0:45metabolic cheat code? The internet,
0:47predictably, has three completely
0:50incompatible answers. Number one,
0:52fasting is an ancient and traditional
0:55cure for modern disease. Our ancestors
0:57figured this out, and that's why we have
0:59Amavasya fasting, Lent, and Ramadan.
1:02Number two, intermittent fasting is
1:04strategic starvation, which puts your
1:06body in a different state that optimizes
1:09for burning fat. Number three, fasting
1:11works only because you eat fewer
1:14calories. So, the timing is irrelevant.
1:17Which one of these answers is true?
1:19Turns out, all three contain a bit of
1:22truth. The problem is only when you
1:25treat any single one as the whole
1:27answer. India has fasted for devotion,
1:30discipline, protest, and penance for
1:33centuries. What changed recently is that
1:35we now log the fast into an app, give it
1:37a ratio like 16:8, and podcasters now
1:40cite studies explaining how and why it
1:42works.
1:44>> [music]
1:46>> So, let's first define fasting. For
1:49starters, it's quite broad. Number one,
1:51time-restricted eating means consuming
1:54all calories within a daily window,
1:56commonly 8 or 10 hours. Number two,
1:59alternate day fasting means eating very
2:02little or nothing every other day.
2:04Number three, a 5:2 fast restricts
2:07calories on two days each week. These
2:09are different interventions. So,
2:11assuming that they're all the same is
2:13like reviewing walking, sprinting, and
2:15running a marathon all under the
2:17category of moving your legs. Religious
2:20fasts differ even more. During Ramadan,
2:22the fast runs from dawn to sunset and no
2:25water. Meals move into the night. An
2:28Ekadashi vrat may exclude grains while
2:31allowing fruit, milk, or one meal. Jain
2:33fasts include everything from Ekasana,
2:36one sitting for one meal, to complete
2:38fasts and avoiding food after sunset. In
2:41general, the purpose of religious
2:42fasting is restraint or devotion, not
2:45optimization of HbA1c. And oh, there is
2:48one category of fasting where people eat
2:50everything from potatoes to sabudana
2:52instead of rice and wheat because the
2:55scriptures only ban grains. That is a
2:59feasting, not fasting. So, a fast can
3:02reduce calories, shift calories to a
3:05different time, change the kinds of food
3:07eaten, remove water, alter sleep, and
3:10happen alone or as part of a community.
3:13Its metabolic result depends on which of
3:15those things changed. Studies on Ramadan
3:18illustrate this well. Meta-analyses
3:20usually find a small weight loss, around
3:231 kg, by the end of the month. Much of
3:25it returns within a few weeks. That does
3:28not make the ritual unsuccessful. Weight
3:30loss was never the point. It shows why
3:32you cannot go in search of medical
3:35evidence for a cultural practice that
3:37easily because you also have to check
3:39for other variables. What people ate
3:41after sunset, when they slept, what are
3:44their genetics, etc.
3:46>> [music]
3:49>> When you eat food, your digestive system
3:51breaks it down into nutrients, which are
3:54absorbed into your blood in the small
3:56intestine. Think of your bloodstream as
3:58a delivery van with a strict rule. It
4:01can only carry about 1 tsp of sugar at a
4:04time. That's it. 5 g or so circulating
4:08in roughly 5 L of blood. Now, when you
4:11eat just two idlis, you have just added
4:1420 g of glucose worth of starch into a
4:17system built for five. So, the body has
4:20a problem, and it has about 20 minutes
4:23to solve it. Why only 20 minutes?
4:25Glucose is sticky and reactive. If you
4:27leave it circulating in blood, it
4:30latches onto proteins and slowly
4:32caramelizes you from the inside, which
4:35is the long game behind damaged blood
4:37vessels, kidneys, nerves, and retinas,
4:41also known as diabetes. Your body treats
4:43a high blood sugar reading as an
4:45emergency for good reason. Here is how
4:48it solves it. The intestine breaks your
4:50food down to its smallest useful pieces.
4:53Glucose from the rice, amino acids from
4:56the dal or egg or meat, fatty acids from
4:58the ghee and oil, and pushes them into
5:00the blood. The pancreas notices the
5:03glucose spike and releases insulin,
5:06which is a doorman. Insulin goes around
5:08knocking on cells and telling them to
5:10open up and take the delivery. Without
5:13insulin, the doors will not open. The
5:15liver takes a big order of glucose and
5:18chains the molecules together into
5:20glycogen, which is essentially sugar in
5:23bulk storage format. Muscles do the same
5:25with their own private stash of
5:27glycogen. And whatever energy is still
5:30left over after all that gets packed
5:32into fat cells because fat is the body's
5:36long-term savings account. About 9
5:38calories per gram versus four for
5:40carbohydrate, and it stores without
5:43water. Evolution chose the densest
5:46format available. And then, you stop
5:48eating, and the whole thing runs in
5:51reverse. Insulin falls, the liver starts
5:54snipping glucose molecules off its
5:56glycogen chains and releasing them into
5:58the blood drip by drip, keeping that 1
6:01tsp topped up while you sleep. Remember,
6:05your blood needs to have this amount of
6:07glucose all the time or you will die in
6:10your sleep. So, first principles, you
6:12are technically fasting every single
6:15night of your life. As the fast
6:17continues and liver glycogen falls, fat
6:20cells release fatty acids. The liver
6:22converts some of those fatty acids into
6:24ketone bodies. Intermittent fasting
6:26influencers will tell you that there is
6:28a magical ketosis switch after 16 hours
6:30of fasting. That is not true. It varies
6:33with the previous meal, your physical
6:34activity, muscle mass, and glycogen
6:37stores. Ketones begin rising after an
6:39overnight fast and become much more
6:41prominent during longer fasts. Just
6:44fasting long enough to burn fat during
6:47the fasting hours also does not mean
6:49confirmed weight loss. If a person eats
6:51back the same calories during the
6:53feeding window, the body can store fat
6:55again. Net fat loss still ultimately
6:58depends heavily on the energy balance
7:01accumulated across days and weeks.
7:04Fasting also changes signaling pathways.
7:07Low amino acid and insulin availability
7:09reduces mTOR signaling. AMPK activity
7:12can rise. Cells may increase maintenance
7:15processes including autophagy, the
7:17system that degrades and recycles
7:20damaged cellular components. Autophagy
7:22has become the fasting influencer's
7:25favorite magic word. The strongest
7:27mechanistic evidence comes from yeast,
7:29worms, flies, and rodents. Measuring
7:32autophagy inside a living human organ is
7:34difficult. One marker cannot tell us
7:37whether more cellular material has
7:39entered the recycling system or whether
7:41the exit became blocked. A 2025
7:44exploratory analysis measured flux in
7:46blood immune cells after 6 months and
7:49found that an intermittent fasting plus
7:51time restriction program may increase it
7:54compared with a standard routine.
7:56Interesting, but it was an exploratory
7:59outcome in one tissue under one combined
8:01program. It does not establish that a
8:0316-hour fast cleans every human organ,
8:07prevents cancer, or extends life. But,
8:09influences go beyond just autophagy.
8:12They claim intermittent fasting will
8:13extend your life. Unfortunately, the
8:15evidence for that does not quite exist
8:17yet. Which brings us to the most
8:20important question in this video.
8:26We now have enough randomized human
8:29trials to give a fairly unexciting and
8:32disappointing answer. Usually, no. The
8:352025 BMJ Open Network meta-analysis
8:38combined 99 randomized trials involving
8:406,582
8:42adults. Intermittent fasting and
8:44continuous calorie restriction both
8:47reduced weight compared with eating
8:49without any restriction. Against
8:51continuous calorie restriction,
8:53alternate day fasting produced about 1.3
8:56kg more weight loss. That was the only
8:59intermittent approach with a clear
9:02advantage, and the difference appeared
9:04mainly in shorter trials. Meaning, if
9:07you eat modestly over a long period of
9:09time, the difference vanishes. A
9:11particularly useful 2022 trial assigned
9:14139 adults with obesity either to daily
9:18calorie restriction alone or the same
9:20restriction plus an 8:00 a.m. to 4:00
9:22p.m. eating window. After 1 year, the
9:25regular calorie restriction group lost
9:276.3 kg, and the intermittent fasting
9:30group lost 8 kg. The 1.8 kg difference
9:33is not statistically significant, and
9:36changes in waist, fat, lean muscle mass,
9:38glucose, lipids, and blood pressure were
9:41similar. That experiment clearly
9:43answered this. When calories are
9:45deliberately reduced in both groups,
9:47does adding a clock produce a large
9:50bonus? It did not. Another trial called
9:53TREAT asked 116 adults to eat between
9:56noon and 8:00 p.m. without other dietary
9:59instructions. After 12 weeks, their
10:02weight change was not significantly
10:04better than the control groups. And the
10:06researchers detected a reduction in
10:08appendicular lean mass. Now, that one
10:11finding does not prove that time
10:13restriction reduces muscle. It does
10:15remind us that protein, weight training,
10:18and the size of the calorie deficit all
10:20matter. A 6-hour eating window
10:22containing tea, two biscuits, one dinner
10:25featuring lemon rice, and inadequate
10:27protein is going to reduce your muscle,
10:29however impressive the fasting
10:31discipline looks. There are also
10:33encouraging trials. In 75 adults with
10:35type 2 diabetes, a known to eight window
10:38without calorie counting produced 3.6%
10:42weight loss over six months versus 1.8%
10:45in the purely calorie restriction group.
10:48HbA1c fell in both intervention groups.
10:51The study was small, and calorie
10:53restriction is famously difficult to
10:55record accurately because people um
10:58lie about what they eat. So, by now, I'm
11:01sure you're confused. What are you
11:02trying to say? But, that is why usually
11:05no, and not just a simple no. So, the
11:07best conclusion is across trials,
11:10intermittent fasting generally produces
11:12modest improvements in weight, waist
11:14circumference, and several metabolic
11:16markers compared with unrestricted
11:19eating. When researchers compare it
11:21fairly with an ordinary calorie deficit
11:23without time restriction, meaning you
11:25eat at any point of time you want as
11:27long as you eat roughly the same number
11:30of reduced calories during a day, most
11:32differences shrink or disappear.
11:39So, is that the end of the video? Timing
11:41does not matter for most outcomes
11:42related to weight, lean mass, blood
11:44glucose, and lipids. And calories in,
11:47calories out explain much of the result,
11:50but meal timing may contribute something
11:53of its own. Human metabolism follows
11:56circadian rhythms. Glucose tolerance and
11:58insulin sensitivity are generally better
12:01earlier in the biological day. Melatonin
12:03rises at night and can suppress insulin
12:06secretion. Night shift work and eating
12:08late at night repeatedly push food into
12:11a phase when the body handles glucose
12:14less effectively. You can watch my video
12:16on the circadian rhythm to understand
12:18this better. In a tightly controlled
12:202018 study, eight men with prediabetes
12:23ate enough food to maintain their
12:25weight, first across 12 hours, and then
12:28within a 6-hour early intermittent
12:30fasting window, essentially 9:00 a.m. to
12:323:00 p.m. After 5 weeks, the 9:00 to
12:353:00 eating improved insulin
12:37sensitivity, beta cell responsiveness,
12:39and blood pressure despite no weight
12:41loss. Yes, eight men is a very small
12:43sample, but the controlled feeding
12:45design demonstrated that timing can
12:48affect physiology independently of
12:51weight. Also note, men is a very
12:53important distinction here. Women's
12:56bodies have an extra layer of hormonal
12:58scheduling that fasting research has
13:00mostly ignored. Insulin sensitivity
13:03shifts, not just during the day as part
13:05of the circadian rhythm, but also across
13:08the menstrual cycle, usually dipping a
13:10little in the luteal phase. So, the same
13:138-hour eating window will end up having
13:15different effects depending on the week.
13:18More importantly, the hypothalamic
13:20machinery that decides whether
13:22reproduction is affordable this month is
13:25exquisitely sensitive to energy
13:27availability. Reproduction is
13:29metabolically the most expensive thing a
13:32human body can adapt. So, evolution
13:34built a conservative chartered
13:36accountant that notices energy
13:38shortfalls. There is some evidence from
13:41small studies that aggressive fasting,
13:43menstrual irregularity, and issues with
13:45reproductive hormones in lean, active
13:48women, while other studies find nothing
13:50much. And everything changes again after
13:52menopause, when the estrogen that helps
13:55women generally handle glucose better
13:57than men on average stop showing up. The
13:59honest answer is that we do not know
14:02enough, and that is partly a failure
14:05that goes back decades. The FDA's 1977
14:08guidance kept women of childbearing
14:10potential out of early-phase drug
14:13trials, officially to protect fetuses,
14:16practically by treating the male body as
14:18the default human. It took until the NIH
14:21Revitalization Act of 1993 for including
14:25women in federally funded clinical
14:26research to become a legal requirement
14:29rather than a suggestion. Nutrition
14:31science inherited that habit, which is
14:33why so many tidy metabolic studies have
14:36sample sizes like eight men with
14:38pre-diabetes and conclusions generalized
14:40to everyone. If you are a woman
14:42wondering whether these findings apply
14:44to you, the correct response is be a
14:46little bit skeptical of fasting related
14:49content on the internet targeted at
14:52women. Now, back to the effects of
14:53fasting on blood sugar. A meta-analysis
14:56of 18 controlled studies covering 1,169
15:00participants found improvements in HbA1c
15:03and insulin with time restriction.
15:06Reductions in fasting glucose appeared
15:07in the early window subgroup. The
15:10studies were heterogeneous and often
15:11short, so the direction makes biological
15:14sense, but the size and durability of
15:17extra benefit still remains uncertain.
15:20Then came a larger 2025 Nature Medicine
15:23trial. Nearly 200 adults with overweight
15:26or obesity received Mediterranean diet
15:29advice and were assigned an early, late,
15:33and self-selected 8-hour window or usual
15:35calorie restriction. After 12 weeks,
15:38none of the time restricted groups lost
15:40significantly more visceral fat than
15:43usual dieting. All three windows were
15:45feasible and some secondary measures
15:47improved, but the supposedly perfect
15:50early window did not produce a dramatic
15:52victory. This is common in nutrition
15:54science. A clean mechanism, circadian
15:56rhythm, technically works on paper, but
15:58the effect becomes smaller when it meets
16:00free-living humans with commutes,
16:03children, different levels of stress,
16:05genetics, and Indian grandmothers who
16:07consider refusal of a second helping a
16:09personal attack. But broadly, regardless
16:12of whether you are just on a diet or
16:14diet plus restricted eating window, if
16:17timing is flexible, finishing dinner a
16:19few hours before sleep probably makes
16:21more sense than skipping breakfast and
16:24concentrating most calories in the
16:25second half of the day. But, saying that
16:28you will eat most of your calories
16:29during breakfast and very little at
16:31dinner might not work for a lot of
16:33people for purely social and cultural
16:36reasons.
16:37>> [music]
16:40>> Calorie counting asks for hundreds of
16:43small decisions. How much oil is in this
16:45restaurant? Usually a lot. Was that
16:48spoon 15 g? Does one bite from a child's
16:50plate count? Of course, it counts. The
16:52mitochondria have not signed a parent
16:54exception. But, time restricted eating
16:56can replace those hundreds of decisions
16:59with one simple, easy-to-follow rule.
17:02Outside these hours, I do not consume
17:05calories. Behavior researchers call
17:07rules like these a reduction in decision
17:10points. There is no tyranny of choice. A
17:13fixed time also acts like an
17:15implementation intention. When the
17:17kitchen closes at 8:00, the default
17:18answer to late-night snacking has
17:20already been chosen. It's no. That can
17:23be liberating for someone whose main
17:25problem is constantly snacking from
17:27breakfast until midnight. It can be
17:29miserable for someone who trains early,
17:32works night shifts, takes medication
17:34with food, or values a late dinner with
17:37the family. Restriction can also provoke
17:39compensatory eating in some people.
17:41Sometimes enduring hunger all day makes
17:44that leftover biryani in the fridge look
17:46extra delicious at night. Intermittent
17:48fasting trials often report good
17:50adherence over a few months because
17:53participants receive reminders, apps,
17:56counseling, regular contact, and
17:58sometimes food. In the 2025 BMJ review,
18:01longer studies showed mixed outcomes.
18:04One whole-day fasting trial fell from
18:0674% adherence at 6 weeks to 22% at 1
18:10year.
18:11Another year-long study reported roughly
18:1384% adherence for both time restriction
18:16and continuous calorie restriction.
18:18There is no universally easier diet.
18:21There are people for whom a clock is
18:23easier than doing continuous
18:24mathematics. That's all. The appetite
18:27evidence is similarly ordinary. A
18:29meta-analysis of randomized trials found
18:31no clear difference between intermittent
18:34and continuous restriction in hunger,
18:36fullness, or desire to eat. Some people
18:39adapt. Some spend the morning thinking
18:41about lunch with the intensity normally
18:44reserved for a first day first show of a
18:46Rajini film. But, all this apart, the
18:49quality of your diet remains an
18:51independent critical variable. You can
18:54use any method, but if you do not add
18:56enough fiber, protein, or
18:58micronutrients, you are not helping
19:00yourself. Okay, now the million-dollar
19:03question. Should I try it?
19:08For starters, if you have a medical
19:10condition, talk to your doctor before
19:12you start. This is important because
19:14fasting is not a neutral act for a body
19:17that is already being managed. If you
19:20are on metformin, a sulfonylurea,
19:22insulin, blood pressure medication,
19:24thyroid replacement, medication for
19:26depression, or anything that needs to be
19:28taken with food at a fixed time,
19:30changing when you eat changes how those
19:32drugs behave. Comes with serious risks.
19:35And if you've had bariatric surgery,
19:37your eating window is already a medical
19:40prescription, not something for you to
19:41experiment with. Again, I am not saying
19:44do not consider fasting if you're on
19:46medication. The point is to do it with
19:48supervision and a plan from your doctor,
19:51not a video on social media. So, start
19:54basic. Measure your current eating
19:56window for a week, from the first chai
19:58to the last bite. See how long that is,
20:00and start by reducing that. Many people
20:03can reach a 10-hour window by removing
20:06late-night snacks or moving dinner
20:08slightly earlier without skipping an
20:10entire meal. For example, if your dinner
20:12time is 9:30 p.m., try make it 9:00, and
20:15then eventually 8:00 p.m. A consistent
20:188-to-10-hour daytime eating window by
20:20itself has way more clinical evidence
20:23than multi-day complicated fasts. Also,
20:26place the eating window where it fits
20:28into the rest of your life.
20:30Earlier is probably metabolically
20:32preferable, but consistency, sleep, and
20:35shared meals also affect health. If you
20:37tend to eat breakfast together as a
20:38family, great. Move most of your
20:40calories to breakfast and dinner much
20:42lighter and earlier. But, if you eat
20:45dinner as a family, then adjust
20:47appropriately. The best diet is the one
20:49that you will follow consistently.
20:51Inside the eating window, don't assume
20:53you have the license to go crazy. Eat
20:55adequate protein at at least two to
20:57three meals, particularly if you're
20:59older or doing weight training. Watch my
21:01videos on those topics to understand it
21:03better. Prioritize vegetables, fruit,
21:05pulses, and whole grains. Drink enough
21:07water. Black coffee or unsweetened tea
21:10contributes little or no calories, but
21:12caffeine late in the day can damage
21:14sleep in some people, which can defeat
21:16the point of the diet, because sleep is
21:18when the actual magic happens. Worth
21:21considering decaffeinated tea or coffee
21:23in that case. Finally, track a metric
21:26that matters to you. Weekly average body
21:28weight, waist circumference, glucose
21:30readings if prescribed, hunger, exercise
21:33performance, and whether the arrangement
21:35is not making normal social life
21:38awkward. If nothing improves after 8 to
21:4012 weeks, you know that you need to
21:42change. In most situations, it's likely
21:44to be what you are eating in the window
21:47rather than the window itself. For most
21:50healthy adults, intermittent fasting is
21:52neither a miracle nor a scam. It is a
21:55schedule that can remove opportunities
21:58to eat and may work better when its
22:01hours align with daylight. Its long-term
22:04advantage over an equivalent simple
22:07calorie deficit is small or absent in
22:10current trials. But, humans do not eat
22:12inside science experiments. We eat
22:14around work, worship, sleep, family, and
22:17habit. India's fasting traditions
22:19understood that abstinence gains power
22:22from a rule, a ritual, a time, and
22:24shared meaning. Modern nutrition
22:26borrowed the rule at the time. That's
22:28what intermittent fasting is. It should
22:30consider borrowing the humility as well.
22:32Choose the pattern that makes good food
22:35easier to repeat. The clock can help. In
22:38the same way MS Dhoni did in that famous
22:412007 T20 match against Pakistan. If you
22:44notice, rather than stand, he was on his
22:47knees and placed himself in a way that
22:50gave the Indian bowlers an easier target
22:53to aim at. They were aiming at solid
22:55blue instead of three thin sticks in a
22:58background of green. The clock can be
23:00like MS Dhoni.
23:02It can make it easier and more
23:04convenient for you to hit your calorie
23:06goals. But you still have to have the
23:09discipline and focus of Sehwag,
23:11Harbhajan, and Robin Uthappa
23:13to still get the job done.

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