Fed or fasted training? Fung, Sims and Galpin split on empty-stomach workouts

The Short Version

Whether to train on an empty stomach depends on who you are and what you want from the session. Dr. Jason Fung argues fasting is compatible with performance, even for athletes, especially in the off-season, and that the body protects muscle while it burns stored fat [1]. Dr. Andy Galpin says fasted training simply doesn't matter for fat loss, because total energy balance decides that, though he does use occasional fasted sessions to train fat use [2]. Dr. Stacy Sims, speaking on both Huberman Lab and The Mel Robbins Podcast, says active women in particular should put a little fuel in before training, because fasted exercise raises cortisol, disrupts hormone-signaling neurons and costs them muscle [3][4]. Most of the split disappears once you see that Fung speaks mainly to metabolic disease, Galpin to fat loss and fuel flexibility, and Sims to the hormonal responses of active women.

Table of contents

What They Said

Jason Fung: fasting fits performance, especially off-season

Dr. Jason Fung comes to fasting from metabolic disease. His main argument, in a conversation on The School of Greatness, is that type 2 diabetes is caused by too much sugar in the body and can be reversed by putting less sugar in and letting the body burn it off through intermittent fasting [1]. He compares the body to a sugar bowl that overflows into the blood once it's constantly refilled [1].

From there he attacks the habit of eating all day. The six-small-meals idea, he says, keeps insulin spiking constantly so the body never taps its fat stores, holding it in growth mode [1]. He urges a return to the pattern of the 1970s: three meals, no snacks, long breaks between eating [1].

He describes what happens during a fast: insulin drops, fat burning begins, autophagy (cellular cleansing) is activated, growth hormone rises and mental clarity improves [1]. After 14 to 16 hours of fasting, he says, the body shifts its fuel from glucose to fat, and after two to three days hunger disappears and energy rises [1]. To the common worry about muscle, he answers: "Your body is not that stupid" [1]. The body uses stored fat first, not muscle, and as long as you stay active it keeps its lean mass [1].

On athletes, including elite marathoners and powerlifters, his view is nuanced rather than absolute [1]. In season, athletes may need more carbohydrate and energy, so fasting is harder [1]. In the off-season or during recovery, fasting helps heal inflammation, improve autophagy and repair tissue [1]. He points to MMA fighter Georges St-Pierre, who uses fasting during training because it sharpens his focus and performance [1].

Fung's source here is a summary only, without transcript passages, so his reasoning on training is reported at a fairly high level. He doesn't address sex differences.

Andy Galpin: fasted or fed doesn't decide fat loss

Dr. Andy Galpin, a kinesiology professor whose PhD is in human bioenergetics, attacks the idea that fasted cardio is a fat-loss shortcut, on Huberman Lab [2].

How fat actually leaves

His starting point is where fat goes. Carbohydrates and fats are both chains of carbon; metabolism breaks carbon bonds to make ATP, and the freed carbon leaves the body as exhaled CO2 [2]. The only real levers for fat loss are taking in less carbon or expelling more through activity [2]. That, he says, is why so many different diets and training styles work: steady cardio, intervals and weights all increase respiration [2]. "There are no free passes in physiology" [2]. What matters is total intake and consistency, not whether a given session burned fat or carbohydrate [2].

Burning fat is not losing fat

The core confusion, Galpin says, is between the percentage of fuel that comes from fat and the total amount of fat you lose. As exercise gets harder, a greater share of fuel comes from carbohydrate and less from fat, which is called the crossover concept [2]. But sleep has the highest percentage of fat burning of any activity, so if low-intensity "fat burning" were the key, sleeping would be the best fat-loss strategy [2]. Low-intensity work burns a high percentage of fat but few total calories; fat never supplies more than about 70% of your fuel, while very hard exercise can be almost all carbohydrate [2]. In his words: "Maximizing fat burning and maximizing fat for exercise and maximizing fat loss over time are not the same thing at all" [2].

Applied to fasted training, the conclusion is direct: for most people, exercising fasted doesn't matter for fat loss, because glycogen and blood glucose stores are ample [2]. If you burn carbohydrate in a workout while eating in a calorie deficit, the body routes incoming carbohydrate to refill glycogen and draws on fat for other energy, so fat is still lost [2]. He compares carbohydrate and fat metabolism to the front and back gears of a bicycle: complementary, not either-or [2]. Carbohydrates are meant to be flexible, while "fat is not meant to be flexible it is meant to be unlimited" [2].

Where fasted sessions do fit: training fuel flexibility

Galpin isn't against fasted training. He uses it as a tool and a test. He says metabolic flexibility has been co-opted to mean maximizing fat burning, when it really means using the optimal fuel at the optimal time [2]. One practical test: you should be able to do your usual workout fasted with little drop in performance and without needing caffeine; another is being able to eat about 50 grams of carbohydrate without crashing 30 minutes later [2]. To improve fat use, he suggests training sometimes after eating fat or fasted, accepting slightly lower top-end performance; to improve carbohydrate use, eat carbohydrate before higher-intensity training [2].

He also warns about the opposite mistake. A large dose of fast carbohydrate right before exercise can crash blood sugar in some people, which he calls the "insulin glucose double whammy," so race-day fueling should be practiced in training [2]. And for top performance, carbohydrate is indispensable: even elite two-hour marathoners burn up to about 80% carbohydrate because fat is too slow [2]. Very low-carb diets down-regulate carbohydrate-burning enzymes and slow top-end performance, so they suit people who don't train intensely [2]. He adds that the idea that carbs at night make you fat is outdated; his teams often use high-carbohydrate evening meals even with athletes cutting weight [2].

His limits come from the liver as well. Muscle glycogen isn't usually the true limiter: people feel strong fatigue signals with muscle glycogen still around 75% full and often quit around 50% [2]. Liver glycogen usually isn't a real problem until about two hours of exercise or extremely high intensity, the classic marathon wall [2].

Stacy Sims on Huberman Lab: why fasted training backfires for active women

Dr. Stacy Sims, an exercise physiologist and nutrition scientist who specializes in women, takes up the question Huberman gets constantly: do intermittent fasting and fasted training work the same way for women? [3]

Fasting versus time-restricted eating

Sims first separates two things. Intermittent fasting, such as holding a fast until noon or longer, is different from time-restricted eating, which she calls a fancy way of describing normal eating in line with the circadian rhythm [3]. For active women, she says, the first is very detrimental unless there is PCOS or another metabolic condition [3]. She cites population research suggesting that people who fast until noon and eat until 6 p.m. have worse obesity-related outcomes than those who eat around 8 a.m. and finish by 4 or 5 p.m. [3].

Two reasons: muscle fibers and the brain

Her first reason is metabolic. "We as women have more oxidative fibers," which makes women already more metabolically flexible than men, so the flexibility benefits often cited for fasting matter less [3]. Exercising without fuel raises cortisol, and without fuel women can't reach intensities high enough to trigger the post-exercise growth hormone and testosterone responses that bring cortisol back down [3].

Her second reason is the brain. Men have one area of kisspeptin neurons while women have two, one tied to appetite and luteinizing hormone and the other to estrogen and thyroid [3]. Starting the day under stress without fuel perturbs and downregulates these neurons, and she says that after four days there is thyroid dysregulation and a change in luteinizing hormone pulse [3]. Because cortisol peaks about half an hour after waking, women who don't eat stay more sympathetically driven, feeling tired but wired [3].

Huberman's confession and a point of agreement

Huberman admits his own routine: "I don't eat until 11 a.m. I like to hydrate and caffeinate before I train in the morning," and several female training partners adopted it with him. "Now I'm thinking that's probably the worst way to go" [3]. Sims adds it gets worse with age: in perimenopause, baseline cortisol rises anyway, and fasted training "increases that cortisol drive and that sympathetic drive" [3]. Since this is a stage when women need polarized training (very hard and very easy) to change body composition, "not having any fuel before high intensity work out puts them in moderate intensity" [3]. She says unfueled women in perimenopause lose about 2 to 5 percent of their top lifting load [3].

On fat loss specifically, Sims and Galpin agree. Huberman says that, while the percentage of fat used as fuel may rise when training fasted, "overall in terms of loss of body fat, it doesn't matter if you train fasted or you train fed," and Sims answers, "Correct" [3]. Huberman adds that people who prefer to train fasted can still do so, but that women should probably take in at least some high-quality protein, perhaps as a shake [3].

How little fuel is needed

The fix is small. Sims has a double espresso with sweetened almond milk and a scoop of protein powder before an ocean swim, and just the protein in coffee before the gym [3]. Citing Abbie Smith-Ryan's work at UNC, she says a strength session needs only around 15 grams of protein beforehand, which also raises post-exercise oxygen consumption, and cardio up to an hour adds about 30 grams of carbohydrate [3]. A real meal about half an hour before is fine too [3]. The goal is to raise blood sugar enough to tell the hypothalamus that nutrition is on board, then eat a real breakfast within 45 minutes after [3].

After training, she recommends around 35 grams of leucine-rich protein within 45 minutes for women in their reproductive years, and 40 to 60 grams from perimenopause onward, because the body becomes more anabolically resistant with age [3]. She argues the idea that there's no recovery window comes from male data: "Women it's within 60 minutes and for men it's up to three hours" for metabolism to return to baseline [3]. The longer women withhold food after exercise, the more the brain senses low energy, and the first thing lost is lean mass, which is very hard for women to build [3].

Energy deficits, done right

Sims isn't against fat loss or small deficits. She sets the tipping point for endocrine dysfunction at 15 calories per kilogram of fat-free mass for men and 30 for women [3]. A deficit of 150 to 200 calories "at night away from training" helps fat loss without costing lean mass or recovery, she says, because "It's the fueling in and around the stress, meaning the exercise stress, it's really important" [3]. She traces the myth that hard training disrupts periods to underfueling: "It's the act of not fueling appropriately for it" [3]. And she names the cultural driver: "women have been so conditioned to not eat and not take up space to be small" [3]. Her aim is "you train hard, you eat well and your body responds in kind" [3].

Stacy Sims on The Mel Robbins Podcast: "Women are not small men"

The request frames Mel Robbins's guest as a separate voice, but it is Sims again, on The Mel Robbins Podcast, giving a more everyday version of the same case [4]. So this is one expert's view in two settings, not two experts converging.

Her mantra is "women are not small men": most fitness and nutrition advice comes from research on men and is simply applied to women, a practice the industry calls "shrink and pink" [4]. "When we look at the trends of the fasted training, don't eat before it's all on male data," she says [4]. Robbins realizes this is exactly what she and her husband did; he got results while she felt "puffier and fluffier" and tired [4]. Sims says women following their partner's program often end up "tired but wired": after four weeks, the man is leaner and fitter while the woman feels fatter, tired and no fitter [4].

The sex difference explained plainly

"So a man can get by with the fasted training," Sims explains, because going without food "stimulates the little molecular structures and the muscles to use more fat" in men, whose fiber types differ: "men have what we call more glycolytic or fibers that use glucose and not as many oxidative or the fibers that use fat as a fuel" [4]. Women already have more fat-burning fibers and a different brain feedback loop, so the same approach backfires [4]. "Women's bodies don't respond well to fasted training" [4]. Women's brains, especially the hypothalamus, are highly sensitive to blood sugar; exercising without fuel is treated as stress, and one of the first things the body breaks down is muscle [4]. Food after waking helps bring the morning cortisol spike down; without it, women stay in a heightened stress state, chewing into lean mass and holding onto body fat, particularly with age [4]. Robbins sums it up as not eating "signaling your body to stay in fight or flight, which makes your body hold on to fat," and Sims answers, "Yes" [4].

Asked what happens if a woman keeps skipping food before exercise to lose weight, Sims is blunt: "You lose muscle" [4]. From the late 30s, women also get a hormonal signal to store more visceral fat [4]. She says DEXA scans of women proud of fasting until noon on coffee can reveal low bone density, more visceral fat, worse cholesterol, more inflammation and insulin resistance, and she warns of women over 40 becoming "skinny fat" [4].

The small fix, and when to skip it

Her image: "If you think about trying to drive a car and rev it up and get it on the highway to at speed with it on E, it's not going to get very far" [4]. Even before a walk, she wants a little fuel, "the protein coffee," "a couple of tablespoons of yogurt, half a banana. It's not a lot, but it's enough to bring your blood sugar up until your brain" gets the message [4]. Many women at a recent competition told her eating beforehand changed their training and energy [4]. Front-loading calories also improves sleep, she says, since many women's night awakenings come from low blood sugar [4].

She also gives a rule for timing across the day. First thing in the morning, always eat before training [4]. "If you are exercising the afternoon and you had lunch maybe two hours before, then you don't necessarily have to eat right before, but you do have to eat afterwards" [4]. For high-intensity or cardio sessions, a small snack before and real food after helps; for strength training, "you have around 15 grams of protein before any kind of strength training session," which keeps metabolism elevated longer and improves the training response [4].

Her own solution is a protein coffee made the night before: protein powder in almond milk with a double espresso, about 30 grams of protein [4]. For women who aren't hungry in the morning, she suggests splitting breakfast, for example overnight oats with chia, half on waking and half after a walk or workout [4]. On circadian timing, she says newer research shows metabolic benefits for people who break their fast around 8 a.m. and stop eating by 6 p.m., but not for those who hold a fast until noon or later [4]. (On Huberman Lab she gave the finishing time as 4 or 5 p.m. [3].)

She also reframes diet trends, intermittent fasting included, as attempts to get health outcomes that exercise delivers, and recommends normal eating: breakfast, lunch and dinner, no snacks after dinner, mostly unprocessed food [4].

Where They Agree, and Where They Don't

Everyone agrees fasted training is not a fat-loss hack. Galpin says fat loss is decided by total energy balance, not whether a session burned fat [2]. Sims agrees that "it doesn't matter if you train fasted or you train fed" for fat loss [3]. Fung's case for fasting is about insulin and metabolic disease, not about fasted sessions burning more fat [1].

Fung and Sims disagree most directly on fasting itself. Fung sees intermittent fasting as a natural biological reset that lowers insulin, raises growth hormone and spares muscle as long as you stay active [1]. Sims says holding a fast until noon or later is very detrimental for active women, raising cortisol, disrupting kisspeptin neurons and eating into lean mass [3][4]. Part of the gap is who they're talking about. Fung's audience in this conversation is people with prediabetes, type 2 diabetes and chronic weight gain [1]; Sims explicitly exempts women with PCOS or other metabolic conditions from her warning [3]. Part of it is sex: Fung doesn't discuss sex differences, while Sims says the fasting research is "all on male data" [4].

On muscle, they disagree outright. Fung says "Your body is not that stupid" and uses fat before muscle [1]. Sims says one of the first things women's bodies break down in unfueled exercise is muscle, and that fasted training costs women lean mass [4]. Fung's claim is about fasting in general and staying active; Sims's is about training hard without fuel, specifically in women.

Galpin and Fung partly overlap on athletes. Fung says fasting is harder in season when athletes need more carbohydrate [1]; Galpin says top-end performance relies on carbohydrate and that very low-carb diets slow it [2]. Both would steer an athlete toward fuel when performance counts.

Galpin and Sims differ on metabolic flexibility. Galpin treats occasional fasted training as a way to improve fat use, and being able to train fasted without much performance drop as a sign of good flexibility [2]. Sims says women already have more oxidative fibers and more metabolic flexibility, so they gain little from fasted training and pay a hormonal cost [3]. Galpin speaks in general, not by sex; Sims argues the general case is built on men.

Galpin and Sims agree on fueling hard work. Galpin says carbohydrate before higher-intensity training improves carbohydrate use and warns against training depleted at top intensity [2]; Sims says unfueled women drift into moderate intensity and miss the hormonal payoff of hard work [3]. They also agree on the value of practicing fueling, though Galpin warns that a big dose of fast carbs right before exercise can cause a crash [2], which fits Sims's preference for a small, protein-based pre-workout snack [3][4].

Why It Works

Fat loss is a carbon budget. Fat leaves as exhaled CO2, and only total intake versus total activity changes that budget; the fuel mix in one session doesn't [2].

Percentages mislead. Low-intensity or fasted exercise can burn a higher percentage of fat but fewer total calories; sleep burns the highest percentage of all [2].

Insulin and fasting. Fung's model is that constant eating keeps insulin high and blocks access to fat, while fasting lowers insulin and, after 14 to 16 hours, shifts fuel toward fat [1].

Cortisol and the morning spike. Cortisol peaks about half an hour after waking; food signals the brain to bring it down [3][4]. Without fuel, women stay sympathetically driven, and without enough intensity they miss the growth hormone and testosterone responses that pull cortisol back down [3].

Kisspeptin neurons. Women have two areas of kisspeptin neurons, linked to appetite, luteinizing hormone, estrogen and thyroid; Sims says fasted stress downregulates them, with thyroid and luteinizing hormone changes after four days [3].

Muscle fiber types. Men have more glycolytic fibers, so fasted training pushes their muscles to adapt toward fat use; women already have more oxidative fibers, so there's less to gain [3][4].

Energy availability. The threshold for endocrine dysfunction is higher for women (30 calories per kilogram of fat-free mass versus 15 for men), so the same deficit can tip women into hormonal disruption sooner [3].

Where It Applies, and Where It Doesn't

Fung's fasting view applies most to people using fasting to address insulin resistance, prediabetes or weight gain, and to athletes in the off-season or recovery [1]. It applies less to in-season athletes who need carbohydrate [1], and the source doesn't address active women specifically.

Galpin's view applies to anyone chasing fat loss: fasted or fed doesn't matter, so choose what lets you train consistently and eat well overall [2]. His occasional fasted session applies to people wanting to test or build fat use, not to sessions where top-end performance matters [2].

Sims's view applies most to active women, especially those training hard in the morning and those in perimenopause, when cortisol is already rising [3][4]. She excludes women with PCOS or other metabolic conditions from her warning against intermittent fasting [3], and she distinguishes an afternoon session two hours after lunch, which needs no extra pre-workout food [4].

It also applies to men. Sims says men can "get by" with fasted training [4], and Huberman and Sims agree fasted training doesn't help fat loss for anyone [3].

The biggest risk is copying a routine built for someone else, as Huberman's female training partners did with his fasted mornings [3], or as Robbins did following her husband [4].

If you have diabetes, a metabolic condition, PCOS, an eating disorder history or are on medication, get a doctor's advice before changing when you eat or fast.

What To Do With It

1. Stop using fasted cardio for fat loss. Pick the timing that lets you train hard and consistently; total intake and activity decide fat loss [2][3].

2. If you're a woman training in the morning, eat a little first. A protein coffee, a couple of tablespoons of yogurt or half a banana is enough [4]. Before strength training, aim for about 15 grams of protein; for cardio up to an hour, add about 30 grams of carbohydrate [3].

3. Make pre-workout fuel easy. Mix protein powder into almond milk with a double espresso the night before for about 30 grams of protein, or split overnight oats into two halves around a workout [4].

4. Eat soon after training. Women: about 35 grams of leucine-rich protein within 45 minutes, rising to 40 to 60 grams from perimenopause onward, plus a real breakfast within 45 minutes of a morning session [3].

5. Time the day's eating to daylight. Sims favors starting around 8 a.m. and finishing by late afternoon or evening rather than holding a fast until noon [3][4]; Fung favors three meals with no snacks and long breaks between them [1].

6. Put any deficit away from training. A 150 to 200 calorie deficit at night supports fat loss without costing lean mass or recovery [3].

7. Use fasted sessions only as a deliberate tool. If you want to build fat use, do an occasional easy session fasted and accept slightly lower top-end performance; fuel with carbohydrate before hard sessions [2].

8. Test your fuel flexibility. Can you do your usual workout fasted without a big drop or caffeine, and eat about 50 grams of carbohydrate without crashing 30 minutes later? If not, work on the weak side [2].

9. Practice race-day fueling in training, and avoid a big dose of fast carbs right before exercise if it makes you crash [2].

10. If you're an athlete, match fasting to the season. Keep fuel up in season; consider fasting in the off-season or during recovery, as Fung describes [1].

Sources

1. The Science of Intermittent Fasting (Foods To Live Longer & Reverse Type 2 Diabetes) with Jason Fung · The School of Greatness
2. Dr. Andy Galpin: How to Build Physical Endurance & Lose Fat · Huberman Lab
3. Dr Stacy Sims Female Specific Exercise Nutrition For Health Performance Longevity · Huberman Lab
4. The Body Reset: How Women Should Eat & Exercise for Health, Fat Loss, & Energy · The Mel Robbins Podcast