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.
“ 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?
Hi Jeff! Yes for sure fat oxidation is key in all endurance sports and in an Ironman. It’s important in the marathon as well, but the longer the race, the more likely we are to run out of glycogen stores if we can’t be metabolically flexible and use plenty of fat as fuel at various intensities. Same considerations for the 50 km, I think working on my metabolism was the only reason that led to a recent breakthrough for me in that distance (3 h 43’), see this blog https://marcoaltini.substack.com/p/trainingtalk-winschoten-50-km-2025
Thank you again Marco, this sharing is so inspiring! Do you finally find an answer to your question « Can we periodize nutrition less and still have 90% of the gains? When do we start to make it worse? ». Where do you find how to design your meals for that experimentation : nutritionist? UESCA course ? 🙏
thank you. Regarding the 90% story, it's simple but also complex, what I mean is that sure, you can be flexible, but you still need to periodize your diet and eat lower amounts of carbohydrates on most days if you are in a situation like mine metabolically. Letting go here and there, in modern society, is a slippery slope, as we are bombarded with all sorts of crap wrapped as food (enter any cafe or bar in Italy, and there isn't anything that is not either fried or a sugar bomb, or ultraprocesesed - not really the Mediterranean diet!). We spend little time talking about the quality of the foods more than the macronutrients, and these matter a lot, as there are large hormonal differences in response to the same calories and even the same macronutrients. For my meals I keep it simple, breakfast either with greek yogurt, nut butter, nuts, and berries or an omelette with avocado and cheese, lunch is always a salad with a bit of everything, from legumes to farro to possibly some fish or chicken, and dinner is similar but typically with cooked vegetables. I do have more fish now than I used to, and tend to snack mostly on nuts and yogurt. For higher carb days I also try to keep it very similar but just add some fruit, potatoes, oats for breakfast, and occasionally I'll have a pizza or eat out and have some pasta, but again, living here if you don't make an effort you end up eating like that every day, and that's not going to work for my fat oxidation and associated performance.
Absolutely brilliant. Thank you very much for sharing your insights.
You write: ‘The exogenous intake of carbohydrates does not conserve muscle glycogen, so you can only conserve glycogen through a high rate of fat oxidation, regardless of what you eat during the race.’
But in your calculations, you include the carbohydrates consumed. I’m not quite following.
How do exogenous carbohydrates work? What is the mechanism behind such high intakes? How is this processed by the body? It seems that, for professional athletes, this has certainly been the key to the leap in performance in recent years.
thanks Bruno! Much appreciated. Exogenous carbohydrates can help us in a few key ways: 1) maintain blood glucose level 2) replenish liver glycogen (hence it does not impact our muscles directly, but our liver storage), 3) brain-mediated improvements (think mouth rinse which shows improved performance even without getting the carbs anywhere near our digestive system). None of these things require high rates though (maybe 3 can require a steady supply, but that could be done with small sips), and in a way I would like also to challenge the idea that the elite have shown better results because of this :) See for example the recent sub-2 marathons, one of the two was much talked about because of their high rate, but the other one had about 50 grams per hour and still ran sub-2! Carbs are essential for long distance endurance performance, but the optimal ranges are likely much lower than currently advertised in this ‘high carb era’ (even the winner of UTMB reduced his rate in the last year). We need individualization, and for some people running for very long at a very high intensity (e.g. a fast 100km or a fast 100 mile running race), it can make sense to fuel very high rates, since they burn so much energy (because of the speed/power output), but the way this gets automatically extended to the average person like me, I think is quite absurd. At my speed/power output for a race that long I can use a completely different approach (relying more on metabolic flexibility and fat oxidation), since that will be enough to cover the energy requirements of the intensity I’m running at. I still need carbs to maintain blood glucose, for centra/brain-gains and simply for calories if it’s a long race, but that can be done with 30-60 grams per hour. Especially for people that struggle to eat much or often have GI issues, I think it’s a much better idea to work on metabolic flexibility and add more tools to their toolbox, so that in a race, they can modulate carb intake and the body can perform optimally at different rates. I’ve added some of these thoughts also on a recent blog post about my preparation for Chianti, see also that one if interested and thank you again!
Thank you very much, Marco, for your detailed response. I especially like your approach of following a low-carb diet in principle and strategically consuming carbohydrates (for intense workouts and races). My experiences with this approach are still a bit mixed, but I’m slowly finding my way. A few really good training sessions (4–6 hours on the bike) have convinced me to stick with this approach. I struggled a bit with the more intense workouts (threshold), but the HIIT sessions went surprisingly well. I’m not as structured and analytical as you, and I know I can’t compare myself to you. But still, most amateurs and ambitious hobby endurance athletes like me are probably better off with this approach. Especially once you reach a certain age (I’m over 60) and after decades of carb-loading.
I’ll read your post about preparing for Chianti with great interest. All the best!
Thanks again for a fab post Marco, I love your insights and your detailed explanations. I did two multi day stage races this year: one in Feb and one in Sept, both around 275km over 6 days. A change I made from the first to second was more carbohydrate in the post-stage recovery, and reduction of fat sources (I feel nauseous with too much fat) and that def helped. In race fuelling was pretty light (30-40g/h) and felt good for most of the race (except on the sand dunes at 1am on the longest day lol). I love hearing the n=1 combined with science as you share your experiences.
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!
More seriously, after trying to figure out how to rise FatMax/FatOx with training only (pushing or pulling…) without success, your journey is really inspiring !
When you say 160g of oats you mean dried oats ?! I ask this because with 50g of dried oats + milk I’m full !
One key aspect of quality diet is fibers. Whether it is through microbiota change or carbs availability, I feel (no met cart sorry) that having a high fiber breakfast for example that makes me satiated till lunch without the urge to snack on something. And I hope that this phenomenon is also a sign of better fat oxidation between meals.
agree on the fiber, I think that similarly, fat and protein help (a lot of it it’s about glycemic control after all, if we snack on sugar 24/7 - which is what people do, let’s not pretend carbs to be “some extra veggies” - we get quickly out of whack hormonally, which has implications even for the same caloric content. Regarding oats, agreed, I eat a lot of them before a big day (marathon or ultra), but it’s a task. Normally it works out if I do it about 3 hours before the race.
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?
Hi Jeff! Yes for sure fat oxidation is key in all endurance sports and in an Ironman. It’s important in the marathon as well, but the longer the race, the more likely we are to run out of glycogen stores if we can’t be metabolically flexible and use plenty of fat as fuel at various intensities. Same considerations for the 50 km, I think working on my metabolism was the only reason that led to a recent breakthrough for me in that distance (3 h 43’), see this blog https://marcoaltini.substack.com/p/trainingtalk-winschoten-50-km-2025
Thank you again Marco, this sharing is so inspiring! Do you finally find an answer to your question « Can we periodize nutrition less and still have 90% of the gains? When do we start to make it worse? ». Where do you find how to design your meals for that experimentation : nutritionist? UESCA course ? 🙏
thank you. Regarding the 90% story, it's simple but also complex, what I mean is that sure, you can be flexible, but you still need to periodize your diet and eat lower amounts of carbohydrates on most days if you are in a situation like mine metabolically. Letting go here and there, in modern society, is a slippery slope, as we are bombarded with all sorts of crap wrapped as food (enter any cafe or bar in Italy, and there isn't anything that is not either fried or a sugar bomb, or ultraprocesesed - not really the Mediterranean diet!). We spend little time talking about the quality of the foods more than the macronutrients, and these matter a lot, as there are large hormonal differences in response to the same calories and even the same macronutrients. For my meals I keep it simple, breakfast either with greek yogurt, nut butter, nuts, and berries or an omelette with avocado and cheese, lunch is always a salad with a bit of everything, from legumes to farro to possibly some fish or chicken, and dinner is similar but typically with cooked vegetables. I do have more fish now than I used to, and tend to snack mostly on nuts and yogurt. For higher carb days I also try to keep it very similar but just add some fruit, potatoes, oats for breakfast, and occasionally I'll have a pizza or eat out and have some pasta, but again, living here if you don't make an effort you end up eating like that every day, and that's not going to work for my fat oxidation and associated performance.
Absolutely brilliant. Thank you very much for sharing your insights.
You write: ‘The exogenous intake of carbohydrates does not conserve muscle glycogen, so you can only conserve glycogen through a high rate of fat oxidation, regardless of what you eat during the race.’
But in your calculations, you include the carbohydrates consumed. I’m not quite following.
How do exogenous carbohydrates work? What is the mechanism behind such high intakes? How is this processed by the body? It seems that, for professional athletes, this has certainly been the key to the leap in performance in recent years.
thanks Bruno! Much appreciated. Exogenous carbohydrates can help us in a few key ways: 1) maintain blood glucose level 2) replenish liver glycogen (hence it does not impact our muscles directly, but our liver storage), 3) brain-mediated improvements (think mouth rinse which shows improved performance even without getting the carbs anywhere near our digestive system). None of these things require high rates though (maybe 3 can require a steady supply, but that could be done with small sips), and in a way I would like also to challenge the idea that the elite have shown better results because of this :) See for example the recent sub-2 marathons, one of the two was much talked about because of their high rate, but the other one had about 50 grams per hour and still ran sub-2! Carbs are essential for long distance endurance performance, but the optimal ranges are likely much lower than currently advertised in this ‘high carb era’ (even the winner of UTMB reduced his rate in the last year). We need individualization, and for some people running for very long at a very high intensity (e.g. a fast 100km or a fast 100 mile running race), it can make sense to fuel very high rates, since they burn so much energy (because of the speed/power output), but the way this gets automatically extended to the average person like me, I think is quite absurd. At my speed/power output for a race that long I can use a completely different approach (relying more on metabolic flexibility and fat oxidation), since that will be enough to cover the energy requirements of the intensity I’m running at. I still need carbs to maintain blood glucose, for centra/brain-gains and simply for calories if it’s a long race, but that can be done with 30-60 grams per hour. Especially for people that struggle to eat much or often have GI issues, I think it’s a much better idea to work on metabolic flexibility and add more tools to their toolbox, so that in a race, they can modulate carb intake and the body can perform optimally at different rates. I’ve added some of these thoughts also on a recent blog post about my preparation for Chianti, see also that one if interested and thank you again!
Thank you very much, Marco, for your detailed response. I especially like your approach of following a low-carb diet in principle and strategically consuming carbohydrates (for intense workouts and races). My experiences with this approach are still a bit mixed, but I’m slowly finding my way. A few really good training sessions (4–6 hours on the bike) have convinced me to stick with this approach. I struggled a bit with the more intense workouts (threshold), but the HIIT sessions went surprisingly well. I’m not as structured and analytical as you, and I know I can’t compare myself to you. But still, most amateurs and ambitious hobby endurance athletes like me are probably better off with this approach. Especially once you reach a certain age (I’m over 60) and after decades of carb-loading.
I’ll read your post about preparing for Chianti with great interest. All the best!
Thanks again for a fab post Marco, I love your insights and your detailed explanations. I did two multi day stage races this year: one in Feb and one in Sept, both around 275km over 6 days. A change I made from the first to second was more carbohydrate in the post-stage recovery, and reduction of fat sources (I feel nauseous with too much fat) and that def helped. In race fuelling was pretty light (30-40g/h) and felt good for most of the race (except on the sand dunes at 1am on the longest day lol). I love hearing the n=1 combined with science as you share your experiences.
thank you Mikki, also for sharing your insights, lots to learn!
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!
Thanks for this Marco, very interesting round up and great to see your thoughts alongside the examples of your own training and racing.
thank you Steve!
Superb. Can’t wait to read your next post about double labeled water and 100k events ;)
More seriously, after trying to figure out how to rise FatMax/FatOx with training only (pushing or pulling…) without success, your journey is really inspiring !
When you say 160g of oats you mean dried oats ?! I ask this because with 50g of dried oats + milk I’m full !
One key aspect of quality diet is fibers. Whether it is through microbiota change or carbs availability, I feel (no met cart sorry) that having a high fiber breakfast for example that makes me satiated till lunch without the urge to snack on something. And I hope that this phenomenon is also a sign of better fat oxidation between meals.
agree on the fiber, I think that similarly, fat and protein help (a lot of it it’s about glycemic control after all, if we snack on sugar 24/7 - which is what people do, let’s not pretend carbs to be “some extra veggies” - we get quickly out of whack hormonally, which has implications even for the same caloric content. Regarding oats, agreed, I eat a lot of them before a big day (marathon or ultra), but it’s a task. Normally it works out if I do it about 3 hours before the race.