Dr. Stacy Sims on How to Lose Weight for Women and Men
Steven Bartlett interviews Dr. Stacy Sims about sex-specific differences in weight loss and metabolism.
Summary
Dr. Stacy Sims explains why men and women require fundamentally different approaches to weight loss, centred on differences in the hypothalamus—the brain's control centre for appetite and metabolism. Women's hypothalamus contains more nutrient-sensitive neurons that interpret calorie restriction or fasted training as potential famine, triggering muscle breakdown and metabolic slowdown rather than fat loss. Men's hypothalamus is less sensitive to nutrient scarcity, allowing them to thrive on strategies like intermittent fasting and calorie restriction. Sims argues that the popular "calories in, calories out" model works for men but fails women because their bodies require nearly double the baseline calories per kilogram of lean mass to maintain metabolic and hormonal function. This evolutionary adaptation protected women from pregnancy during food scarcity whilst preparing men to hunt.
Key Takeaways
Full Transcript
Steven Bartlett: If a man and a woman came to you and said they want to lose weight—they're 200 pounds and would like to lose some weight—would you give them different advice on what to do?
Dr. Stacy Sims: Absolutely. It comes down to something we see on social media all the time: calories in, calories out. When we're looking at calories in, calories out, that algorithm can work well in men. The reason for that is the hypothalamus. The hypothalamus is an area in the brain that controls appetite and our endocrine system.
For men, they don't have as many of what we call kisspeptin neurons activated. These neurons are responsible for firing when we have nutrients coming in—they signal that we've got enough nutrition coming in that we can now accommodate developing muscle and losing body fat. For women, we have more areas that are very sensitive to nutrient density.
When I say this, when we're talking about four grams of carbohydrate that come in—say they're carbohydrate from fruit and vegetables, not from ultra-processed stuff—those four grams of carbohydrate will affect the bodies differently between being a man and a woman. For a man, those four grams of carbohydrate coming in primarily will go to blood sugar and then be stored as liver and muscle glycogen. For women, it's blood sugar, but it doesn't get stored because for women to store muscle and liver glycogen you have to have an activation of some enzymes from the liver as well as some enzymes within the skeletal muscle itself to signal that we want to store this, we don't want to circulate it.
Then we start looking at how the brain is perceiving that. If the brain is saying we can store this because there's still enough muscle tissue around, there's still enough blood glucose that we can keep going and survive the day. But for women, the blood glucose sits there, and when it starts being used, the hypothalamus asks where's the extra food that's coming in so we can keep going and counter the stress that's coming in.
The best way from a numbers perspective to look at it is when we're examining baseline calorie intake just to exist and not get into any kind of endocrine or hormone dysfunction and appetite dysfunction. For men, it's 15 calories per kilogram of fat-free mass. For women, it's 30.
We start to see men do really well on things like fasted training. We see men do really well on calorie restriction because the hypothalamus is not as sensitive to lower calorie intake or to low carbohydrate intake or to high protein and high fat intake. But for women, because the hypothalamus has more areas that are sensitive to nutrient density—
Steven Bartlett: What does that mean? Sorry, I'm not even sure what the hypothalamus is.
Dr. Stacy Sims: The hypothalamus is an area in the brain, and it's sensing. You have blood that circulates through the brain. It senses temperature, how hot your blood is.
Steven Bartlett: Like the thermostat or something of the body?
Dr. Stacy Sims: It is a thermostat. It's the appetite control centre. It's how your body responds to salt, how your body responds to protein, carbohydrate, whether you need more or need less. It's like the control centre for the most part.
For women who come in and do fasted training, the hypothalamus responds with alarm. It registers that we don't have any blood sugar, we don't have enough carbohydrate to actually do this kind of training. What it does is create a little bit of dysfunction and start downturning all the other systems that need the same kind of fuel because there isn't enough just to do these muscle contractions.
Steven Bartlett: That means you could end up losing muscle?
Dr. Stacy Sims: Absolutely. If a woman comes to me and says she wants to lose weight and has been doing fasted training—she gets up, has a black coffee, goes to the gym, does lifting and cardio, then isn't that hungry because she did a hard workout at the gym, maybe has a protein recovery shake and holds off eating her first meal until noon—I always turn to them and ask why they went to the gym. Because all they've effectively done is burn through their lean mass.
Your body needs to have some fuel and the first thing that goes is lean mass because it's a very active component of the body. It would be better for you as a woman to have maybe 15 grams of protein if you're going to do strength training, or 15 grams of protein with 30 grams of carbohydrate if you're going to do cardio and strength, because this is just enough to raise your blood sugar to circulate to the hypothalamus that there's some nutrition coming in. You're able to get that blood sugar working, you're able to get that blood sugar into the muscle, you're able to stimulate the mitochondria in the muscle to actually use some more free fatty acids, you're able to tell the liver that you can actually get through this and use these free fatty acids instead of storing them. It only takes a little bit of food to then have benefit for what you're doing.
For a man, if he comes in and says he has a black coffee, goes to the gym, does strength training, might do a little cardio, has protein afterwards, and then might delay his meal—that's all right because you have a longer window for recovery. The hypothalamus isn't as sensitive. You're not burning through lean mass. You're developing a stress on the body and we know that it's really good that you had that protein post-exercise because that's going to create some muscle protein synthesis and hold you over until you have your meal.
Steven Bartlett: I'm going to try and explain this to you like I'm a 10-year-old, which is the exact level of IQ I have on this subject matter. You've got this hypothalamus in the brain, which is basically this sensor. It's trying to figure out, make sure everything is in homeostasis. Everything is level. And a woman's hypothalamus is more sensitive. So if my partner wakes up, goes to the gym, has her black coffee, goes to the gym, does a big workout as she always does, her hypothalamus is going to panic a little bit more because it's going to assume that there's stress on the body now and it's going to look around to see if it has sufficient blood glucose levels. It's not going to because she's not had anything for a while. She's not going to have the sufficient blood glucose levels. So it's going to start burning her lean muscle mass. Which means that she's essentially—it's like one step forward, one step back. Super simplified. For a guy who has his black coffee in the morning, goes to the gym, does the workout, the body looks, and because the hypothalamus is less sensitive, it's less requiring of there to be higher blood sugar levels, doesn't care as much. So it can also tap more into liver and muscle glycogen stores. So it's going to say we have a little bit of blood glucose, we need a little bit more, so let's tap into those stores and pull them out. So it's less reluctant to go straight for my lean muscle mass as an alternative fuel source.
Dr. Stacy Sims: Exactly. That's interesting.
Steven Bartlett: What's the evolutionary story of this? Why does this make sense?
Dr. Stacy Sims: When we look tribally—and I might get hit by some sociologists saying this isn't completely true, but for the exception there are some tribes that didn't fit into this—but for the general idea from a biological evolutionary standpoint, when we had times of low calorie intake, when we had to go find the beast or go out and find calories, it was at a disadvantage for the woman to be pregnant or to have a baby, an extra mouth to feed.
In times of low food intake, the reproductive system or the endocrine system of a woman would wind down. She would become amenorrhoeic or lose her menstrual cycle for a while. But it didn't affect men in that same way because they had to lean up and get fitter and faster because they had to go fight the beast or go find the calories and bring it back.
When we're looking from that evolutionary standpoint, in times of low calorie intake or low food intake, a woman's body will start to conserve and wind down because it thinks that there's a famine coming. But for men, they're not as sensitive and the body responds by thinking that not a lot of calories coming in must mean there's a fight to prepare for. So the body leans up, addresses all the fuel systems so that it can tap into all these alternative fuel systems to have the energy to be able to go and fight the beast to bring the calories back.
When there's adequate calories available, we see that women will lean up. They'll become more acutely aware. Cognitive function comes up. Carbohydrates are really important. We see that there is a development of egg maturation, we have better endocrine pulse. That means that our hormones that pulse on a daily basis actually have the full pulse and return to baseline to encourage the body to have a really robust endocrine system. That's thyroid, that's our menstrual cycle, it's all the things. But when we start pulling the calories back, all that stuff winds down.