Got a great set of questions recently pushing back on some popular claims about women, fasting, and metabolic flexibility. It’s the kind of stuff that gets repeated so often it starts to sound like settled science. So let’s go through it piece by piece, because when you actually go looking for the data behind these claims, a lot of it isn’t there.
Claim #1: Women shouldn’t fast beyond 12–14 hours or it triggers a “starvation response”
This is one I hear a lot, usually attributed to Stacy Sims: that fasting beyond 12–14 hours can lead to a starvation-type response in women that disrupts cycles and hormones. Short answer: I don’t know of any data that actually supports that.
Now, if we’re talking about women in general tending to run lower on calories and being more subject to energy deficiencies, yes, that can be true. I’ve seen it in guys too, just not nearly as often. But I can’t find human data showing fasting itself is the trigger.
There’s a kisspeptin angle here too. Kisspeptin neurons being sensitive to energy availability is a real mechanism. The rodent work shows Kiss-1 suppression involving multi-day fasts. But the human data is about acute caloric restriction and chronic low-energy availability, not about a longer fasting window. And rodent metabolism is different enough from human metabolism that a “multi-day fast” in a rodent doesn’t translate directly. In humans, that’s genuinely multiple days, not 14 hours.
So is the trigger the fasting condition itself or total energy availability? As far as I can tell from the data, it’s total energy availability, which tends to be lower in women. I can’t find data that fasting itself is the trigger.
In practice, if I have women on very low calories for a while who are having performance issues, maybe amenorrhea, I’m not going to have them fast because fasting is another stressor on top of an already low energy intake. But if I have women who are eating adequate calories, at maintenance or above, they tend to tolerate fasting just fine. It’s individual, not really a gender difference.
Claim #2: Women are less able to break down glycogen, so they should eat carbs and protein before and immediately after training
I can’t find any data supporting the idea that women break down glycogen less effectively than men.
And honestly, if someone’s eating carbs every few hours, I’m struggling to see how they’d ever meaningfully deplete glycogen unless they have massive output. That kind of eating pattern is more likely to just keep insulin chronically elevated. It depends heavily on the person. Is this someone who’s metabolically inflexible and in a caloric surplus? Then no, I wouldn’t have them snacking on carbs all day.
Flip side: I’ve got an athlete right now, a guy, actually, whose calories are so low we’re just trying to get him to eat more whenever possible. I’ve run into that with a few females too. Is that normal? No. But it happens. If someone’s a serious multi-session-per-day athlete, frequent carb intake can actually be a great way to induce metabolic flexibility. It really depends on the athlete’s metabolic state more than anything else.
Claim #3: Women rely more on fat for fuel than men, so they deplete glycogen less
Poll AI, poll most people, even most exercise physiologists, and you’ll get the classic answer: women use fat better than men. I can pull multiple textbooks off my shelf that say exactly that.
So I went back and spent way too long (again) pulling together the actual studies on this. Short version: in sedentary individuals, there’s a meta-analysis showing that, yes, this can be true. In athletes, I’d say probably not. We’ve got some pretty good studies on that.
The two biggest confounds in this research are: (1) what phase of the menstrual cycle the women were in when measured, which does make a difference, though probably not enough to justify training entirely around your cycle; and (2) how fat versus carb use is even being determined. Most of this comes from a metabolic cart measuring RER, but there are other approaches, including tracer studies or looking at blood glycerol as a marker of lipolysis (which tells you how much fat was liberated for energy, not necessarily how much was burned).
When I normalized the data per body mass, and specifically lean body mass since males in these studies are often just bigger, and accounted for VO2 max, the picture changed. The key papers here: DeVries (2006), building on Tarnopolsky (1990); a meta-analysis; and probably the best one, Roepstorff (2002), which used metabolic carts, absolute fat oxidation and tracers together.
Bottom line: once you normalize for VO2 max and lean body mass in athletes, there’s not much difference between men and women. Statistically, in some studies, yes, there’s a difference. But outside of one study, we’re talking a few grams of fat. Are there individual differences? Absolutely, and I’ve seen massive ones. But that’s individual variation, not a reliable sex-based effect. The only way to actually know for an individual is a proper metabolic cart analysis looking at maximal fat oxidation (MFO).
Claim #4: Women should never train fasted because it raises cortisol, which drives fat storage
I genuinely can’t follow the logic on this one. I even ran it by a couple of physiology colleagues.
Here’s the actual mechanism: if you’re in a caloric surplus, fasted exercise can free up fatty acids and glycerol through lipolysis. If those aren’t oxidized, yes, they can get re-esterified back into triglycerides and stored. But that’s a calories-in/calories-out issue, not a cortisol issue.
Cortisol’s primary role is to mobilize energy substrates. It increases lipolysis and helps you burn more energy. It’s catabolic; it breaks things down for fuel. So the idea that cortisol from fasted training is what drives fat storage doesn’t hold up. If there’s an effect here, it’s almost certainly just calories.
Between you and me, I genuinely don’t know how Stacy Sims gets to some of her claims. She’s notorious for citing rat studies and presenting them like human data. Maybe it’s just that the more extreme soundbites from a three-hour podcast are what circulate on social media and end up in my inbox. But a lot of it seems pretty far out there.
So what about meal timing and metabolic flexibility?
On the fasting-between-meals question, yes, I generally like spacing meals out if someone can handle it. High insulin can slow gut transit time, which can be a factor in things like SIBO. Spacing meals lets insulin come back down and lets you use a bit more fat for fuel.
There’s also a practical accuracy issue: people snacking between meals have a much harder time tracking those calories, even when they’re diligently logging everything else. I’ve had plenty of people cut out between-meal snacks for a week, and suddenly their logging accuracy goes way up.
If I’m pulling out all the stops: some kind of 3–4 hour break between meals and 12–14 hours overnight to let insulin come down. That’s a solid approach for metabolic flexibility, provided the person isn’t a high-level athlete and we’re okay putting them in a caloric deficit.
But it’s not one-size-fits-all. For people who need more energy, often younger, more insulin-sensitive, higher-output athletes, I’ve actually encouraged the opposite: more frequent eating, even drinking more calories. It depends entirely on the person’s activity level and energy needs.
For the average sedentary person eating around the clock with low muscle mass and a poor VO2 max, yes, cut the snacks, get some protein in, build some metabolic flexibility. For a high-level athlete, more often than not, I’m looking at their intake thinking they just need to eat more, period. Females tend to be worse about this than males. The upside: once women get on board (especially with a team leader or captain modeling it), compliance tends to be excellent because they quickly notice they can eat more, perform better and still get leaner over time. Their caloric output goes up without breaking the calories-in/calories-out equation. They’re just using it more efficiently.
If someone’s struggling to go any length of time without eating, that’s usually a sign they need to work on it gradually. Pull back meal frequency from five to four to three over time rather than jumping straight to extended fasting windows.
Bottom line
Most of these “women must train and eat differently” claims don’t hold up well against the actual physiology literature once you dig into the studies being cited. There are real individual differences worth paying attention to, including energy availability, menstrual cycle phase and training status, but broad sex-based rules for fasting and glycogen use mostly don’t survive contact with the data. Individualize based on the person in front of you, not a rule pulled from a podcast soundbite.
