The Hidden Dangers Of A Low Carbohydrate Diet


If you’re a frequent visitor to this website, or listener to the BenGreenfieldFitness podcast, you’ve probably gotten the idea that I’m a pretty big fan of limiting your carbohydrate intake.

And you’d be right.

To understand why low carbohydrate eating can bestow some significant health and performance advantages, check out my Perfect Health Diet interview with Paul Jaminet, or listen to the perils of constantly elevated blood sugar levels in this episode with Nancy Appleton: Which Foods Contain Hidden Sugar That You Didn’t Even Know About.

Or go read about how physically active individuals may be able to actually benefit from strategic low carbohydrate intake in my article 4 Reasons To Think Twice About Eating Carbohydrates Before A Workout or (if you’re a Rock Star Triathlete Academy member) you can read 5 Ways to Get A Big Carbohydrate Restricting Performance Advantage.

In a nutshell, pun intended, as you begin to increase carbohydrate consumption above the levels that you need for survival or periods of intense physical activity, you lose your ability to rely on fat burning mechanisms, and you experience the damaging effects of chronically elevated blood sugars, including neuropathy (nerve damage), nephropathy (kidney damage), retinnopathy (eye damage), increased cardiovascular disease risk, potential for cancer progression (tumor cells feed on sugar) and bacterial or fungal infection.

Unfortunately, whether due to a misinterpretation of what low carbohydrate dieting actually is or an “all-or-nothing” approach to restricting carbohydrates or perhaps the influence of low-carbohydrate-done-wrong diets like Atkins, many people (and especially athletes) try or attempt to try a low carbohydrate diet and end up messing the whole thing up, experiencing the hidden dangers of a low carbohydrate diet and hurting their bodies.

So what are the hidden dangers of a low carbohydrate (AKA “ketogenic”) diet?

Here are the low carbohydrate risks, in ten steps:

1. Your body stores carbohydrate, mostly in your liver and muscles, in the form of glycogen. Depending on your size, you can store roughly in the range of 1500-2000 calories of storage carbohydrate (although that number is fairly variable based on your fitness and size).

2. If you’re sedentary and don’t really exercise much (which I don’t encourage), this amount of storage carbohydrate is more than sufficient to get you through a typical day. Really, your body only needs a maximum of 600 calories of carbohydrate to survive each day – and that carbohydrate can be derived from diet, or from you own storage glycogen.

3. But if you’re active and at the same time consuming a low carbohydrate diet, you can easily burn through your liver and muscle glycogen stores in anywhere from 2 days to a couple weeks. The nice part about this, if you’re trying to lose weight, is that since glycogen carries up to four times it’s weight in water, a low carbohydrate diet can quickly shed 5-10 pounds (or more), which seems quite satisfactory. But the problem is, most of what you’ve lost is A) energy to sustain intense physical activity and B) water.

4. So now you have very little storage carbohydrate and are potentially dehydrated. If you’re an athlete or a physically active individual, this means that you’re limited to utilizing fat as a fuel for energy. Fat, through a process called “beta-oxidation”, can provide tens of thousands of calories of readily utilizable fuel, but the problem is that it burns far more slowly than carbohydrate.

5. This means that if you’re on a strict low carbohydrate diet, you can say goodbye to intense weight training, track intervals, or just about any activity that would be consider “tempo”, “threshold”, or “intervals”. And this is the stuff that adds lean muscle to your body, boosts your metabolism and gets you fit fast – compared to a slow and sluggish slog in your “fat-burning zone”. This is not negotiable by your body. It is simple physiology. When you deplete muscle glycogen, there is a directly proportional increase in muscle fatigue, and also an increase in muscle catabolism (direct metabolism of your body’s own muscle protein, or conversion of that protein into glucose via gluconeogenesis). Many people on a low-carbohydrate diet simply stop exercising, because it can suck so much.

6. As you lose muscle mass, your already handicapped metabolism drops even more. I will acknowledge that muscle fibers don’t burn as many calories or boost your metabolism as much as we all like to think, but this is still an important consideration for those trying to maintain lean muscle mass or tone.

7. For active people, this trouble may all be “in vain”. Since physically active individuals and athletes are far more sensitive to insulin and less susceptible to blood sugar fluctuations, any attempt to eat low carbohydrate in conjunction with exercise, for the pure purpose of “controlling blood sugar levels” could be a mostly unnecessary endeavor anyways.

8. Low carbohydrate diets, if implemented improperly, result in low fiber intake from a sharp reduction in plant-based food consumption, which can increase risk of digestive cancers and cardiovascular disease, and also leads to constipation and bowel issues. In addition, a drop in fruit, vegetables, legume and grain consumption can result in inadequate phytonutrient, antioxidant, vitamin C and potassium intake. Many (but not all) low carbohydrate diets have these problems.

9. Typical “low carbohydrate” meal replacement bars and shakes, ice creams or ice cream sandwiches, and other low carb or sugar-free snacks often contain potentially unhealthy ingredients like maltitol, and are chock full of preservatives and highly processed ingredients. If your low carbohydrate diet involves boxed, wrapped and packaged food, it probably falls into this category.

10. There can be long term health issues as your body is chronically carbohydrate depleted over extended periods of time. Your liver is exposed to extra stress as it is forced to assist with manufacturing glucose from fats and proteins, potentially toxic amounts of ammonia are produced as proteins are converted into glucose, your body has a more difficult time producing mucus and the immune system becomes impaired as risk of pathogenic infection increases, and your body loses the ability to produce compounds called glycoproteins, which are vital to cellular functions.

So is it possible to “do a low carbohydrate diet right”?


And later this week, I’ll be posting more about low carbohydrate diets, potential benefits, and how to do a low carbohydrate the right way.

But until then, feel free to leave your questions, comments and feedback below.


51 thoughts on “The Hidden Dangers Of A Low Carbohydrate Diet

  1. I understand that people who are into triathlons and marathons may need more carbs, but what about people who have brief moments of intense training like cross-fitters?

    The main focus for them is high insensitivity weight training and many of them are on ketogenic diets.
    Isn't then this type of diet more beneficial for people who are skinny-fat and want only to get leaner and more muscular and do not care about competing in endurance sports?

    1. Yes, but muscle glycogen is required for Crossfit intensity, unless you want to be a "back of the packer". You gotta be burning carbs to do well at Crossfit, at least acutely (during your WOD).

  2. Very interesting, and a topic I would love to hear more about (especially in the context of losing fat while maintaining muscle and improving strength)

  3. great topic. looking forward to the article and would love a podcast on what YOU think on the matter. my attempt at low carb is very high in vegetables, which hopefully limits the carb depletion you have talked about. excited to hear what you have to say! keep up the great work

  4. Interesting opinion, but that's all it is, and it's full of problems. You say low carbers suffer from high amonia byproducts because they produce too much glucose, yet they suffer debilitating effects from not getting enough glucose. Both sides are rubbish and abviously both can't be true. Produce one case of too much amonia please. Our bodies need very little glucose, and burning fat is not that slow. It does become slow when you overload yourself with insulin, but that;s why HFLCers do what they do. They become fat burning engines. Many athletes are now converting, and nobody is shrivelling away. Grok on! The hardest argument to take is your disease card. There's absolutely no proof that fiber does anything but make you poop more often and smell more. There's no proof that fat causes heart disease. There's no proof that fiber reduces cancer risks. Nothing! On top of that, you'll find that most low carbers eat much more plant fiber than the average person. It's not all bacon.

    1. Afraid you have it wrong, John. I never said ammonia byproducts were from producing *too much glucose*, but rather breaking down *too much protein*. I also acknowledged in beginning of article that you only need 600 cal of glucose per day, whether from glycogen sources or diet.

      Next, a high fat or high protein diet produces just as much insulin as a high carbohydrate. I encourage you to do your research on that matter.

      Finally, if you go read or listen to anything I've written on fat, I have already pointed out that fat does not cause heart disease. And there are literally hundreds of research studies that demonstrate chronic disease reducing benefits of fiber.

      Like I said (and you would realize if you had actually read my article carefully, instead of skimming and jumping to the comments) – there is a way to do a low carb diet right. But lots of people don't do it right, especially athletes.

      1. >>Next, a high fat or high protein diet produces just as much insulin as a high carbohydrate. I encourage you to do your research on that matter.

        A high fat diet results in just as much insulin as a high carbohydrate diet. Are you sure about that, it's counter to everything else I've read regarding low carb diets?

          1. That article says ZERO about fat produces insulin. Also the studied participants were on a mostly carbs, somewhat fat and low protein diet. Not keto. The body produces it's own glucose only through carbs and protein not fat and insulin can be spiked with protein and carbs because of this. Fat and carbs are pretty much the only things the body uses. Carbs cause issues as humans aren't built for the intake we get with the level of average activity of a human being.

            The only real dangers with keto is if you already have or family has a history of heart and/or liver problems definitely consult with doctor first or just lower carb intake, but don't do keto if you want to lose weight. And most importantly DRINK LOTS OF WATER. You need to flush out excess ketones to avoid ketoacidosis. Which is a type of insulin resistance such as diabetes II presents.

    2. Top answers John. So much error in some peoples understanding of low cabs… or should I say Crap Carbs ? As not one of the Low Carb guys including Atkins say to you need to be Carb free, just mainly crap processed carb fee, and things like High Fructose Corn Syrup. I have been a Low Carber now for 12 years, and at 61, 6 feet tall, I weigh in at 175 lbs and have a 35" waist. I have no illnesses at all and my blood pressure is just 110/70.
      So keep up the good work of standing on what you believe to be a good diet of natural food that does nto kill or maim you.

    3. Many people who are on low carb diet experience constipation,incluidng me,7 days constipated and I was not eating THAT low in carbs,around 80 grams of carbs daily.
      It is impossible to eat more plant fiber than the average person when you are on low carb,I know I am using something that counts all my carbs and etc.Even though I will still eat less carbs than the average person,I will not go very low in carbs because it seems that many people including me had very bad experiences.Moderation is the key.

  5. Looking forward to your podcast, I did Atkins and it worked for me loosing weight, the second wind of energy comes I think after week of being on it, and you are spot on with having limited energy reserves I did notice this. I mixed up my meat with lots of veg and did my best to stay hidrated, the whole thing seemed right but wrong at the same time and something aimed more up the middle and balanced is probably the answer to this kind of fueling for the body.

  6. From your comment #4, you seem to insinuate that your body prefers to burn carbs for fuel and will do so as a priority until the carbs are depleted, after which it will resort to burning fat for fuel. I came across a similar insinuation during a lecture in my Human Biology class the other night as well. This confuses me. I had thought if you're in an aerobic state your body would burn fat for fuel and not resort to burning carbs until you're in an anaerobic state. If carb stores are full, lipid stores are full and protein stores are full what fuel source does your body choose when in resting up to aerobic state.

  7. Great article, Ben. I trained for an NPC Bikini competition last year following traditional wisdom, you know, oatmeal, lots of protein shakes filled with chemicals, depleting water, etc and got down to 112 lbs at 5'3" and felt like death. Afterward, I went carb crazy and gained back 5 pounds by New Years. Back in the spring, I was reading a lot about low-carb Paleo and started doing that. I didn't lose an ounce. If fact, I got up to 130 lbs, all the while doing HIIT and weight training 5 days per week. After a couple months, I started eating more plant foods and cutting back on the protein and fat. I feel much better now, but I can't lose any weight. I've been at 134 lbs for over two months now and my digestive tract is not what it used to be either. (I'm taking digestive enzymes for that.) I feel like my body just went haywire on low-carb. Is it possible that some people just are not meant to "run on fat" like the Paleo crowd preaches?

  8. Thanks for this Ben! I am a relatively new female triathlete who is 5'5" 117, pretty lean to begin with. I just started Paleo this week and lost 3 pounds in 3 days. I like the diet, but am concerned that I may lose too much. Any ideas you might have for the lean endurance athlete eating paleo? What types of high-fat foods might make more sense to be sure to include daily? Thanks again!

    1. Most of what you lost is glycogen/water weight. That's pretty typical! Don't worry – that rate of weight loss will subside after 5-10 days. I'll be posting more on this Friday about high fat foods, etc.

  9. Looking forward to your article on Friday. I am 13 weeks out from IMAZ (are you doing that race btw?) and would like to drop 10-15 lbs before then. I did the TIm Ferriss slow carb diet from Oct to Feb of last year and lost 30 lbs and dropped 25 minutes off my half marathon time. Is that diet good for IM training? I did a lot of beans, mixed nuts, chicken, eggs, etc.

  10. You are wrong on some very key points here Ben. Carbs are not required by the body to survive, and you are not going to "run out of glycogen" as your body will make more. What you are saying might be true for elite athletes, but for the "average joe", even an active one, it's simply false.

    There are plenty of elite athletes who do not eat carbs and who survive just fine, including Triathelete Jonas Colting.

    The fact is if you are eating a high fat, moderate protein, low carb diet your body will run just fine, even doing training that you have suggested would be impossible.

    1. Mathew, I'm afraid YOU are wrong. The body does need glucose for several small organ functions (although not as much as common nutrition advice would lead us to believe). Re-read my article. I didn't say you need much glucose, but you do need trace amounts from dietary sources.

      1. Thats what gluconeogenesis is for. What on no carb high fat low protein your body converts proteins and fats into a very small amount of glucose for your brain. This is called gluconeogenesis and it is done so without eating any carbs so ben you are wrong

  11. I have a question. I did a low carb diet for about 3 months and I lost 4 kilos. I work out a lot, running and weight lifting. After about 3 months all my veins started showing really bad. It looks awful and I hate it. (I am a female) It is like my skin has turned into glas, all veins show. I also feel that my muscles are less toned. Why is this? How can I fix it and get my body back ? Please help – I am desperate!!

  12. I long should it take to remedy the "see-through" skin problem. I am 40 years old. I have now started eating plenty of good carbohydrates and drinking lots of water. (I have to admit that I was not drinking much water at all earlier so I think my skin got severely dehydrated) Can I get my skin back and hide my veins again?

  13. I spoke with a doctor last December 2011, cosmetic surgeon, and she suggested 30g of carbs per day for women over 55 years old like me trying to lose weight with no success. I resent her suggesting that since I've been reading that could've caused serious complications and bad side effects like kidney stones. What are your views and can any damage from being on this regimented diet be reversed? I eat more carbs than that now but was restricting carb intake, from fruits mainly and whole grains, for several months.

    Thanks for the feedback!

    1. The kidney stones would only be an issue if you replaced that carb intake with PROTEIN rather than fats…you simply need to ensure that you are eating a high fat, moderate protein, low carb, and not traditional "Atkins style" high protein.

  14. Hello. I am a 34 year old female and suffering from the effects of a low-carb diet. My naturopathic doctor put me on the Paleo diet due to me being insulin resistant. After four plus months of following this diet, I weigh 100 pounds and am dealing with anxiety attacks, depression, low immunity (just trying to get over the cold/flu), no energy (no exercise), etc. I am now eating carbs such as rice bread, baked potatoes, millet, oatmeal and of course fruits and vegetables. Any further advice on how I can reverse the effects of this terrible diet? Thank you.

  15. My husband and I are starting a diet which is only going to involve lean protien (chicken, turkey mostly) and only getting carbs from vegetables that we will blend in our ninja blender. I know we must stay hydrated but I was told to only drink distilled water with electrolytes such as Smart water. Is distilled water best?

  16. Ben my girlfriend has just dumped me and is a no carb diet. She has become emotional for no reason at all. Does this have anything to do with the diet?

    1. Hard to say. The "carb flu" can make people emotional if they aren't compensating correctly. Especially if she was VERY addicted to carbs and went cold turkey. Or maybe you are looking for a scapegoat? Jus sayin'…

  17. Hi–I'm 2 weeks into a ketogenic diet prescribed by my doctor (traditional western medicine, believe it or not!) in an attempt to help my lipid profile, without increasing my statin dose. I have a familial dyslipidemia problem and the women in my family die before 60 of sudden cardiac death, so. . . I'm willing to do this. I've always eaten quite low carb (~100g/day) but all this fat is very new to me.

    I'm doing tons of research trying to weigh risks and benefits of this approach. Once I'm sure I've keto-adapted (i started spilling ketones pretty quickly after starting –I think b/c I've been low carb so long and workout hard), I think I'll try doing some kind of carb cycling–for my workouts (Crossfit–which sucks bad right now), if nothing else.

    But right now, my question is about the information about long-term ketosis being dangerous for our immune systems. You write:
    "our liver is exposed to extra stress as it is forced to assist with manufacturing glucose from fats and proteins, potentially toxic amounts of ammonia are produced as proteins are converted into glucose, your body has a more difficult time producing mucus and the immune system becomes impaired as risk of pathogenic infection increases, and your body loses the ability to produce compounds called glycoproteins, which are vital to cellular functions."

    I haven't been able to find an research on this–can you provide some links to the studies that explain this and document the evidence for it?

    Thanks in advance!

  18. After being a total butt towards my family for the past week and sick. I realized it was this low carb diet that did it. I would never recommend the keto/atkins diet to anyone ever again . I would rather be healthy happy than thin and unhealthy.

  19. Low carb high fat diet have one more problem. I think this is the big problem. I had before couple of months of a strickt LCHF diet for about 3 years. I do hidrocolontherapies once a year. My therapist says that people that eat too much meat and protein as all have big chances to stick foot in the gut or colon. We need to eat potatoes/starches/ only for one reason , our body cant digest starch and the main purpuse of the gut is to remove these starches from the body. And the starch is used as transport for the other food that is in the gut. Also other foods are not digested and used for transport – cellulose, onions and others.
    Ben, i think will be good if you make one podcast with theme hydrocolon therapies. This is the first step for everyone that want to be healty and want to live longer. With these therapies the body is cleaning at cellular level! And after that the body digest and absorb better and more micro and macronutrients at all.

  20. Hi, Ben. I’ve been on an extremely low-carb diet for two weeks, eating 1000-1200 calories per day. I lost 10 pounds in the first week, and am now 138 pounds. I’m a 6ft tall, 18-year-old male. For months I had been compulsively overeating sugary foods, going from 98 pounds to 155 pounds in just a few months. Now I’ve quit cold turkey, but realize from your article that this type of deprivation can be injurious to my health. Have I done too much damage already in 2 weeks, or does my metabolism have yet to significantly slow down?

  21. Thank you for the low carb. Article. I have a neighbor friend that has been on the Atkins diet for over 15 years. He used to cook sausage 3xweek and I talked him into the 1xwk or less because of the high content of fat. He is now diagnosed with severe depression. Didn’t know if their might be a correlation.

  22. hello. i really need your help in this. I have been on a protein diet now for 3 months..eating exactly everyday: oatmeal(1 egg, 2 tbl spoons oats, and 2 table spoons zero fat yogurt) with cheese and thyme and 1 tbl spoon olive oil and coffee with zero fat creamer for breakfast… grilled chicken breast for lunch… and 2 grilled eggs for dinner..some times i add another cup of coffee with creamer or cadbury highlight…. i have lost 6 killos… which is a lot for me as i am only 54kg now so i wasn't fat just chubby…i still have 2 to 4 killos i wanna lose but i am kind of facing plateau… i m an athletic person doing cardio training… yesterday i started doing weight lifting and will be stopping cardio for a month ( a program i got from… i wanna build muscles and burn the last few kilos of fat..but i am afraid of my diet as i dont take carbs… will i be able to do so by staying on this diet? i want to note that first my metabolism is a mess as i spent my life dieting…and if i should eat carbs now how should i do that..since as u know after a long protein diet any little amount of carbs will be stored easily… and i really cant gain back weight…i did a very hard job losing those kilos…plz hekp me..thank you

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