Why Low Carb Diets Lower Physical and Mental Performance

Fab Fit Over 40 with some commentary on low carb diets and why they lower physical and mental performance.

There is absolutely no doubt one can reduce bodyfat to single digit levels by reducing carbohydrate intake while increasing protein and healthy fat consumption.  But there can also be a distinct trade off when it comes to maintaining optimal performance.

Your body needs full muscle glycogen stores to power through its weight lifting workouts.  The only way you can ensure your muscle cells are full of glycogen is by getting enough clean carbohydrates in your diet.  If your goal is to gain and or maintain muscle, you must consume enough clean burning carbs to support your training intensity.

If you consistently eat a low carb diet, your peak performance will suffer!

Low Carb Ketogenic Diets

Low Carb Diets

Many people (including Fabulously Fit Over 40) have used low carb diets including Ketogenic Diets and Cyclical Ketogenic Diets to maximize fat burning and attain a ripped six pack.  Unfortunately, a hard training bodybuilder, fitness athlete or anyone looking to tone up/look better naked CAN NOT use a low carb diet over the long term.(Atkins etc)

Why? They would grow tired of never having enough energy for their training performance.  What can be worse than trying to perform your workout on super low muscle glycogen stores?  If that isn’t bad enough already, low carb dieting can also significantly reduce your brain’s performance.

Why  do low carb diets alter mental focus and or mental performance?  Our brains and central nervous systems prefer to run on glucose(carbohydrates).  The brain is thought to use close to 100 grams of carbs daily in its normal functions.  Not giving it enough carbs is akin to putting cheap gas in your car.   This is the noticeable feeling people have of brain fatigue or lack of focus when in ketosis when on either a Ketogenic or Cyclical Ketogenic Diet.  This effect is also felt when the body is running on very low glycogen stores from severely restricted carb diets not fully Ketogenic in nature.

There is no doubt the brain can run on Ketones for fuel.  This is a subject of much debate and proponents of Ketogenic diets continue to maintain the brain becomes more efficient moving into Ketosis and burning ketones for fuel.(especially as the body becomes more efficient emptying it’s liver glycogen stores.)  But most people we know- including Mike from Danger & Play suffer cognitively when using severely low carb and or Ketogenic Diets. It is also well understood, after long periods of low carb dieting, the body becomes sensitive to the reintroduction of carbs and often retains water providing a puffy or bloated appearance. This is a normal but very unwanted side effect from the body trying to regulate it’s insulin sensitivity.

In our experience (already mentioned above) training intensity MASSIVELY SUFFERS on low carb and Ketogenic diets.  Two to three days with minimal or no carbs and the body is sluggish and laggard when training compared to when muscle glycogen stores are topped off.  Training on free fatty acids as a primary fuel source isn’t the same as full muscle glycogen stores.

To summarize why LOW CARB DIETS LOWER PHYSICAL and MENTAL PERFORMANCE in the long term.

  • People suffer cognitive decline when on low carb and/or ketogenic diets due to the brain not getting enough glucose from their diet.
  • Re-introducing carbs after long periods of low carb dieting wreaks havoc on insulin metabolism causing unwanted water retention and a bloated appearance aka Carbface. (this can and often does normalize over time)
  • In order to maintain intensity in the gym and for specific sports performance the body has to have ample muscle glycogen stores to fuel the activity. This can only be provided by adequate and/or ample carb consumption.

Fab Fit Over 40’s bottom line:

In order to optimize fat loss and maintain training intensity, a MILDLY REDUCED carb diet, high in protein and EFA’s with targeted CLEAN CARB consumption (Carb Cycling) around training sessions works best.

Stay tuned for a highly detailed and informative article and podcast on how to use Carb Cycling to dramatically transform your body this summer .

Be the BEST YOU at Any Age!


  • Mike H

    You clearly don’t know much about the Keto diet.
    Over time On Keto your cells produce more Mitochondria. This eventually leads to equal performance vs carbs.

    You convert enough protein to sugar to take care of your nerve cells and brain. There is no decline in Keto because 90% of your brain can use Ketones for fuel.

    • Jay

      Brother..no offense..but I have FORGOTTEN MORE ABOUT KETOGENIC DIETS than you could ever comprehend.

      If you would like to meet me in a public setting to debate me, I would be happy to meet you. We can videotape it for the world to see. It will not end well for you.

      As to your ABSURD COMMENT about the cells producing more mitochondria, please provide me a peer reviewed study showing this. I am very familiar with the metabolic changes which occur in the absence of carbs while in Ketosis and when out.

      If you would have actually bothered reading the article, you wouldnt be debating anything I wrote. In fact, I am trying to be professional in my response but you dont deserve it because you offer no reason or logic to the discussion.

      If you would like to come back and provide reasonable commentary I would be happy to engage you in an intelligent discourse.

      • Mike H

        Then why would the US Army be looking into it?

        • Hey Mike! This is the kind of intelligent discourse I was looking for. Thank you for it. My hubris sometimes gets the best of me.

          I will agree (as I experienced it myself) there are adaptations which occur with long time Ketogenic diets which allow *some people* to function effectively. But there is no way MOST PEOPLE can perform high intensity exercise in the absence of carbs for long periods of time. I don’t need any research from anybody to tell me otherwise. Can people do OK aerobically? Probably. But they can’t lift weights or train explosively. The biggest issue is the likelihood of injury from a lack of carbs in the diet. Glycogen replenishment and tissue restoration isn’t coming from pure fat and protein.

          Now if they are doing a CKD and training during or around their carb re-loads, then it can work. But again it’s not optimal. We’ll just have to agree to disagree.

          Like I have said at various other points in this thread, Keto or CKD’s are not optimal long term because they are too restrictive. How is anybody going to avoid carbs forever? It’s inhumane. And when they do make a choice to stay on for a long time and then *ADAPT* their bodies suffer and go into a metabolic haywire mode. Insulin sensitivity is altered as is many other metabolic systems because the body is inefficient at regulating it’s own Insulin production.

          Lastly…I stayed on a CDK for about 19 months consecutively. I got ripped to shreds but hated the way I felt by Thursday and Friday before my carb-up-re-glycogenation phase. And trust me brother-my mind was razor sharp while undergoing gluco-neogenesis. No issues there. But I was tired as an active 26 year old guy because my muscles and brain had not enough circulating glucose. Been there-done that. Just isn’t the right choice for a long robust life filled with enthusiasm and energy.

          IMO>CARB CYCLING with bouts of IF is the optimal way to go. Thank you for your point of view and your reading of my blog.

          • Mike H

            I think a lot of the problems with empirical evidence is diet related.
            There are so many variables that affect Keto it is silly.

            Protein. Since 1/2 amino’s are keto and the other half are carb and no protein source has the same balance.

            Nutrients. Keto definately requires a different balance of nutrients than carb. No one has pinned down exactly what, when, and how much is needed. Everyone is still slogging in the dark.

            Individual metabolism’s. Everyone’s body treats everything slightly differently and many problems have very similar side effects.

            IMO things should start becoming more clear as far as knowledge goes since popularity in keto has taken a huge leap in the last few years.

            Mabye there is some magic formula that would make it work for just about everyone. no idea.

            And I’m sorry for being a lil cranky. I get that way just like you.

          • It’s all good brother. Different viewpoints are making the universe continue to explode. We definitely don’t know jack shit about molecular biology and learn more by the day.

            I will admit there are probably solutions to the lack of muscle glycogen production in the absence of carbs-we just don’t understand them yet.

            I don’t know about Keto taking a huge leap…lots of people were using these diets in the 70’s and 80’s. Just more underground. But definitely the ability to see things at a molecular level has improved greatly and only recently.

            My mantra since the late 90’s and will remain so to my dying days (5000 years from now) “Biochemical Individuality is the ultimate determinant”.

            Stay in touch!

  • Josh

    Hey Jay,

    Last summer I went OD ketogenic (bulletproof coffee, grass fed beef with MCT oil for dinner) and I was honestly KILLING it when it came to cognitive performance.

    My anxiety levels went to nil (I’m attributing much of that to ketosis, I can’t find the Paul Jaminet article that linked to some PubMed stuff about ketosis and anxiety but I’ll link it when I do), my migraines disappeared completely (that I will definitely attribute to not eating gluten, although ketosis does help clear excess glutamate in the brain more rapidly according to some research I’ve read and that probably also contributes) and my ADHD and focus were improved dramatically.

    However I did start working out hardcore for the first time in 9 years this year as I said in a previous post and I can attest to the fact I have horrible workouts when I haven’t eaten carbs in a couple of days.

    I think the decline in weight-lifting performance during ketogenic dieting is almost beyond debate (IE if you lift hardcore you gotta eat carbs, long duration cardio is another story) but I believe the cognitive decline is highly personal.

    Not trying to troll, just wanted to see if you agree it could work better for some and not for others. I am an endomorph so maybe I can handle no carb diets better than someone like yourself who is a meso/ecto naturally.

    • Hey Josh! Thank you for such a well written and elucidated post. I couldn’t agree with you more. I used a CKD for almost 3 years and got into the best shape of my life at 26 years old. I was ripped to shreds (pre gear usage) and weighed about 187 pounds. Think the grainy hard abercrombie look. I also didn’t suffer the mental/cognitive decline most people feel while in Ketosis. It definitely is an individual thing in the way people respond to being in Ketosis.

      My personal pet theory is many people of Northern European descent (who have more slow twitch fiber typing and are bigger meat eaters) developed metabolisms which were highly efficient at processing meat and fat especially in times where there were limited supplies of grains/fruits and vegetables. (Think Tundra conditions with nothing alive growing on the ground)

      There is no doubt endomorphs do better with fat loss and maintaining a lean toned look while minimizing carb consumption. So it stands to reason endomorphic people will do well or typically better than Ecto/Mesos on severely carb limited diets.

      The problem with CKD’s is coming off. You can’t switch back and forth without serious repercussions to your insulin sensitivity. Plus when one is in Ketosis and is running efficiently the window of opportunity for eating carbs without negative consequences is minimal. It’s obvious though right? When your body never gets carbs…and then all of a sudden you eat 250 grams at one sitting…you experience haywire mode with your metabolism. CKD’s are effective for fat loss and offer some individuals improved cognition but overall for best overall performance both in and out of the gym-cycle your carbs. Plus it’s just better psychologically to never deprive yourself of any one food source.

      • Jeff
        • Wow. My psuedo scientific intellectual mind came through again. 😉 Thanks Jeff. Very interesting article. I appreciate you reading my blog.

          • Jeff

            Funny how that works, isn’t it? Your unconscious mind seems to recall the characteristics of a group of people, and the science just confirms what you already knew.

            Something I am very interested in right now is the confirmed fact that genes themselves have a memory. This is just the tip of the iceberg:


          • Brother…I’m more than happy to travel down the rabbit hole with you. There is plenty of information “not provided” by “Modern Science/Study” whose funding is purposely controlled. You want to read an amazing book? Read this one: http://www.amazon.com/Slave-Species-Gods-History-Anunnaki/dp/1591431514/ref=sr_1_1?ie=UTF8&qid=1401389848&sr=8-1&keywords=slave+species+of+the+gods

          • Josh

            Hey Jay,

            That’s an awesome theory!

            My dad is 1000% ectomorphic naturally and eats a Mediterranean diet with pretty low carbs naturally (he’s from Morocco.) He tried total keto just because he saw me doing it and knows all about how insulin resistance is bad and can contribute to aging and Alzheimer’s (he’s 65.)

            Terrible results. Brain fog, fatigue, typical stuff. Same with alot of people I know who are naturally pretty lean and have heard me extoll the virtues of low-carb dieting and how insulin is the reason we age (especially the days when I read a bunch of Ron Rosedale and go on a rant lol)

            And it was so weird you said how they can cause insulin resistance, I’ve almost never heard people explain that properly.

            A few years ago I read something by Berardi talking about how ketogenic diets are great for fat loss, but they can cause temporary insulin resistance. He said something along the lines of “It doesn’t matter when you’re eating no carbs. But once you come off a contest diet or extremely ketogenic diet you have to do it carefully or you will end up eating 10 boxes of cereal and blacking out from a sugar rush because of such a ravenous appetite.” That’s why he created the “Get Shredded Diet” as well as the “Get Un-Shredded Diet”.

            I think for most people, even myself, I feel great on keto, but I can get most of the same benefits while eating around 50g of carbs a day.

            Anyway, awesome blog, great posts and keep up the awesome work here and at D&P.

            I’m only 26 but given the complete dearth of intelligent people in America (and particularly when it comes to diet and exercise) it’s rare for me to find someone who knows alot more than me. You and Mike are a goddamn breath of fresh air.

          • Thanks bro. There is so much more coming. We appreciate you spreading the word. Tomorrow’s article is gonna be ‘different’ from what’s been put out there but one I’m proud of.

  • GM

    I personally know that low carb dieting is disastrous for me, even as an endomorph.

    After just a couple of days of eliminating higher-GI carbs (so only eating greens etc.), I have trouble standing and need to steady myself against a wall, I can’t think properly and the only thing I can do is lie in bed and vegetate.

    I don’t know why this is, but for some reason this is how I respond. I had a low carb week last week and it affected me so much I still haven’t fully recovered. Perhaps it wreaked havoc with my insulin sensitivity? Who knows. I’m eating carbs now, but trying to limit them to once a day.

    • What is your ethnicity? The further away from the Northern climates your ancestors were, the more sensitive to carb depletion you become. Its very strange but I have observed it in many different people.

      • GM

        Eastern European.

      • Omar Torres

        it might be the blood type, in Northern climates the majority is A or B. Apparently O+ people are more sensitive to carbs.


        • Blood types and how they correspond to the population strata across the globe is a fascinating topic. Much is hidden about it but it’s in depth study could probably reveal a ton on human metabolism and optimal cell functioning.

          • Omar Torres

            According to some sources http://www.effectivebrain.com/primal-ketogenic-diet-brain/,

            when the ratio Omega 3:Omega 6 is higher in Omega 6, the body feels lethargic; it should be 1:1

            And about the blood types, indeed it is a very interesting subject, taking into account that most studies are made in Europe/USA where the majority of people is A. So the studies might not “work” in B or O people.