Dear Mark: CrossFit and Carbs; How to Lose Weight as a Teen

CrossFit FemaleFor today’s edition of Dear Mark, we’ve got two questions. First, I handle one from Heidi, a full-time CrossFitter with low body fat, a love for carbs, and a desire to burn more fat than sugar. Find out whether her love of CrossFit is compatible with her desire to stick to a low carb Primal way of eating. Next, I cover a question from Sam, a 14-year old teen who wants to drop about 15 to 20 pounds. He’s currently eating Primally and following the P90x routine, but is there another, perhaps better way?

Let’s find out:


I just recently joined your subscriber list and downloaded your ebooks. I have a question regarding the transition from burning carbs to burning fats.

First a little background: I am a 40-year-old woman who has been pretty active her entire life. I have been doing CrossFit for the past two years and really enjoy the intensity. I am 6 foot tall and 150 pounds. And last I checked my body fat percentage is about 14%.

I absolutely love carbs and figure that my body is addicted to carbohydrates and uses them for fuel. Last year I tried the paleo diet and after about 10 days I got extremely sick. I had a severe migraine and could not function and craved carbohydrates uncontrollably.

What is your suggestion for helping my body make the transition to burning fat versus carbs? And how long do you think that would take?

My end goal is just to maintain a low body fat percentage, nice muscle tone, and overall health.

I respect the information that you are providing and look forward to your response!


Glad to have you aboard! Thanks for following.

CrossFit is a notorious carb-burning modality. That’s fine, but it’ll be hard to maintain a very low carb or ketogenic diet while engaging in near-daily CrossFit workouts. That’s just the nature of the beast. Sometimes, carb cravings should be heeded. Sometimes, you just need more glucose. This sounds like one of those times. Cravings aren’t always an example of your body sending unreliable signals. I’ve always said that eating some carbohydrate after a high-intensity, glycogen-depleting workout (like a CrossFit WOD) is the best time to eat it. You’re more likely to fill glycogen reserves that way and your muscles are insulin sensitive and thus require less to do the job. Exercise even up-regulates something called non-insulin dependent glucose uptake, a glycogen-repletion pathway that allows carbohydrate utilization without any insulin at all.

However, there’s emerging evidence that given adequate adaptation time to a ketogenic diet, you can maintain or even improve power and strength output. For instance, there was the group of trained gymnasts who lost body fat and maintained or even improved their strength levels (hanging leg raise, pushup, dip, pullup, squat jump, countermovement jump, and continuous 30 second jump were all tested) on a zero carb, meat/fish/olive oil/green vegetable-based ketogenic diet. They were given a full thirty days to adapt. Most people don’t give themselves an entire month before declaring a diet useless or dangerous or bad for physical performance, but this study suggests that they should.

There’s also the cyclic ketogenic approach, where you eat lower carb most of the time but load up on Primal safe starches and fruit post workout. And by “load up,” I don’t mean the 400-500 gram carb fests that people embark upon after doing yoga for 45 minutes. Try to find your minimum effective dose of carb repletion. 200-250 grams should be plenty for most CrossFitters on a WOD day. Start at 200 and see if you need more – or less.

Also, you might consider increasing your body fat percentage. That’s a low enough percentage that many, if not most, women in that range begin feeling and seeing negative symptoms, including carbohydrate cravings and low leptin levels. Remember that body fat secretes leptin, a satiety-promoting hormone. Ample leptin (and good leptin sensitivity) is an indicator of adequate energy availability to the body. Low leptin (caused by inadequate body fat stores) indicates inadequate energy availability to the body; food cravings are a natural response.

If you’re balking at increasing body fat, 14% is very low body fat for many women and looks quite different than 14% on a man. This isn’t scientific, but here’s a picture showing what 15% actually looks like on a woman; here’s one showing 15% on a man. Consider shooting for 20-30% body fat levels. That sounds like a lot to you, I’m sure, but you might feel better with fewer cravings. Here, take a look at what those levels look like on a woman. You obviously know how to get back down to 14%, so if the higher levels don’t work for you, you can always cut down. It’s worth a try to see if your cravings subside and performance maintains.

Bottom line, though: if you want to become more of a fat burner, you have to cut back on the exercise because the CrossFit is exhausting your glycogen stores and demanding that they be replenished with more carbs. In most people, fat simply can’t support that level and volume of intensity. That’s okay, if that’s what you’re going for, but I suspect you’d be happier and overall healthier if you toned down the workouts, kept the bulk of your carb intake on workout days, and did a lot more easy, slow movement – walks, hikes, even pure strength training, as opposed to 30-minute WODs, are more conducive to fat-burning. Don’t give up CrossFit. Don’t give up the WODs. Just do it differently (maybe your box has a strength training or Olympic lifting-focused day that you can try out instead of the metcons for a change).

Good luck!

Hello Mark,

I am a 14 year old boy that is 15 to 20 pounds overweight and I am doing the Primal Blueprint diet and doing the P90X program 5 to 6 days a week. I only do 30 to 40 minutes of each video and I do abs every day. I was just wondering if you had an estimate for the time it would take for me to get in as good shape as an average high school boy my age.


Well, seeing as how about 21% of teens aged 12-19 are obese (as of 2012) and even more are “just” overweight, I’d say you’re doing better than the average high school boy. The fact that you’re reading this blog, asking questions, and going forward with an exercise plan of your own puts you in rarefied territory. Hats off to you, Sam. It’s a great start.

You’re probably about to start full blown puberty. That’s where your voice deepens, you get some sweet body hair (if you’re genetically inclined to grow body hair), and, most importantly for your situation, your testosterone levels skyrocket. Thanks to the testosterone boost, it’s also where pudgier preteens begin to lean out and build more lean mass. This happens without you doing anything in particular. It’s purely hormonal; your body is just following the script. If you’re eating well and exercising, the effects will be even more pronounced. I suspect you haven’t hit your testosterone phase.

If I were you, I would make a few changes to my workout routine. P90x is great at improving endurance, cutting fat, and increasing your pain threshold, but it’s not amazing at building lean muscle. Instead of 5-6 days a week, try 2-3. And cool out on the abs. You don’t need to do them every single day, and the heavy emphasis on sit-ups and crunches can negatively affect the stability and health of your spine, especially when you’re 20 minutes into your fourth straight day and your form goes out the window. Planks, hanging leg raises, ab rollouts, and even squats and deadlifts are excellent ways to strengthen your torso (or “abs”) without the volume of P90x.

Lift some actual weights, if you have access. Although people will tell you differently, no research has ever shown that weight training actually stunts growth in youth. Kids who lift enjoy stronger bones (which extends into adulthood), increased lean mass, improved insulin sensitivity (which will help you lose body fat), and overall better health – just like adults who lift. Lifting heavy things will also build some nice stuff to be revealed once you lose the body fat.

Primal Blueprint Fitness, primarily a bodyweight program that can be modified to include weights, is a great place to start. Plus, it’s free if you sign up for the newsletter (which is also free).

There’s also non-workout stuff you can do. You’re 14, which makes you an incoming freshman, right? How about going out for a sports team? If you don’t feel up to it (high school can be intimidating, especially if you don’t feel you’re at your physical best), you can always play on the side and then come back next year ready to dominate. Consider non-traditional sports, like Ultimate Frisbee, swimming, or maybe lacrosse. They’ll get you really fit without the level of scrutiny applied to basketball, football, and baseball.

You should try active commuting to school, if it’s feasible for you. Walk or bike to school instead of taking the bus or getting a ride. Remember, attaining and maintaining optimal weight is much more about integrating constant physical activity throughout the day, rather than focusing on punctuated bursts of intense activity. Since you’re probably sitting down for most of school, walking/biking to and from campus can really help mitigate all the inactivity. Throw a few workouts in a week, some play/sports, a weekend hike or two, and you’ll be in great shape.

Let me know how it works out for you, Sam.

That’s it for today, folks. Thanks for reading and be sure to chime in with your words of advice or encouragement for Heidi and Sam (and anyone else who might be in a similar position)!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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42 thoughts on “Dear Mark: CrossFit and Carbs; How to Lose Weight as a Teen”

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    1. When I was a youngster we had to rely on the three television networks for information, also it was the fashion to wear an onion on our belts.

      1. I remember when onions cost a nickel, and in those days, nickels had pictures of bumblebees on ’em. “Give me five bees for a quarter,” I’d say.

        1. Now where were we? Oh yeah: the important thing was I had an onion on my belt, which was the style at the time. They didn’t have white onions because of the war. The only thing you could get was those big yellow ones…

    2. I studied martial arts as a kid, which was great. But I knew nothing at all about weight lifting or eating until I was 27, which is sad.

      I could have spent my early 20s as something other than a 6-foot, 130 poound weakling!

      1. Spoken like a 20-something who doesn’t realize all the great living they have ahead of them! The 30’s are better anyway, and if you’re Primal (and don’t get too much sun on your face) you’ll LOOK like you’re in your 20s. Carpe diem, man.

  1. Looking at pictures of women at different fat levels, my gut reaction to the 10-12% is “Knarly, dude!” And 20-22% starts to look good *to me*, and 25% looks great. But based on prehistoric figurines, Grok would have fantasies about 50%. Not saying that women *should* aim for 25% – It’s just that I went through adolescence back when women had curves. Sam might not feel the same way.

    1. +1.
      This sickness of being too skinny has got to go. Every man I know wants a gal with some meat on the bones.

    2. Women’s figures have evolved. If you watch a few of the old black and white movies and take notice, you’ll see that women’s butts were much bigger in the 40’s and 50’s than they are now. In fact, they were bigger all over, though not considered fat. Now days healthy women who aren’t overweight are less curvy and more muscular. We’ve lost that farm-fed, hourglass look. I don’t think this is necessarily a bad thing. IMO, much of it can be attributed to more exercise. Women of my mother’s generation didn’t hit the gym. They rarely even ventured out for a walk.

      1. First, I wasn’t passing judgment. I like the figures of women in those old movies because . . . that was my era. I’m 71.

        Second, after watching Esther Gokhale’s presentation at AHS2014, I realize that, as in so many other areas of health, people now don’t understand how to stand. Don’t tuck in your butt; be proud of it and stick it out.

        Third, most women don’t hit the gym, or they don’t do it productively.

        Fourth, farm women are muscular and work hard.

        Finally, most young to middle aged women today are probably 35-50%, usually with the fat around the middle.

        1. Harry, I wasn’t criticizing your comment. Just pointing out, in the same thread, that women’s bodies have changed over the years, likely because food and exercise norms have changed.

          Also, the concept of an attractive female body has changed among the younger generation. Thankfully we’ve moved away from the emaciated Twiggy look into a more healthy phase, but the overblown hourglass look of previous eras is gone too. Frankly, I see no great loss with either one.

          While I do agree that far too many women carry their fat in the wrong places these days, I disagree that women don’t hit the gym. Or the yoga, pilates, kickboxing classes–whatever. Many of us do. It’s far more common for women to do these things now than it has ever been, and it does contribute hugely to a leaner, more sculpted appearance.

  2. Hi Sam. Do you read comments? Mark gave you such grandgrand advice, and if you follow it you’ll probably look like a bazillion bucks in no time. But always remember to be kind to yourself. pleaseplease. 🙂

  3. Dude! Are those pictures accurate? I’m 45 and 23% body fat and I look nothing like that 25% picture! When I was in my late 20s, I was 19% body fat and looked more like the 22-23% photo (without the arm definition). Maybe my scale is wrong and my body fat percentage is actually a bit higher? I don’t need to be 15%, but 19-20% would be nice (and I’d still be plenty curvy). I’ve been doing primal/paleo for 2.5 years and assumed that I would eventually lower my body fat. The result? I’ve just put on more around the middle, regardless of whether I’m moderate carb or ketogenic. Is 20-30% what I’m doing to myself by following these principles?

    1. Kim

      I’m 47 and what I have discovered, along with many in the Forum, is age makes a huge difference. Also there are very few scales that measure body fat accurately.

      The female body as it approaches menopause has different hormone requirements and they play an integral role in body fat. In fact, going through menopause with a few percentages more is helpful and protective as it balances out the loss of oestrogen from the ovaries. The body naturally tends to more fat retention at this time, for good reason, and I think it’s why many of us have struggled with a little more than we consider ‘ideal’.

      I’m trying to use wellness as my gauge now and try not to sweat the figures, or being a little more curvy.

      I’m really hoping Carrie’s Primal Woman will address some of these issues in her forthcoming book.


    2. I used to use one and my % never jived with what really fit people’s %’s were. Then someone told me how inaccurate body fat scales are and I did some research…Google it and you’ll find a ton of information on how inaccurate they are.

      They’re good if you want to track your progress (not good if you want your true body fat %) and only if you do it at the same time each day under the same type of hydration…as being a little dehydrated can skew the numbers.

    3. I’m 44 and the primal diet works for me health wise. But if I want to look athletic and fit, I found out the only thing that works is lifting heavy things and doing body weight resistance training regularly. That really does the trick. No amount of hiking and slow movement improves the way I look, it is just lifting heavy. I learned to accept it and now I like it.

      1. Frequent movement, lift heavy things, sprint. Yes that is the Primal Blueprint.

    4. Most of those scales use electrical impedence to determine body fat, and they do it by having you stand on electrodes… This means that they only measure from one foot to the other, by the shortest path… so your lower body(below the waist) is 23%. If you are carrying more weight in your upper body then that won’t be reflected… Conversely if you have very little in your upper body that also won’t be reflected.

  4. From my experience, people can CrossFit on very low carb and keto diets as long as they listen to their bodies and not try to PR every time they are in the gym. If you are looking for peak performance, you’re not going to be able to do it on a keto diet. However, if your goal is different than peak performance, then aiming to become fat adapted might be something to shoot for. Listening to what the body wants is the biggest thing here though.

  5. I was in full-blown puberty around 11 or 12. I thought that was normal?

    1. For women yes, for men it is later. Hard to tell from your name/ picture which you are, but assuming you are a woman based on your comment!

  6. Saying you’re “addicted to carbs” is like saying you’re “addicted to air.” Your body absolutely needs glucose to function every single day. They are absolutely crucial for a healthy metabolism, let alone a healthy digestive system. Your gut microbes outnumber your own body’s cells 10 to 1, and they need healthy, fermentable carbohydrate to function. Going unnecessarily low carb constantly is a great way to ensure healthy bacteria die off and pathogens overtake your digestive system, leading to allergies, food intolerances and poor glucose control. Eating lots of healthy, fermentable carbohydrate from whole foods is crucial to ward off allergies, food intolerances and remaining insulin sensitive.

    That being said, there is a huge difference between real food and refined garbage. Just make sure to get your carbs from real food, like potatoes, sweet potatoes, plantains, fruits, honey, etc. Y’all know the drill. Your body cries out for carbs because it needs them. Carbs are better at producing ATP than fats, so that’s why they better fuel workouts. Healthy fats certainly have their place in a diet, but carbs are more efficient than fats and it’s why your body prefers to metabolize them. Embrace them.

    1. Carbs can be made from protein. The Inuit must be from outer space. Longevity in mice studies based on calorie restriction may be more to do with insulin secretion over a lifetime: than calories alone. I eat carbs, they taste good and are cheap. I try and keep them low(ish) because I am overweight and (let’s say 32.5% on Marks examples) they give me indigestion (and I have other problems that are reputed to be carb created). Just sayin’.

  7. I think I was a pudding at 14…

    I was also awfully tired and headachey for about a month when I started eating fats instead of carbs (generally speaking). I think I was doing way too much exercise, not getting nearly enough calories, and still shirking fat a little .. .so do the opposite of that!

  8. I’m female and I started early puberty at 10 and was in full blown puberty at 12-along with a four inch growth spurt in three months. My ex husband didn’t hit full blown puberty until he was nearly eighteen. He was 5’6″ all through high school and grew 6 inches at 18 when he was in the army in boot camp. It is really variable. Both my daughters went through puberty a bit earlier than myself and my son is still years away from puberty. I have noticed that many of the boys get a bit pudgy right before they hit their growth spurt when it just goes away.

  9. 14% seems like super low body fat for a woman, I can’t imagine that’s something she’d feel well on for the long haul regardless of approach to her diet. Also as a runner who eats paleo but not low carb it’s definitely possible to add in a few extra carbs to build glycogen when needed while sticking to real food.

  10. Heidi,

    The transition from burning carbs to fat is tough. Think of it like the process of stretching a hamstring muscle. It hurts at first but you know you need to do it. Once you can comfortably hold your toes, it feels great and your whole body is better off for it.

  11. I love the advice to the teen! It’s easy to over do workouts and start down a bad path that builds a lifetime of bad habits!

  12. Such great advice! The beauty of PrimalBlueprint is how easily you can adapt it to your personal goals. Good luck Heidi & Sam!

  13. Great advice for Heidi – although it may be hard to hear, after years of thinking leaner is better, this may seem very counterintuitive. Having seen those comparison photos before, it made me rethink my attitude to leanness and overall female health. And I feel a shedload better for it. I hope you’re open to it, Heidi.

  14. I am also a Crossfitter, and want to say thank you for this response. I also want to say thank you for the You Tube videos lately they have been great and I love listening to them, even after reading the books.

  15. I am a p90x er myself. Usually after my workout, I will have a grass fed whey proteing shake. I use almond milk and I throw some frozen mixed berries and a banana in there. If usually does the trick. I will rarely have bread or pasta. I never have toast or a wrap with my breakfast. I have a buddy who is a serious cross fitter and also has celiac. He eats steel cut oats every day. I used to eat them a lot, but i’ve cut them out the last few months. I love Alton Browns recipe ( u can search for it , not sure if im allowed to link to it here). I hover btwn 11-12% bf and I find that the carb intake from fruits and veggies are enough for me. I cant lie tho. In a moment of weakness I had 3 of my wife’s brownies saturday night. It was a little planned treat, and they are my weakness 🙂

  16. I’ve found in my personal experience that P90X is too high volume. It just leaves you too drained.

    You might try doing only 20-30 minutes of the DVD each day with heavier weights than you would be using if you were doing the full DVD. I will have to (respectfully and only partially) disagree with Mark on the abs DVD advice.

    My advice for doing the abs DVD is to follow the cues your body gives. If you feel great that day and your abs aren’t fatigued or sore, you are fine to do the DVD. If they ARE sore/fatigued, skip it that day and hit it again the next day.

    Just my $0.02 on what’s worked for me.

    1. i have done all the p90xs p90x 3 is my favorite. the workouts are 30 minutes. There is plenty of workout packed into it, and you won’t be totally burnt after like u are with other p90x workouts

  17. I am a primal female and I do crossfit 5-6 days per week. Cross fit is generally an hour of which 20mins is skill based (lifting, gymnastics, etc) and 12-20mins is WOD (metcon). It is not ever 30mins plus high intensity. I go at 6am and have breakfast around 8pm (an hour after). Breakfast is yoghurt/milk kefir, protein powder, berries and some homemade nut granola. I may not be the fastest in the class but I hold my own and do not feel in any way limited by my primal diet. I had been primal for almost a year before starting crossfit which probably helped. I think you may want to consider giving the wods a miss and doing strength training only whilst you adapt to the diet.

  18. Crossfit looks like as much work as the manual labour I used to do, but you pay them, classic. I like the examples of bodyfat (apart from the obvious of looking at girls in bikinis) but now if I call my missus pudgy and she pokes my beer belly, I can tell her Mark says I have a lower bodyfat percentage. Way to go Mark.

  19. Sam, walking is man’s best medicine (I didn’t coin that by the way). See if you can make it fashionable.

  20. Now I’m getting confused, while some people would say to tone down on carbs and load more on protein. I am trying to squeeze in exercise to my schedule (thanks to my husband for his encouragement). I do not know if a 30 minute exercise (zumba, wishing to move to the next level and do crossfit, somehow) plus protein loading is a good combo.

  21. Hey Mark, you wrote this above…”That’s okay, if that’s what you’re going for, but I suspect you’d be happier and overall healthier if you toned down the workouts, kept the bulk of your carb intake on workout days, and did a lot more easy, slow movement – walks, hikes, even pure strength training, as opposed to 30-minute WODs, are more conducive to fat-burning”

    Above what you stated isn’t the thinking anymore.

    You are correct that long slow training uses fat primarily as a fuel source. However, high intensity training uses just as much fat and more glucose/glycogen than long slow training overall. So, with high intensity training your total caloric expenditure in much a shorter time is higher, with just as much fat burned. The bottom line, you will not burn more fat from doing less.

  22. Hi Mark. Some great advice on your site. It’s great how you give personalized responses, but share so that everyone can benefit. Thanks

  23. ok – just my 2 cents worth:

    Crossfit – Crazy WOD burn out type mentality workouts – invented by a guy named Greg Glassman, who makes a lot of money off the modality and is sponsored by Reebok. Never does cross fit workouts himself. The mode seems to encourage bad form for more half done reps leading to high injury through “death by a thousand cuts” type burnout by not allowing time for muscles, ligaments and tendons to recover.

    Primal blueprint – Involves lots of low level cardio, Heavy lifting max 3 times a week with a minimum 48 hours between heavy workouts, and a weekly sprint, does not recommend daily WOD’s that burn you out (Muscles take 48 hours to recover, that just fact), but does have WOW’s. Formulated by Mark Sisson, through he’s own experience and practice. Mark actively follows this workout style himself.