[TrainingTalk] Learnings From a Year of Periodized Nutrition and Metabolic Testing
Acute and chronic changes in response to dietary modifications, fat oxidation rates, impact of training vs nutrition, shifts in substrate utilization during prolonged exercise, and more.
About a year ago, I started experimenting systematically with periodized nutrition. I wanted to understand whether deliberate modulation of macronutrient intake, combined with consistent endurance training, could meaningfully change substrate utilization and, more importantly, my performance in ultramarathons.
The first phase of this project was described in an earlier post, where you can find the theory, how to execute it, a sample diet, and initial findings.
Since then, I’ve started building my own lab and added several rounds of metabolic testing both on myself and others, gathering useful data to better understand many nuances of the interplay between training and nutrition. What follows is a reflection on some of the most interesting aspects I’ve analyzed (some of them barely discussed in the scientific literature, typically for practical reasons, more than anything else).
In particular, below I look at:
Acute vs chronic effects of changes in diet and macronutrient distribution.
Impact of short-term (<48 hours) high carbohydrate intake.
Impact of longer-term (7-10 days) high carbohydrate intake.
Fat oxidation vs running economy vs performance.
Absolute and relative changes in fat oxidation rates depending on such dietary modifications.
Associations (or lack thereof) with lactate, body weight, and training load.
Substrate oxidation changes during prolonged exercise up to 6 hours and implications for race fueling.
Race fueling.
Resulting performance in ultramarathons.
I hope you’ll find this follow-up interesting for your own training and racing, and please feel free to comment below should you have any questions. I’ll do my best to follow up.
What Are We Talking About?
For the ones new to it, periodizing nutrition simply means manipulating macronutrients based on the demands of training. Hard workout? Higher carbohydrate intake for breakfast. Long and hard workout? Higher carbohydrate intake for breakfast and the previous dinner. Easy run? Lower carbohydrate intake. It can get a bit more complicated, but this is the core of it.
The reason for doing so is that by periodizing nutrition, you will be able to execute optimally high-intensity training (due to high carbohydrate availability) but also develop higher rates of fat oxidation (due to lower carbohydrate availability on days in which intensity is lower), therefore improving your ability to spare limited glycogen resources and improve performance in long-distance events.
This is the essence of metabolic flexibility: training the body to use fat and carbohydrates depending on the demands (intensity, duration) of the event. In this blog, I won’t address in more detail the basics, for which I encourage you to spend time on this blog, where I go into a lot of detail, while below I will cover slightly more advanced topics.
I will conclude this introduction by saying that while you might have heard that as endurance athletes we are already good at oxidizing fat, my own data and the data I’ve collected on others with similarly high-carb-all-the-time diets, shows the opposite (more about this in the next sections). Metabolic flexibility can be improved, but the work must be done in the kitchen.
In addition to total “daily” nutrient intake, the elite athlete must, therefore, adjust their energy and carbohydrate intake in a meal-by-meal and day-by-day manner (i.e., carbohydrate periodization) in accordance with the energetic demands and training objectives of each specific training session. - UCI Sports Nutrition Project.
p.s. I’d remove “elite” from the sentence above, and agree on the rest of it.
Acute vs. Chronic Effects of Dietary Manipulation
We know a few things about the impact of acute (e.g., pre-training or for a few days as in carbo-loading pre-race) and chronic (as in our diet) carbohydrate intake on fat oxidation, but given the usual high individual variability in responses, testing can clear a lot of unknowns for our own case.
This whole experiment started because my fat oxidation rates were particularly low after all, otherwise I would not have necessarily gone this way to improve my performance. With my first testing about 9 months ago (2 months after starting to periodize my nutrition), I could answer the most important question: 8 weeks of periodized nutrition were sufficient to go from not oxidizing any fat, to oxidizing plenty, even at sustained intensity (e.g., between marathon pace and 50km race pace, fat oxidation was still 0.8 grams/minute, hence we ar not talking about an easy jog but a meaningful change in substrate utilization at race pace). Initially, my expectations were low, I thought maybe I’d improve a bit my rates during an easy jog and not much else, but the data showed that what was a weakness turned into a strength (we’ll see more about the implications of this in the sections below in the context of race fueling and performance).
At that point, many other questions arose:
What is the impact of acute carbohydrate intake? Do you need to eat low carb for fat oxidation to do its thing, and do we suppress it meaningfully when doing otherwise? (e.g., during carboloading pre-race, consider the implications of this for racing).
How much can fat oxidation change for an individual? Did I get to 0.8 grams/minute, as that’s a genetic ceiling for me, or another 2, 4, 6 months, could lead to higher numbers?
Where is the optimal? And here I mean both in terms of living a life that doesn’t require being obsessed with what I eat every meal, and keeping metabolic flexibility where I want it to be for my performance. Can we periodize nutrition less and still have 90% of the gains? When do we start to make it worse?
The answers to these questions are likely individual (because once again, individual variability is huge), hence only testing could answer them for me.
In the past 2-3 months, I’ve done plenty of testing and experimenting to answer such questions, and below I report my findings.
Impact of short-term (<48 hours) high carbohydrate intake
The first question I had was on the impact of acute / short-term high carbohydrate intake. You hear a lot that substrate oxidation is impacted acutely by what we eat prior to testing, but it’s never so simple, and my previous tests had already shown that having 2 gels prior to testing made no change. The next step was to look at a more substantial increase in carbohydrate intake. Hence, the data below was collected during a maximal test for which I had a high-carb dinner the prior day and a high-carb breakfast the morning of the test (> 120 g). My fat oxidation rates were equivalent to the ones I have seen after a low-carb day and breakfast (see below for some data):
After many of these tests, in my own data, I can say that acute carbohydrate intake has minimal / negligible impact on fat oxidation, given that my diet is heavily periodized. I tested this many times, as it’s rather easy and can be done with a submaximal test (actually, it should be done with a submaximal test, to ensure we reach steady state). On most days, I cannot see any difference in fat oxidation regardless of what I ate for breakfast (e.g., an omelette with avocado and cheese vs 120 grams of oats) or the previous dinner, because fat oxidation and metabolism in general are robust, given chronic exposure to a certain diet.
The implications are important. Working on improving fat oxidation will require training under conditions that stimulate that pathway (i.e., with lower carb intake, glycogen depletion, etc.), but on race day, you should not worry about having more carbohydrates: your work will stay with you. This is somewhat obvious: if it were the other way around, our diet would be irrelevant, and the only thing that mattered would be our last meal before an event, which, of course, makes no sense.
Our diet matters, and we can have the best of both worlds by periodizing our diet and then having higher carb intake on race day or prior to race day to ensure full glycogen stores, stable blood sugar, etc., when we compete.
Now, there’s of course a limit to the considerations above. How far can we push it in terms of increased carbohydrate intake before we start deteriorating our fat oxidation?
Impact of longer-term (7-10 days) high carbohydrate intake
In my original experiment, it took only 8 weeks to develop strong fat oxidation (maybe less, but I could test only after 8 weeks, as I did not own a metabolic cart back then), hence reverting back to a daily high-carb diet for long enough will shift things in the other direction and deteriorate metabolic flexibility. How long does it take to get there?
I did this experiment recently, eating a diet similar to what I used to eat in the past (i.e., high-carb daily on most meals), for about ten days, and indeed, fat oxidation was reduced by about 50%, to values that at this point I’d consider quite low and detrimental to my performance in ultramarathons:
The measurement above was useful as I had previously seen no impact of a short carboloading window (e.g., 2 days) on my fat metabolism, but a longer stretch did the job and reduced it from about 0.8 grams/minute to half of that or less (0.4 grams/minute), at the same intensity.
After those tests, I wanted to see if I could revert it in a rather short time, since, in theory, now my body has been there, and should be able to get there again faster than in the past. Hence, I went back to a more focused nutrition in the two weeks before the last race of my season. Nothing crazy, just higher quality food, almost no ultraprocessed foods, and in general I went lower on total carbohydrate intake when not training or training easy (still, eating plenty, as I cover later, weight has only increased in the past 6 months, make sure not to confuse “eating fewer carbohydrates” with “being in a caloric deficit”, they are very different things!). I’ll add an extra note here that even when eating fewer carbohydrates, I still eat plenty of high-quality foods that include carbohydrates, e.g., vegetables, berries, legumes, farro, etc. - maintaining my fiber intake rather high. I think it is key for the focus of this whole process to be on the quality of the food we are eating first, and the macronutrients second. The same can be said equally importantly for days in which carbohydrate intake is higher, which doesn’t mean eating pastries all day, but adding some oats, potatoes, etc., to my standard diet.
Anyways, back to our topic, after these two weeks of periodized nutrition, I was back in my lab to test if there were any changes, and I was quite surprised to see that I was back at 0.9 grams/minute (tested at 65-70% of VO2max, or 5’15”/km). Equally interestingly, this happened despite a week with 2 hard days (3 hard workouts) and 4 days off in terms of training, or in other words, the opposite of what is supposed to promote fat metabolism. This additional experiment gives me useful information: I can let go of the diet and pick it back up in a matter of weeks, regardless of training.
Nutrition clearly beats training on this one.
Fat Oxidation vs Running Economy vs Performance
I want to briefly discuss how fat oxidation can come at the expense of running economy. While a reduction in economy can be seen when on a low-carb diet, periodized nutrition maintains the required carbohydrate intake, avoiding this issue. However, if we go through large changes as the ones I have documented for my own data, I think there are still implications to consider.
For example, as my focus is at the moment ultramarathons, it makes more sense to me to eat high-carb only a day or two prior to the race, as I want to maximize fat oxidation if I’m going to be out there 5-9 hours. On the other hand, for a marathon, I might want to eat differently a few days earlier and reduce fat oxidation slightly, if the reduction in fat oxidation comes with a noticeable increase in running economy, as can be the case. This is because glycogen stores are limited and will certainly be depleted in an ultramarathon, and as such, the more fat I can burn, the more I will delay running out of glycogen, hence, fat oxidation has higher priority in that context. Somewhat similarly, the intake I need to sustain a given intensity could be reduced without compromising performance, and therefore, there will be lower gastrointestinal distress (which is a typical issue for me personally).
For a marathon, assuming fat oxidation is still decent at race pace, a reduction won’t require particularly high exogenous carbohydrate intake because the duration is much shorter (e.g., ~3 hours), and economy tends to have a greater impact. As you can see, there isn’t a perfect diet (or training) that works for every distance, but we need to appreciate the nuance and use the tools at our disposal in relation to our specific goals. Keep in mind that it’s never so simple, and while I wanted to add these considerations to give you the full picture, all of these aspects also need to be tested repeatedly, which I have not done, given that my goal race is not the marathon anymore.
Absolute and Relative Changes in Fat Oxidation
I mentioned in my previous blog how my fat oxidation rate was pretty low at about 0.2-0.3 grams/minute, even at low intensities. Was it bad genetics on this one too, or was it just a bad* diet?
Well, if I look at the data of a few other endurance runners on a chronically high-carb diet, what I see is a similar picture. Low absolute fat oxidation rates, and no cross-over point, i.e., there isn’t a single intensity at which they burn more fat than carbohydrates, not even when jogging:
I’ve shown in my report how I could go from these low rates up to about 0.8 grams/minute, which is on the opposite side of the bell curve for this parameter, even when looking only at endurance athletes (and how it is not uncommon now to see data in the 0.9-1.0 grams/minute range when I test on myself).
Hence, it seems clear that diet and nutrition are the most important levelers when it comes to improving our metabolic flexibility. And sure, there will always be genetically gifted individuals oxidizing high rates of fat while eating crap all day. That’s just the way it is. However, if we don’t belong to that category, we can work on it and dramatically change our metabolism through proper nutrition.
Given that if there is one valid criticism of periodizing nutrition is that it is not the most practical thing to do when sharing meals with others, both Alessandra and I adopted the same diet and mostly aligned our hard workouts so that it would make life easier. At our lab, we also tested her fat oxidation, which went from the same 0.2-0.3 grams/minute to 0.6 grams/minute, showing similar improvements to mine. I am now in the process of working with a few athletes who target ultramarathons to bring these same changes to their metabolism, something I will document in the future, but so far, it is clear that we can at least double our fat oxidation rates in a few months of periodized nutrition, when starting from particularly low values. The implications in terms of glycogen stores are huge, as I’ve detailed here.
*We can argue forever on what this means. For the sake of my argument here, I mean “bad for fat oxidation”, not bad for health, which is another important conversation to have, just not today.
Links with Lactate, Body Weight, and Training Load
There are a few misconceptions on the relationship between substrate oxidation and lactate, body weight, and training load, which I’d like to briefly address here.
Lactate
The relationship between lactate and fat oxidation is weak. If you want to look at fat oxidation, please measure oxygen consumption and carbon dioxide production, because otherwise you’ll be easily fooled. As an example, when in the best shape of my life, my lactate curve had moved in the right direction, my first lactate threshold was as good as ever, my absolute lactate values at low intensities weren’t moving from 1 mmol/L, and yet, my fat oxidation was pathetic. My inability to oxidize fat was not visible in lactate data. I get it, lactate is cheaper than indirect calorimetry. Just like PPG is more convenient than actually measuring things that we care about, like blood pressure. Cost or ease of measurement, however, don’t make these methods valid; they just make them cheap or convenient.
Here, I want to reiterate a simple fact: lactate is not what you measure if you want to look at fat oxidation rates. There isn’t a single scientific article that investigates fat oxidation rates in relation to training and / or nutrition and that uses as reference lactate. Once again: if you want to look at fat oxidation, please measure oxygen consumption and carbon dioxide production. While my metabolism has changed dramatically in the past year, nothing has changed in lactate data: same resting values, same values at low intensities, same curve during an incremental test.
Please also note that this does not mean that lactate is not useful, there are uses, of course (see here and here for example), but fat oxidation is not one of them. Not in absolute terms, not in relative terms.
If you can measure something, measure it; don’t try to guess it from somewhat correlated variables.
No data is better than made-up data.
Body Weight
In my original experiment, I had lost plenty of weight. I documented the change because it would not have been fair not to, since it did happen, even though I was certain the change in body weight had nothing to do with metabolic changes and oxidation rates. How could I be so sure? I had already been that weight and body fat, and I had already collected metabolic data, and I had measured pathetic fat oxidation rates back then as well. Hence, to me, it was obviously about the macronutrient distribution, not about my body weight or body fat.
Fast forward 8 months, and I have even more evidence in support of my statements: I have now gained 5 kg / 10 pounds (and some adipose tissue to go with it), and my fat oxidation is still great or even better than it was in February. Weight is a whole topic and has its role in performance, but in the context of metabolic flexibility, it’s not what drives the changes.
Training Load
At this point, it should be clear also that training load impacts oxidation rates minimally for well-developed athletes. My training volume has been overall lower this year, even though intensity has been a bit higher as I’ve introduced ways to do more work (e.g., using the bike or hills, to limit muscular damage). Hence, both chronically and acutely, my training load has been going in the opposite direction with respect to what you’d think stimulates fat oxidation. However, as mentioned earlier, even testing on a week in which I barely trained (and only trained hard when I did train), resulted in some of my best fat oxidation rates, since I had focused more on nutrition in that period.
We burn what we eat.
Substrate Oxidation During Prolonged Exercise
How does fat oxidation change during a very long event?
This is a somewhat underinvestigated aspect that piqued my interest only recently and that I now find extremely interesting. In particular, I was sent this article, showing how even at race intensity, and fueling with plenty of carbohydrates (and while being on a high-carb diet), prolonged exercise (> 6 hours) eventually leads the body to shift towards using fats as the main fuel source even at race pace. The ironman in question, a world record holder, was running even faster in the second part of the test (3’37”/km, which requires great energy turnover), while relying mostly on fat. That piece of data alone goes against what you hear from most people, pretty amazing how our metabolism can adapt.
I had previously done some simple math to estimate my carbohydrate requirements in a long race given full glycogen stores and fat oxidation rates assessed during short tests (such as the one I linked earlier). However, it is clear that the way things work is very different, and fat oxidation rates can increase dramatically during a long event. As someone who struggles to maintain high intake during a race becasue of nausea (or worse), I found the implications of the study above really interesting and possibly able to explain what I had experienced in my recent races, i.e. the ability to maintain high power output, race strong, and eat little especially in the second half of the race when nausea would be more present (please note that I am not arguing that this is how it should be done, but it is a way to do it. Are you looking to improve your race performance or to maximize the number of gels you can eat per hour?).
Given my profile and target races I decided to do a little science experiment and replicate the study above, to see how my metabolism would shift substrate use during a long race. I went to my lab, measured fat oxidation, then ran 6 hours and a half not far from race pace for an event of that duration, and went back to the lab to measure fat oxidation again. Eventually, there was a massive change in substrate utilization, in my case even more marked than what was documented in the paper, reaching 1 gram/minute in terms of fat oxidation.
Race fueling
I will add that in today’s climate, it’s difficult not to get in the order of ideas that we need to eat 100s of grams of carbohydrates per hour in order to perform. That’s all I hear from most people. Even many nutritionists will now so easily say that you should get to around 90 grams/minute as a default statement (regardless of your physiology, and even regardless of your power output. Are you running at Tom Evans’ pace? And even then, did you know that he actually reduced his intake this year? That’s what I thought..). Even I, despite the change in diet and metabolism, recently felt “the pressure” and increased intake, leading to more nausea, just like every other time I tried. If anything, these tests after prolonged exercise put my mind at ease again: while before changing my nutrition I could not run well during ultramarathons, because I was burning through glycogen stores in no time, my body now can clearly run well at the intensities I need to run for my target events while relying more on fat. Carbohydrate intake remains key to maintaining blood glucose levels and sustaining performance. I wouldn’t go out without any intake for long events, obviously, but the optimal rate is highly individual, and for me personally, it can be lower than “recommended”, now that I’ve done the work required to improve metabolic flexibility. Do we really think that there is a magical number that brings optimal performance to every human? Really?.
With this last paragraph, I just want to give you some food for thought. There is large individual variability, and there are different roads to reach our optimal performance. I’ll also add that the considerations above would change once again if you were to target multi-day events, in which it might help to increase carbohydrate intake to speed up recovery before the next stage. Find what works for you and your event.
Performance Outcomes: From the Lab to the Race
All that I have discussed above is intellectually stimulating, but at the end of the day, we also want to run faster, don’t we?
As I’ve been a runner for a while, and I train consistently at high volume (say, 5-6000 km/year already for many years), and with consistent high intensity, the days of big improvements are long gone (or so I thought). For anything that goes from a short race, say a 5km, to a half-marathon or even a marathon, my performance has been rather constant in the past 2-3 years. Sure, I ran a few seconds faster here and there, but again, no dramatic changes.
This year, I can say the same about those distances, and this is of course expected. If you are targeting an event shorter than 3 hours, improving fat oxidation is unlikely to make a large difference. However, if the event is longer, then we start to enter a territory in which glycogen sparing does matter, and that’s what I’ve seen in my performance as well. In particular, I took 10 minutes off my personal bests over 50 km this year, on different courses, thanks to improved metabolic flexibility (races discussed in more detail here and here).
In other words, despite the same VO2max, same lactate threshold, and same running economy, changes in metabolic flexibility have translated into better durability (an improved ability to sustain higher intensities for longer in ultramarathons) and therefore better performance, which was the whole point for me to go this way.
Periodized nutrition brings a more efficient fat metabolism and preserves glycogen, which in turn extends muscular endurance. In ultras, that’s often the difference between holding pace or fading late. The reason why I am sticking with this diet (apart from at this point enjoying it!), is that my performance is better, not that my fat oxidation is better. I hope that’s clear.
Final Thoughts
After a year of applying a more periodized approach to nutrition systematically, both in myself and with other athletes, I’ve seen how metabolic flexibility is a highly dynamic trait (both between workouts and within the same workout!), mostly driven by our diet. The way I’ve been improving my physiology and ultramarathon performance is quite far from the current hype. It is not better, it’s better for me. Individualization is key, and there is no recipe that works for everyone, given large individual differences, goals, strengths, and weaknesses.
When asked by my athletes if they should do something similar, I start by looking at their training data and race requirements. If available, I also look at their metabolic data. If their race requirements do not need large fat oxidation rates, there is no need to go there. If they do (e.g., their goal is an ultramarathon), and their metabolism already supports it with good fat oxidation rates, either because their diet or because of their genes (likely a combination of both!), great, no need to go there either. Somewhat similarly, if they have no gastrointestinal issues and feel that energy-wise things could go better, we can try higher rates of carbohydrate intake to see what helps the most in terms of their performance.
This is all to say that if you have a profile similar to mine, we might try similar things. If you don’t, we’ll do something else. My only training philosophy is individualization. Pretty simple, isn’t it?
Alright, that’s a wrap for this follow-up on all things periodized nutrition and fat oxidation. I hope there was something useful in there for you, and happy training!
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Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching. He is a certified ultrarunning coach.
Marco has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine. He loves running.
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Regarding this statement:
“ If you are targeting an event shorter than 3 hours, improving fat oxidation is unlikely to make a large difference. However, if the event is longer, then we start to enter a territory in which glycogen sparing does matter”
I think the paper you link supports this idea but wanted to ask for clarity: Do you think the above quote applies only to running events or would Ironman (or any endurance event that is 6-9+ hrs) also fall into this category?
What about a road 50k that’s around 4 hrs?
Thank you for this great update! As an integrative GP and runner (up to marathon distance) the idea of metabolic flexibility has always made so much sense to me. You’re right - it varies for each individual but you give some good general guidelines without extreme measures. The most important point you make is the quality of the food, which for many athletes is unfortunately not good- especially around race time and after. Lots of poor quality processed carbs. All the best!