Dear Mark: Ketosis and Testosterone, Dehydration Hormesis, and Isomalto-Oligosaccharides

For today’s edition of Dear Mark, I’m answering three questions. The first one concerns a potentially combative and controversial topic: ketogenic diets. What’s the deal with their effect on testosterone? You can find keto anecdotes across the web both inspiring and flaccid, but what, if anything, does the science say? Next, might there be a way to derive beneficial hormetic effects from acute bouts of dehydration? It seems like every other stressor can actually make a person stronger, so perhaps an otherwise wholly negative one like dehydration might as well. And finally, is the prebiotic fiber known as isomalto-oligosaccharide safe and/or good to eat?

Let’s go:


There is a ton of conflicting information on the internet about whether the standard ketogenic diet increases or decreases testosterone levels. What is your opinion?



In women with PCOS, a ketogenic diet lowers testosterone. Sounds bad, right? In PCOS, lower testosterone is actually an improvement as the disease is characterized by unnaturally high testosterone levels. What about in men, the population worrying about T levels most — does keto improve testosterone in them, too?

Unfortunately, there aren’t many studies that examine the effect of ketogenic diets on testosterone in males. We have a bit of data about the opposite — an increase in carbs and a reduction in fat. In an older study, men who customarily ate a 40% fat diet were placed on a 25% fat diet; the lower-fat diet also had a higher PUFA/SFA ratio and more fiber. Their total and free testosterone levels were measured pre- and post-intervention. After going lower-fat and higher-carb, total T dropped from 22.7 to 19.3 and free T from 0.23 to 0.2. Resuming their old diets partially restored their lagging T levels. Again, this isn’t a keto diet study, but it gives us a small inkling. Higher-fat, lower-carb appears to be safe and perhaps beneficial for testosterone.

Another study in men compared a higher-fat (41%, PUFA/SFA ratio of 0.6) diet to a lower-fat (18.8%, PUFA/SFA ratio of 1.3) diet, finding that serum T was 13% higher on the higher-fat diet. Both diets were calorie matched.

But neither of these papers looked at ketogenic diets. Low-carb just isn’t the same as full-blown ketosis. We can’t extrapolate. So let’s speculate.

How ketogenic dieting might both lower and raise testosterone: Ketogenic diets tend to be inadvertently hypocaloric, which is why so many people have so much success using them for weight loss. When you eat fat and protein to satiation, reduce carbs, and avoid hyperinsulinemia, your appetite normalizes and you’re actually able to tap into stored body fat for energy. For many people, this makes it hard to eat as many calories as before. This is great if you want to lose weight. Indeed, in obese men, a low calorie diet will improve sexual function and raise testosterone. The effect of weight loss (and consumption of many thousands of calories in stored body fat to make up for the “missing” food) is enough to boost testosterone production.

If you’re lean, the effect likely changes. You don’t have the luxury of eating pounds of your own body fat. Ultra low calorie diets – which for some, ketogenic diets become – take their toll on hormonal health. Lots of the CRON crowd — the calorie restriction for longevity guys — suffer from poor libido, likely stemming from low testosterone levels. Also recall what happens to the hormonal profile of bodybuilding competitors as they approach competition and embroil themselves in heavy training and dieting. Their testosterone plummets.

As I’ve said before, I’m suspicious of chronic ketosis for most people. I’ve heard incredible stories from people who thrive on it, but I’ve also heard disappointing reports from people who’ve failed. What seems to be a safer bet for almost everyone is spending some time in ketosis, not all of it. Everyone can probably benefit from short dalliances with the ketogenic state. Most people don’t need to remain ketogenic for perpetuity, and they might even suffer for it.

The effect ketogenic dieting has on testosterone comes down to a few things, most likely:

Are you eating enough food?

Are you consuming testosterone-supporting nutrients like zinc, cholesterol, magnesium, saturated fat, and vitamin A?

Are you noticing any negative changes in libido?

Are you using it to lose weight — and succeeding?

I’d love to hear from people reading this: how has ketogenic dieting affected your testosterone?

Hi Mark,

Been a long time reader. Found a lot of your articles to be very enlightening and thoughtful. For example, after reading your write-up on sauna health benefit, I’ve been striving to go use sauna 2-3x a week. In a related note, after profuse sweating from working out and sauna usage, I am generally pretty dehydrated. Normally I’d drink some water from the fountain at the gym to rehydrate, but I was thinking if there would be any health benefit via hormesis from temporary acute dehydration. My home is 20 mins away from gym, so it’s not long I’m putting my body in a period of stress. Plus, I have more options of rehydration once I get home. Like to know your thoughts. Thank you.



It may be that short, targeted bouts of acute dehydration improve kidney function. Most research finds that the harder a person exercises, the better their kidney clears creatinine from the blood. Pro cyclists, for example, regularly dehydrate themselves over the course of a race and have better GFRs (glomerular filtration rate, a measure of kidney function) than recreational cyclists and sedentary people.

Whatever you do, don’t feel the need to “drink ahead” of your thirst. By all indications, an athlete’s performance actually suffers when they try to stay ahead of their thirst by drinking before they feel thirsty. Those who use thirst as a guide to hydrate enjoy better performance than those who drink before thirsty or ignore their thirst.

I don’t think you need to abstain from water after a workout. I think you should just replenish your fluids when thirsty. The stress you experience training while thirsty is probably enough to provoke an adaptive response, if one’s going to occur.

For me, I’m rarely thirsty in the middle of a workout, but I’m definitely parched immediately after. My guess is it’s worked, since I can hold my own without constant access to water.

Still, be very careful with this one. Most people would think you’re crazy to even try, but I doubt it’ll hurt. If you’re really dehydrated, you can always stop for a drink along the way (or keep a bottle of water on hand).

Dear Mark,

As a 19-year-old in the digital age, the humor is not lost on me as I type this–asking your opinion on something called isomalto-oligosacchardies (IMO)–when the abbreviation “imo” is commonly used among my peers to communicate the phrase “in my opinion.”

Anyway. IMO: Stumbled across a recipe that called for this stuff. The brand was “VitaFiber.” The blogger touted the ingredient for being an alternative sweetener that is not a gut-disrupting sugar alcohol. In fact, as the name would suggest, it provides prebiotic fiber. I’ve read that it can be made from starch derived from just about anything. I’m assuming IMO made from grains would be out then. But what if it was from tapioca?

imo…tbh (to be honest)…the amount of experimental paleo baking I do, combined with my love of fruits, I’d like an option for sweetening a treat without necessarily relying on raw honey or grade B maple syrup. I know this is like the holy grail for primal beings like me with a stubborn sweet tooth and vivid memories of wheat/sugar/junk-based desserts & candy. So I understand if you’re sick of the sweetener questions. With the overwhelming number of options out there, and all of the businesses looking to dominate the market, I haven’t dwelled too much on the topic myself. I haven’t tried stevia yet, which seems to be one of the more primal-approved options. But I often wonder whether there is in fact a good (not just decent) human diet “hack” for this issue.

Many thanks for your time and all that you do.


Isomalto-oligosaccharides aren’t common in nature, which makes people wary. That’s understandable. But we should do our due diligence and really look at the evidence before condemnation or celebration. Let’s do that now. What does the research say about IMO?

IMO seems fine and even beneficial, IMO. For those not interested in messing around with VitaFiber, Quest Bars use isomalto-oligosaccharides. As far as commercial protein bars go, the Quest ones aren’t half bad. Whey isolate, stevia to sweeten, various whole food ingredients (cocoa, nuts, etc), plus the isomalto-oligosaccharides? You could do worse.

That’s it for this week, everyone. Thanks for reading and be sure to weigh in with your thoughts and responses down below!

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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38 thoughts on “Dear Mark: Ketosis and Testosterone, Dehydration Hormesis, and Isomalto-Oligosaccharides”

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  1. For once I think I disagree with you! You advise against drinking ahead of your thirst, but I’m so bad about drinking enough water throughout the day that if I don’t consciously sit down and drink a glass of water every few hours, then I’ll ‘forget’ and not pay attention to thirst signals when they arise. Living, and running, in Florida makes this a dangerous negligence on my part so I’m happy ‘drinking ahead of my thirst’ in an attempt to curtail any dehydration issues (I can’t tell you how many times I’ve had heat exhaustion training in the summer.)

    1. I concur. Depending on your climate your thirst can lag way behind your actual needs.

      I prefer not to drink during exercise, but I do before ans after. Before I surf in the morning I drink one big glass of hot water, followed by a big mug of green tea and a chocolate Larabar. Then I go surf from 1 1/2 to 3 hours. After I get out I drink some more warm water…maybe 12-16 oz. Not a ton. Then if it’s a planking or spinner bike day, I do that, then drink some more afterward, then have breakfast. Which you can imagine tastes AMAZING after that much exercise.

      I’m convinced that my power drinking before is what enables me to maintained sustained activity without a lot of hydration post workout and none during.

      1. I’m late to the party with this comment, but I just HAVE to add something. I think Mark’s a genius, but this one subject helped me realize that it is unwise to read advice on any subject, and assume it’s true for everybody, in all cases.

        Years ago, when I started to use Mark’s nutritional advice, I jumped on board with his opinion that drinking a lot of water is overstated, and unnecessary. While I enjoyed all the health benefits of changing my diet, I continued to have digestive issues. Things just did not move (sorry), regardless of how much I increased my vegetable consumption. This problem wasn’t while my body adjusted to my new diet. This went on for a year.

        I finally did an experiment, and started drinking a lot of water again. I drink it all day at work, and at home too. The water completely solved my digestive problem.

        When I go on vacation, or change my schedule and I’m not able to drink my normal amount of water, I have the same issues. It’s very clear to me now.

        I just wanted to say that, in case there is someone else out there like me. Some people do so much better drinking more water.

        1. Great to know. I have the same issues and even after upping my fiber to 30+ grams per day by way of berries and vegetables, it make almost no difference. I’ll try increasing my water intake and see if that clears me out.

        2. Second to Erica and Miki! But it really depends on each individual.. i have tried drink after the thirst and before… my body prefer and function better for the “drink before thirst” ! I will experience headache if i drink after thirst >.<

  2. I believe in personalized carbs intake, requirements change from individual to individual, and even within the same individual according to the situation.
    Low-carb diets won’t impact testosterone levels as long as the adrenals are healthy and the blood sugar doesn’t regularly drop below the emergency level (read: doing low-carb and crossfit). If several stressors cohexist, a low-carb diet which triggers ketosis is not appropriate, and it is not just because of the testosterone.

    1. This x 1000. I always guide my patients to a decent carb intake. I usually start them out at about 25% CHO and adjust from there.

  3. Never had libido problems and my erections are rock hard but i was fat blob for a few years. I used dukans diet (which is low fat low carb ketogenic) and lost about 20kg in a month and a half. I did not measure my T but when i adapted to ketosis my erections became hard as a diamond. With return to “normal” food (my biggest regret is that i did not transitioned to paleo/primal) my erections also returned to their normal rock hard state

    1. How can a ketogenic diet be low fat? Wouldn’t that really be a starvation diet?

      1. You need fat to create ketons – to be ketogenic. In low carb and low fat scenario You become source of fat. When one have body fat over 35% it is low fat intake diet but high fat metabolism. With enough body fat, proteins and micronutrients Your body is realy far away from starvation.

  4. Mark, if I were the Quest bars company, I’d fly to California and give you a great big kiss!
    That’s the second time you’ve called their products “not bad” in recent history. High praise, coming from you. ????
    I throw a quest bar in my bag if I’m headed out with friends & not sure where we’ll stop for food. If I have a better option or I’m not hungry, the quest bar will keep. But, it’s a nice little portable insurance policy.
    Odd, but if I have one in my bag, I find I’m just not hungry. If I forget to throw one in there, somehow I’m ravenous!

  5. Interesting to see you mention Quest Bars in that way. They seem to be popular in the weight lifting/modeling arenas. I think they taste great and if we have them around I will usually split one with my husband in the evening if we want something sweet. I am not convinced they “improve” bathroom experiences though, they tend to speed things up for me…which isn’t necessary with my Primal lifestyle… :/

  6. re: As far as commercial protein bars go, the Quest ones aren’t half bad.

    Actually, it might be “half of them are bad”. About half of the flavors needlessly contain sucralose, which I avoid, and their newest flavor contains both sucralose and “soluble corn fiber” (which triggers my suspicion meter).

    The flavors without sucralose are fine, although too low in fat to be a meal replacement, but then, nothing else on the market is more attractive.

    The market is still wide open for a low net carb high-fat meal replacement bar that uses sane fats and avoids adverse other ingredients. I haven’t found one yet.

    1. My emergency bar is a jar of coconut butter and a bag full of macadamias…

    2. I’ve found 2 really great bars recently: Marigold bars and Keto bars. They’re both relish and good ingredients! I personally think Quest bars have too much “junk” fiber and make me feel bloated.

  7. I’ve never had a Quest bar. I have to say I’m a bit wary of the IMOs with my own sensitive digestion, but great to get your opinion!

    1. I have leaky gut and poor digestion. I am highly allergic to Quest bars. I thought it was the whey isolate and that is probably part of it. But I tried a new vegan bar with IMO in it. I am getting the same horrid hystamine reaction as I did with the Quest bars. Unbearable itching. I am extremely sensitive, so I am probably a rare exception, but thought I would add my experience.

  8. Mark, thanks for all the assistance you provide so many people. You’ve helped me a lot. Since getting rid of the grains, sugar, and starches and upping my good fat intake I feel so much better than the bad old days and I’m in my mid-40’s now.

    I have been in ketosis for probably 95% of the time for the past ten months and feel well so far. I was under the impression which could be easily mistaken that it isn’t just the total testosterone number that matters but also the estrogen numbers and that the ratio of the two needs to be looked at. Since body fat produces estrogens I would have thought that having a lower body fat percentage; continuing to eat higher fat to provide the cholesterol hormone base; and lifting heavy things would really boost the testosterone.

    But perhaps I’m mistaken.


  9. i am a type 1 diabetic who has eaten low carb (60-80 g/day) for approx 3 yrs. I recently did a 3 month stretch in ketosis just for n=1 purposes. my libido, which was strong anyway, did not seem to change, and though i did not measure T, in months 2 and 3 i added serious upper body mass like i never have before. i lost 10 lbs in 3 months trying to make the diet as isocaloric as possible

    1. lol, yeah, i was trying to figure that one out for a while…until i realized i was trying to understand another man’s erection before/after ketosis…that’s when you know its time to move on.

  10. “In women with PCOS, a ketogenic diet lowers testosterone. Sounds bad, right? In PCOS, lower testosterone is actually an improvement as the disease is characterized by unnaturally high testosterone levels”

    IMHO, what a ketogenic diet does is NORMALIZE hormones, not lower select hormones. High testosterone is not normal in women, the ketogenic diet balances the hormones, which happens to involve lowering testosterone in women with PCOS.

    So I don’t think the same thing would happen to men on a keto diet–normal hormonal function for them is high testosterone. Since my husband has been following a mostly primal diet, his testosterone levels have come back to normal ranges.

    1. Which I find fascinating… as someone with PCOS and resultant hyperinsulinemia, without about 80g of carbs in my diet I feel dead. I lost about 2kgs in ketosis straight up and then it stalled and nothing ever changed.

      I find the medical community’s focus on lowering testosterone to be interesting because from my own research and my own symptoms and hormone levels it indicates excess oestrogen is the offender, since fat receptors come from oestrogen, as does the acne, (and I also suffer from endometriosis, a known symptom of excess oestrogen flowing around the body). I would’ve thought if I had testosterone dominance I would be lean and muscular. Admittedly I have always been extremely muscular with little effort, considering I’m a girl.

  11. I’m 63 and my erections are massive!
    Actually, I have noticed a positive change in libido when I’m eating clean, (low carb). When I’m eating more carbs and gain a little weight, I see a decline in libido. I’m not sure if it’s from low T or just how I feel looking in the mirror seeing a fatter body. I think GH has a lot to do with it, as does mood. I’ve seen a positive change in libido doing HIT exercise, so there are a lot of factors in play here. I’d like to see a study good study done on this issue. Getting older, it becomes more important to keep the T up.

  12. Drinking ahead of thirst…..

    I do Sumits Yoga, which is a vinyasa flow in 105 to 110 degrees. I believe that I am better off drinking ahead. I routinely sweat off 5 to 7 pounds of sweat in a class, gross as that is. The little bottle of water I take in cannot keep up.

  13. Just a FYI… If you were interested in making your own type of “Quest bar” … There is another corn-free syrup I just got this week called Fiberyum… It’s from tapioca.

  14. Since I cannot at the present do any HIIT I have been sitting in a sauna three times a week to help get my heart rate up a tad, hopefully help a bit with the torn abdominal muscle, and relieve anxiety. Since you are in a wooden box at temperatures potentially in the range of 160 to 190 degrees F, I highly recommend taking a bottle of cold water in there with you (I typically drink fluids at room temperature only, so in this case it is an exception). I sip it from start to finish of my session in the sauna. I’ve heard too many stories of people getting dizzy, passing out, etc. in a sauna. I have no empirical evidence to support the wisdom of doing this, but it has served me well, I come out with a good sweat and not feeling dehydrated.

  15. I would caution those who think states of dehydration after exercise might be good for you – dehydration leads to low blood pressure which can precipitate kidney failure not to mention all the little shreds of muscle tissue that the kidneys get clogged up with.

    Read the part about hydration towards the end at least if the article bores you.

    When people with rhabdo come to the hospital, we tank em up with liters of normal saline. isotonic crystalloids is how the article refers to it.

  16. Hydration: Yeah, I’m pretty sure our paleo ancestors carried a gourd of water in their hands where ever they went.

    Ketogenic diets and testosterone: I’ve read that the best way for a man to increase his natural testosterone is by intense exercise (but not every day) and by sex. That would include solo sex too. There was a study in Germany some years ago that showed that merely getting an erection caused the body to produce more testosterone. (And people think all porn is bad.)

  17. I think “chronic ketosis” is a misrepresentation of long-standing reality and tells our critics they are ultimately right. I think “chronic glucosis” is far more descriptive of current reality. In the ancient world of unsteady food ketosis was a steady source of energy. This is a fantastic advantage. No crashing blood sugar, no brain fog, no running out of energy running down food, or digging up food or making a baby. There is a reason why our fat tank is large and our sugar tank small: The latter is secondary, auxiliary even, for limited use. Ancient man was ketonic nearly all the time and for nearly all his life. There is no such thing as “chronic ketosis.” That just an excuse not to deal with reality. Ketosis is man’s natural and prime metabolic state. To define a state of ketosis as “chronic” is to fundamentally agree with the high carbage crowd.

  18. My wife and I have been eating primal since jan. 1st. To keep this short, we have experienced ALL the awesome effects. And I mean Healthier than we have ever been, etc. BUT we are both seeing pretty serious hair loss and it is devastating, but I don’t know what else to point to except our primal diet. I don’t want to go back to pasta and bread and being unhealthy, but something is going on and perhaps it is the testosterone situation? I’ve been googling my brains out and noticed a few hair loss articles on eating too much fat as a potential hair loss problem, one stating that animal fat increases testosterone especially DHT! I would love any advice. We are eating pretty high fat meals. 1 piece of bacon omlettes cooked in the bacon fat and some coconut oil added, with berries and mango’s in a small dab of high fat greek yogurt. Salads for lunch with chicken or shrimp on it. And dinners with a ton of veggies, broccoli brussel sprouts, but often with grass fed butter and some bacon lard on it with salmon, etc. Basically get all our meals off paleo websites. Califlower pizza, etc. I think we are at about 120 carbs per day on average. Eat a couple brazil nuts a day. Supplements are fish oil,

    Would love ANY input, feel so great but this one thing is kind of devastating. Mark seems to have a nice head of hair.

    1. Watch out for multiple Brazil nuts every day – too much selenium can cause hair loss.

      1. Thanks! Been eating one in the morning and one at night for several months. I probably wouldn’t have considered that small little portion, but I get so diligent and in a routine that perhaps I have some brazil’s with high selenium content and overtime it is built up. Going to cut way back and see what happens. Would love for that to be the problem. Just have never felt better in all aspects of my health in my entire life and I know it is diet related as both me and my wife are experiencing the same thing. She is dealing with perimenopause and if it was just her hair, I would attribute it to hormone imbalances, etc, but with both of us experiencing it, has to be something we are doing. Sincere thanks for taking the time to respond, most wouldn’t take the time!

  19. Just my own experience on ketogenic diets and testosterone…
    I never tried keto until I was 18% or lower body fat, and my testosterone was already on its way down. I never actually had labs, but based on sex drive it was not good for my hormonal balance. I had plenty of energy after the initial lagging as far as daily activities, exercise and training though. Later when I realized that my levels were low and getting lower I tried various dietary and lifestyle changes with nothing having an improving effect on total test or free test, though the best balance of hormones was with a varied diet, high in protein with moderate fats and moderate carbs from many sources.
    From my reading now I suspect that the lower level of fat consumption is much more important in regards to health, too low fat can cause hormonal crashing, poor balance of hormones, while too high is less damaging in that aspect. As I learn more and science discovers more I am sure I will continue to change my ideas.

  20. Great article, from personal experience and loosing 100+ pounds on ketosis I can say chronic ketosis is really beneficial to me personally on a mental level. I ate the typical high carb american diet up to obesity and after battling with diets and workout regiment for years I came upon on Dr. Attia’s and Gary Taubes work and the results have been life changing, aside from the obvious benefit of no longer being obese there has been what a nutritional layman such as myself can only describe as a reset of my hormonal levels. When I return to eating carbs (occasional dalliances back into my old ways) I feel… sick. Not in super significant way but I feel… lazy, slow, can’t focus, and noticeable drop in what I assume to be testosterone levels. It seems as the instant I switch back to the traditional diet, I start gaining weight, get brain fog, seemingly constant cravings, loss in sex drive, you name it. But when I’m on “chronic ketosis” which has just become my reality I feel incredible, in a subtle way but everything body-mind related feels better, I’m rarely hungry, and when hunger does return it comes without any cravings, I can just go for a 24-48 hr fast at the drop of a hat with minimal discomfort. For me the tradeoff of what undeniable can be an inconvenient diet/lifestyle is worth it, because the benefits that at least for me creep into every aspect of my life.

  21. Hi, I have noticed a positive change in weight when I’m eating clean, I mean low carb. Anyway, thanks for the article, I’ve just subsrcibed your blog.

  22. Hello Mark. Is IMO okay on a low carb diet? I don’t consume fruits so use this to make my own protein bar. I train to be healthy and very lean All year round as a personal goal/regime. Would love your valid input..