Once again it’s time for another mailbag.
Mixed Brain Fuel
Question: On a ketogenic diet, how rapidly does the brain flip between glucose and ketones for fuel? Can it use both fuel sources simultaneously?
Answer: The above question sort of encompasses a few different potential things and I’m not 100% sure which you’re asking so I’ll just cover them all. First realize that one fuel that the brain cannot use is fatty acids, at least not directly. This has led to the oft-stated belief that the brain can only use glucose. But this is incorrect as the brain has an alternative fat derived fuel which are ketones (or ketone bodies, the two major of which are beta-hydroxybutyrate and acetyl-acetate).
Ketones are produced primarily in the liver (from the breakdown of fatty acids) and exist predominantly as an alternative fuel source for the brain (they can also be used by skeletal muscle) during periods of low-carbohydrate availability. This probably was originally important during periods of complete starvation; now very low-carbohydrate diets (defined here as any diet containing less than 100 grams per day of carbohydrates) effectively ‘exploit’ this mechanism.
Now, on a carbohydrate based diet, the brain runs essentially on 100% glucose since ketones are generally not produced in significant amounts under those conditions (there are a couple of odd exceptions, one is following very long duration endurance exercise where a post-exercise ketosis can occur due to changes in fuel metabolism). So what happens when you remove most or all carbohydrates from the diet? Does the brain magically switch to using ketones? For the most part, no. Studies done way back when show that there is an adaptation phase that may last about 3 weeks while the brain ramps up its ability to use ketones for fuel.
Even there, after that roughly 3 week period, the brain still only derives about 75% of its total fuel requirements (about 400 calories per day or thereabouts) from ketones; the other 25% come from glucose (which the body can produce through a variety of pathways that I won’t detail here.
All of this is explained in excruciating detail in my first book The Ketogenic Diet). Mind you, this is only relevant on a very low-carbohydrate diet. Even if the brain could still use ketones on a carb-based diet they wouldn’t be produced in large enough amounts for it to be relevant.
So I think that answers at least part of your question: when first starting a low-carbohydrate diet, it takes the brain about 3 weeks to adapt to using ketones for fuel; even then it only gets about 75% of its total fuel from them. This scans pretty well with what many experience on the diet, they don’t feel fantastic for the first 2-3 weeks of the diet (while they are adapting). Some of that, mind you, is related more to mineral intake than anything else (early studies found that sufficient intake of sodium, potassium and magnesium eliminated all of the fatigue and lethargy that occurred on very low carbohydrate diets).
But there is a related question that often comes up which has to do with switching back and forth between fuels (this is especially relevant for some cyclical ketogenic diets such as what’s described in The Ketogenic Diet or in my Ultimate Diet 2.0). Here I am unaware of any research on the topic and most of what I have to say is just based on empirical evidence, what people have reported over the 15+ years they’ve been giving me feedback.
Certainly early in the diet there is often a period where the alternation of high and low carbs often causes some people distress, they get the same headaches and issues going from high-carbs back to low-carbs for a couple of weeks. Probably just a function of ‘interrupting’ the adaptation to ketone metabolism in the brain and there might be some rationale to doing 2-3 straight weeks of a ketogenic diet prior to inserting refeeds or carb-loads.
At the same time, after more extended periods on the diet (perhaps 6-8 weeks), switching back and forth from a carb-based to a ketone-based brain metabolism seems to cause most people no problems. They can sort of drop in and out of ketosis (even throughout the day under certain conditions) and not really notice anything one way or the other. Interestingly, even after extended periods off of a low-carbohydrate diet, most people don’t report the same early adaptation phase that they went through the first time on the diet; they go back onto a ketogenic diet and don’t notice anything.
This suggests to me that there is some type of long-term and/or almost permanent change in the brain in terms of its ability to use ketones for fuel with long-term exposure to them. Again, I have exactly zero research to back this up; it’s just an observation. But even there you’d still expect to see the same basic 75/25 split, just with an easier switching back to ketone metabolism after that initial adaptation phase.
Hope that answers your question.
Does Sprint Training Impair Muscle Growth?
Question: I recently came across the following comment on Bret Contreras’ blog with regard to a figure athlete he is training and a question about sprinting….
“I don’t have her do sprints. I have journal research that shows that sprints interfere with biochemical pathways involved in muscle growth. While sprints will help pack on mass for beginners, it can actually negatively impact more advanced lifters in terms of hypertrophy. That’s why the bodybuilders don’t sprint or do plyos – the risk isn’t worth the benefits.”
The comment did not specifically cite the particular research, nor did it make mention of the sort of volume and distance of the sprints, i.e. whether this was low or high volume and whether the sprints were somewhere in the 10-40 yard range or beyond that range.
To the best of your current knowledge, is there a legitimate reason (biochemical or otherwise) why sprints would interfere with muscle growth? (I realize that with specific parameters and context this may be a bit tricky to tackle) I am not necessarily arguing that figure athletes or bodybuilders need to be or even should be performing them, simply wondering if those who may do so would actually be compromising results as Bret seems to intimate.
Answer: The short answer to your question is that Bret is right; certainly distance probably will have an impact but when people talk about ‘sprinting’ for bodybuilders, it’s usually some form of high-intensity interval training (HIIT) being discussed. That is, longer repeats (typically in the 30-90 second range). Few would be doing short sprints like a track sprinter.
As to the mechanism, Bret is most assuredly referring to AMPk (adenosine monophosphate kinase) and how it impacts on muscular metabolism. AMPk is activated, essentially, by alterations in the energy state of the cell: things that occur in both endurance training and longer sprint type training (AMPk is regulated both by the duration and intensity of activity).
Activation of AMPk has a number of implications: it’s involved in the adaptations to endurance training (stimulating various improvements to occur in the fibers in which AMPk is activated), increased fat burning, insulin sensitivity and a host of others.
Relevant to this specific issue, activation of AMPk has been shown to directly inhibit a molecular marker called mTOR (which stands for the mammalian target of rapomyacin, aren’t you glad you know that now ). mTOR is a key player in muscle growth; for example, the amino acid leucine that so much has been talked about acitvates mTOR directly and this appears to be the major way that leucine/BCAA turns on protein synthesis.
So when you activate AMPk, via long-duration and/or high intensity endurance/interval training, this inhibits mTOR activity; inhibiting one of the prime players in muscle growth. Again, that’s surely what Brett is referring to.
Now, as you bring up, this probably doesn’t hold for very short sprints but those would be uncommon for bodybuilders or physique athletes to do in the first place. For the type of sprint/interval training commonly advocated for such athletes, I agree that the negatives would outweigh any potential positives when maximal muscle growth is the goal (interval training can have potential benefits for fat loss, including stubborn fat loss, as discussed in The Stubborn Fat Solution).
Ketosis, Carbohydrates and the Brain
Question: I’ve been doing the CKD effectively. However, I have a major exam on Friday. Is there any effect on limited carbs on cognitive processes? Does limiting carbs ( 20g / day) have a negative effect or could it retard my performance on a major exam, i.e. MCATs, Series7, etc? Is there any study or suggestion you could give based on your research?
Answer: First a quick definition for anyone who isn’t familiar with the abbreviation: as discussed in the Comparing the Diets Series , a CKD refers to a cyclical ketogenic diet. This is simply a diet that alternates between periods of very low-carbohydrate eating (typically 4-6 days) and very high-carbohydrate eating (1-3 days). Dan Duchaine’s Bodyopus, Mauro DiPasquale’s Anabolic Diet and my own Ultimate Diet 2.0 are all examples of CKD’s. My first book The Ketogenic Diet discusses CKD’s generally in mind-numbing detail.
Now back to the question: does ketosis negatively impact on cognitive function? And the answer is one huge it depends. Certainly early studies found that, in the short-term (first 1-3 weeks), low-carbohydrate diets tend to cause some problems. For this reason short-term studies (usually a week long) tend to report decrements in a lot of things including cognitive performance.
Empirically, as well, many report fatigue, lethargy and a sort of mental ‘fog’ until they adapt to the diet (the brain adjusts to using ketones for fuel over those first 3 weeks). I’d note that supplementing with sodium, potassium and magnesium seems to go a long way towards limiting or eliminating that feeling of fatigue.
So, for most I certainly wouldn’t recommend starting a very low-carbohydrate/ketogenic diet right before some major test or cognitive challenge. Odds are it’s going to cause problems.
But what about someone who has adapted to being in ketosis. There there tends to be huge variance. Some people are sort of neutral to it but I know of many who report far better brain functioning when they are in ketosis. I couldn’t tell you the mechanism, this is just one of those self-reported things. But it tends to be highly variable (and I can’t think of any studies that have examined cognitive performance after long-term adaptation to low-carbohydrate diets).
CKD’s add another complication since dieters are moving into and out of ketosis at some frequency. While this pattern has been examined for endurance performance, I don’t recall any work on CKD’s and cognitive performance. I bring this up as some people do report changes switching back and forth between very low and very high carbohydrate intakes.
Quite in fact, many who find that they feel ‘great’ in ketosis feel a bit dopey or sleepy when they switch back to high carb intakes. This is probably related to either blood glucose swings or a big increase in brain serotonin (which tends to cause lethargy and fatigue) but it does occur.
Similarly, some seem to go through at least a brief re-adaptation (in terms of fatigue, etc.) going back from high carbs to low-carbs. Again, this is pretty variable, many people can switch back and forth from one extreme to the other and don’t seem to notice anything. I have no idea why, just reports I’ve seen.
So back to the question, should you switch out of ketosis for your test? It’s a hard question to answer and you’d have to think back to your previous switches from low- to high-carbs during the CKD. If you find that you’re fully adapted to ketosis and function fine mentally, and that you get dopey going back to high-carbs, I’d probably suggest you stay on low-carbs through the test.
If you’re one of those people who don’t seem to have ever fully adapted to being in ketosis (they do exist), you might want to move back to at least moderate carbs a day or two before your test. Unfortunately, there’s just too much variability for me to give you any advice beyond that.
How Much Cardio is Too Much?
Question: This is a follow up question for your last QA. It is often said that too much cardio on a restrictive diet is “bad”. With NEAT in mind, I wonder exactly what defines cardio in this setting. Playing with your kids for a few hours(playing ball in the yard etc) is this defined as cardio? Does taking a leisurely stroll with a baby carriage for an hour or two
per day count as cardio? Or is cardio defined as something else?
Answer: I think you’re referring to the article I wrote on why combining very low calories and excessive activity can harm fat loss. although I may have addressed the issue in a Q&A as well (I can’t find it). In any case, your question is one that comes up fairly frequently, especially in the context of the Rapid Fat Loss Handbook approach (where I am adamant that excessive activity/cardio can cause the diet to work far less well than expected). People want to know what and how much of certain types of activities will or won’t cause problems.
The primary issue here is this: the body appears to be sensing what researchers are calling energy availability, basically energy in (from food) versus energy out (via energy expenditure). If Energy Availablity (EA) goes too low, the body will adapt rapidly. For example, researcher Ann Louckes has shown that many of the issues that often occur in women in terms of hormones or menstrual cycle dysfunction occur at a threshold of energy availability (and aren’t actually related to body fat percentage as used to be thought). I discuss this extensively in The Women’s Book Vol 1.
In that sense, pretty much all activity can potentially be a problem if that activity results in an energy availability to the body that is too low. Of course the activities you’re listing aren’t really big calorie burners, a walk with a stroller probably only burns a few hundred calories per hour. But done for extended periods it will contribute.
Another issue I discussed in relation to the combination of excessive cardio with large calorie deficits is this. In addition to hormonal issues, often the combination of big caloric deficits and excessive activity (either too much activity, too hard of activity, or the combination) can cause some real weirdness with water retention that masks fat loss.
I’d tend to say that this is more common with more formal ‘cardiovascular’ activities than just activities of daily living. This is just due to the potential for increases in hormones like cortisol; this is especially an issue as the intensity of activity increases. Clearly this isn’t an issue for a leisurely walk but it becomes more of one for more formal cardio activities.
This isn’t really stopping fat loss mind you, but it does drive people crazy because it makes it appear that the diet is not working. I’d note that this isn’t an issue for everyone, certain physiologies (and especially psychologies) seem relatively more prone to problems with water retention than others. This is why some people can get away with massive amounts of activity and not have issues and others can’t.
In any case, I hope that answers your question to at least some degree.
Pre- vs. Post Workout Nutrition
Question: If protein and other nutrients take time to be broken down and utilized, does it really matter whether or not you have a PWO meal, if you’ve had a large meal relatively soon before your training? In other words, can a Pre-workout meal be just as beneficial as a post-workout meal (if not better)? Isn’t it important to have AA in your blood stream when training? And if there is a designated time for digestion, wouldn’t the other nutrients effectively help recovery, even though they were consumed before hand?
Answer: As usual this is going to be one of those longish ‘it depends’ kinds of answers and I’m probably going to go way off track in trying to answer it. As I discussed in The Protein Book, some recent research certainly suggested that pre-workout nutrients (carbs and protein, and I’ll assume the combination from here on out) were superior to post-workout nutrients in terms of promoting protein synthesis.
Other research wasn’t so positive but it did look like having nutrients in the system during/immediately after workout might be better than waiting until afterwards. Some of it depended on the form of nutrients (especially protein consumed); in one study immediate pre-workout essential amino acids (EAA’s) were better than post-workout EAA’s. In another, a whole protein taken right before training wasn’t superior to post-workout; this may have been an issue of digestion time.
I would note that protein synthesis isn’t the only goal here; maintaining high levels of training intensity during a workout is also key and pre- and/or during-workout nutrition can benefit folks there as well. A complication of that research was that most of it was done fasted, that is first thing in the morning, after folks hadn’t eaten for many hours. While that is relevant to some people (e.g. those who train first thing in the morning), many if not most trainees will have eaten something prior to the immediate pre-workout period. This complicates issues.
And the general picture that seems to be developing is that if someone is in the ‘fed’ state, that is they have eaten within a few hours of their workout, pre-workout nutrients don’t seem to provide any major benefit. This mainly has to do with the slow digestion time of whole foods. A relatively ‘normal’ whole-food meal is still releasing nutrients (carbs and protein) into the bloodstream as much as 4-5 hours after you eat it.
So if you’ve eaten within 2-3 hours of your workout, you already have nutrients in the system and probably don’t have much of a need to eat anything right beforehand. On the other hand, if it’s been 4 or more hours since your last meal (e.g. you eat lunch at 1pm and don’t get to the gym until 5-6pm), consuming something immediately before your workout is probably a good idea. Not only will you get some protein into the system that will be available during and right after training, a small amount of carbs will ensure that blood glucose is normalized so that you can have your best workout.
Depending on what type of training you’re doing, you may also find that during and post-workout nutrition is also beneficial. The length of the workout, type of workout, etc. all go into this determination.
So again, the answer is that it depends. Certainly there is some logic to having nutrients in the system during and right after training by consuming something right before workout. But this is complicated by whether or not you’ve eaten a whole-food meal in the few hours before your training. As above, if you’ve eaten within 2-3 hours, I wouldn’t bother with anything before your workout.
If it’s been 3-4 hours since your last meal, I’d suggest at least experimenting with eating something maybe 30 minutes before workout (this could be a protein bar, a small carb/protein drink) to normalize blood glucose and get some amino acids into the system is probably a good idea.
About 0.3-0.5 g/kg (0.14-0.23 g/lb) of both carbs and protein would be a good starting place. For a 180 lb lifter that’s 25-40 grams of carbs and protein combined about 30 minutes before your workout (to give the nutrients time to get into your system).
I would mention that a small percentage of people seem to get rebound blood sugar issues from consuming rapidly digesting nutrients right before a workout; if you’re one of them you’ll want to wait until you start warming up to eat anything. Exercise blunts insulin response and waiting until your warm-up to start eating anything limits the possibility of a negative blood glucose response.
I should also mention that one or two recent studies have intriguingly suggested that training in a completely fasted state might have benefits in terms of activation of some of the molecular markers involved in growth and adaptation to training. So far nobody has looked to see if this truly impacts on growth down the road so I’m remaining on the fence until more research becomes available. My gut hunch says that having nutrients in the system will be superior to not having them available but if research comes out against that, I’ll happily change my mind.
- What Is The Ketogenic Ratio?
- Bodyrecomposition Mailbag 7
- How Many Carbohydrates Do You Need?
- Should Training Determine the Diet or Vice Versa?
- Do Ketogenic Diets Have a Metabolic Advantage?