Sunday, 14 April 2013

The effect of Fat burning on Carbohydrate requirements for an Ultra

I've just completed some preliminary analysis of the effect of Fat burning on Carbohydrate requirements for the Highland Fling, comparing fat burning measurements of Scottish Ultra runner Caroline McKay and the figures published in the paper "Maximal Fat Oxidation During Exercise in Trained Men" (MFO).  The results are startling, demonstrating the low carb diet recently adopted by Caroline and the training she does has had a massive effect on fat and carbohydrate utilization, far eclipsing the reference data for typical trained athletes.

The analysis required Heart Rate (HR) data against speed, total Calorie consumption and amount of fat/carbohydrate matabolized.  I've been hunting such data and in particular was looking for local ultra athlete to give it relevance to Scottish ultra scene, and last week Caroline McKay posted "Quest to become a metabolically efficient fat burning machine!" entry on here blog.  Very kindly Caroline passed on the Metabolic Testing Report for her January session at Napier Uni's.  This report provided HR, speed, calorie and Respiratory Exchange Ratio (RER) values that allowed me to compute the fat/carb values, and as there is only a 6 data points provided I've had to fill in the gaps by fitting a mathematical functions to various elements of the data.

For reference I also mapped the data provided by the published MFO article normalizing to match Caroline's HR and total calorie usage so I could do a representative comparison.

The maths behind this analysis is something I've put together this weekend so isn't in any published article or book, and while I could attempt to explain it all I'll not attempt it in this blog entry, it's really not something I'd expect 99% of reader to ever want to get their heads around, ask me if you want to know more.  As I'm not trying to write an academic paper rather dwell on the details I'll just cut to the chase and show you the base graphs of the various key parameters, so you'll know where the final analysis is drawn from.

First up we have the relationship between HR and speed that Caroline's fitness test found - I've fitted the original test data to the straight line as this matched really well and allows me to extrapolate them to lower ultra-marathon relevant paces.

Next I turned to fitting a curve to the total calorie and fat usage for Caroline's report, and did the same for the MFO data whilst recalibrating the values to match Caroline's HR range and total calorie consumption.  The key was making the comparison of the data representative.  In the HR to Calorie/Fat chart below red data is Caroline's data, and yellow is the MFO reference, while blue is the total Calorie usage rate (Calories per hour).

While these are idealized curves, they are fitted to real data, take your time to digest just what the results are - Caroline is burning fat almost right up to here maximum HR, while the "Trained Men" reference began using carbs exclusively from a HR of 151 upwards - an intensity that Caroline is still getting almost half here energy from fats.  I find this quite simply extraordinary. Awesome job Caroline!

While the graph above is staggering, what does this mean in practice when running an ultra?  Like many in the Scottish Ultra scene the next big race for us is the 53.1 mile Highland Fling that runs along the southern half of the West Highland Way.  Using an estimate of 5,772ft of ascent for the route that John Kynaston kindly provided, and some of my own mathematical models for cost of elevation/descent on running efficiency I estimated a 9% time and energy cost for all those hills.  Then using the HR to speed and HR to energy usage graphs above I then extrapolated how many Calories of Carbohydrates would be required for different finishing times:

There is no way Caroline will be able to sustain her maximum HR for the whole route, but I thought it would be useful to see the effect on carbohydrate needs on the extremes both at maximum speed and at slow enough to just squeeze in before the 15 hour cut off.  Several amazing things drop out of this graph:
  • Normally trained runner requires at minimum 3000 calories of carbs to complete the race, this means consuming at least 1000 calories of carbs as well as burning all their 2000 Calories of stored glycogen.
  • If Caroline was happy to do a slow trot she could get by with just burning 500 calories of carbs.
  • Caroline's 2011 PB of 10:34 would now require less than 1000 calories of carbs.
  • While a normally trained runner would require nearly 3500 calories of carbs for the same PB pace.
  • A course record of 7 hours would require just under 4000 calories of carbs for a fat burner like Caroline, this would entail eating about 285 calories of carbs per hour to make up the extra 2000 calories required over glycogen stores.
  • While a normally training runner to run a course record would require over 6000 calories, and a staggering 571 Caloris/hour
  • I believe these figures suggest that for a 7 hour course record we'll need athletes that are even more fat adapted than Caroline, as well as having the extraordinary speed over distance that we've always expected.
I'm pretty blown away with all these stats.  But we do need to take a step back and remember this is just a quick analysis that makes several assumptions that compromise the accuracy to actual requirements on the day.  Lets run though these:
  • Assumed the fat/carb consumption doesn't change through out the race, I couldn't factor this in without any concrete data so rather than guess I've just left this as an assumption.  The assumption will mean that the above figures are likely an over estimate of actual carb requirements.
  • The hills and rough terrain may well be cost more than 9%, so this might push up/pull down the data a bit.
  • Weather effects fat/carb consumption, again this is large unknown so not possible to factor in so for the analysis I'm assuming that the temperature is the same as for Caroline's test.
  • Consumption of fluids and nutrition will have an effect on fat/carb consumption, again as I can't quantify this I've assumed it's net effect is zero.
  • Pacing is consistent through the race - I'm planning to specifically look at this issue in a follow up article (although as spoiler I can say from a carb sparing perspective the most efficient split is an even split - negative and positive splits will required more carbs overall.)
I'm sure I've made further sweeping assumptions, but I believe the general findings are useful:
  • Food Mix -> Fuel Mix : The low carb, high fat diet that Caroline has been following this year clearly is very effective at supporting high fat utilization across all running paces and especially at ultra race paces. 
  • Typical trained runners with high carb western diets metabolize carbs over fats, even at slow paces and will require large amounts of carbohydrates to finish the Fling, consuming at least 100 calories of carbs per hour will be required.
Further thoughts on what this means when it comes to race day:
  • For capable fat burners like Caroline it should not be necessary to consume large amounts of carbohydrates.  
  • Reducing the amount of carbohydrates consumed reduces the risk of stomach problems
  • Consuming carbs at too fast a rate is likely to induce a insulin response which switches off fat metabolism, so if you don't need them desperately then limiting consumption is likely to be lower risk
  • Might the new wave of engineered starches like SuperStarch help avoid the insulin response? Is anyone selling them in the UK yet?
There is lots more to think about besides, personally I think the results are jaw dropping.  An ultra runner diet looks to be one of most important aspects that we should be working on to improve our performance and enjoyment of races.


Poscript:  Just adding a small note: since writing this blog entry Caroline has run the Highland Fling 53 ultramarathon, setting a new personal best of 9:36 for the race, which an hour faster than her previous best.  Caroline in her blog entry, A Fling with Friends, reports that she felt strong throughout the race despite eating less than in previous years.  This performance provides good evidence that she has achieved her goal of becoming efficient at metabolising fats and as consequence has achieved a very impressive improvement to her ultramarathon race performance.


  1. Robert, this is very interesting and I think does provide quite thought provoking evidence in favour of a low carb diet.

    However, the fact that you plot calories from fat v HR in the second plot is a little misleading as it is energy consumption at a given pace that matters most to the athlete. CM has a relatively high HR at a given pace. My own data acquired when I was fairly fit a few years ago shows that at paces around 8 mph my HR was 17 bpm lower than CM.

    The curve of energy from fat v HR for CM shows calories from fat falling to zero at 167.5 b/min which corresponds to a pace of 9.2 mph. My own data shows that at 9.2 mph my HR was 150. If my ability to use fat was similar to that of the males in the MFO sample, I would still be obtaining a small proportion of my calories from fat at 9.2 mph. I am not especially attentive to diet. I do eat a lot of fruit, veg and cereals so probably obtain a little above average proportion of my calories from carbohydrates.

    I suspect that the most likely reason that CM has a higher HR at a given pace than I do is the likelihood that she has a smaller left ventricular volume. Females usually have smaller hearts than men. Furthermore, the available evidence suggests that I have a fairly large heart relative to my size.

    Low levels of circulating glucose can lead to higher adrenalin levels, which might be another factor accounting for her higher HR values. I think that high levels of adrenalin are not desirable during an ultra.

    Nonetheless, the really impressive thing about CM is the very high proportion of energy obtained from fat at slow paces. If you re-plotted her data with pace on the horizontal axis, you would find that at 6.5 mph, she obtains over 2/3 of her energy from fat, whereas, if my ability to utilise fat matched that of your reference sample, I would be obtaining only about 36% of the required calories from fat at that pace.

    Some of her efficiency in obtaining energy from fat is probably due to her diet, though it should be noted that I had not been training for an ultra and had done virtually no training at paces slower than 7 mph in the preceding year. The really important comparative data would be in fat utilization in a reference sample who had done comparable amounts of training at slow paces.

    On the topic of diet, have you experimented any further with beetroot?

    I hope that your foot has mended well and that everything goes well for you in the Fling.

    1. Hi Canute,

      Caroline's HR values for a given pace a quite a bit lower than mine, especially a lower paces so I guess I must be even more of out liner :-)

      I wouldn't suggested reading too much into specific HR and paces though, everyone is an individual with their own specific HR to pace graphs. The key thing that the data shows is the profound differences we see if fat utilization between individuals and how this aspect of performance is highly trainable via diet and training. I believe the most dominant factor is diet rather than training though, and illustration of going fully keto-adapted has on fat usage is illustrated in the following post online:

      For the second graph I chose HR against Calories rather than pace against Calories as it's one can work out the Speed vs Calories using this graph in conjunction with the first graph, and knowing the effect of HR on Calories is useful when using a HR monitor to gauge the appropriate intensity to run at. Again I'd recommend runners get a feel for their own HR ranges rather than use this graph.

      It would be an interesting study to compare diets with the same training profiles, and same diet different training profiles to find out the effect of diet and training at slower/faster paces.

      W.r.t beetroot I was planning to introduce it in March and look for effects but alas got injured and ill so that went out of the window. Left feet still hurts so I'm in the pool this evening for some cross training. Being fit for the Fling isn't looking likely unfortunately.

  2. I didn't mention in the post, but I would like to clarify the HR to pace relationship varies from runner to runner, these charts are tailored to Caroline's data so shouldn't be directly mapped to your own HR to pace expectations. For myself my resting HR of 45 is higher than Caroline's 39 resting HR, and my HR for most paces is 5+bpm here, while Caroline's max HR is 168, while mine is 189+ (recorded in a playful sprint at the end of 10 miler last week). So there is huge variation from person to person for the exact figures.

    The key thing I hope is taken away from this article is the general principle behind the effect of fat burning on carbohydrate utilization in ultras, and the bearing this has on nutrition strategies it suggests during training period and during the race.

  3. Robert,
    I am sorry to hear that the foot is still being troublesome.

    Thanks for the link to Peter Attia’s intriguing site – intriguing but also frustrating as I found it easy to get sucked via hypertext links into vortices that never quite delivered the answers I sought. The page you directed me to demonstrated that his 12 week experiment with ketosis led to an impressive increase in ability to utilise fat but a potentially devastating decrease in VO2max. He ended that page with the crucial question: is there a way to reap the benefits of keto-adaptation on the aerobic side, without any of the anaerobic cap costs? He promised he would answer this ‘soon’. There was no date on that page, and on account of the convoluted structure of the site, I could not determine whether or not he has answered that question yet. Do you know if he has answered this question?

    1. I haven't seen any follow up articles so don't know if your improved on his VO2max since. Yesterday I emailed him via to see if he had more data that will enable me to do a similar analysis as I've done with Caroline. I also emailed Michael Arnstein ( in the hope that he might have done some testing and could provide another data point that would allow us to scope out the possible effects of various diets.

      As for the decrease in Peter's VO2Max, while I find this interesting finding, I am not so much of concerned as his anaerobic threshold improved so much that I'd expect his performance capabilities to have improved for any race further than 10k. This just my expectations though, he hasn't published any race performance results before/after going Ketogenic so we'll have to wait for him or others to publish their results. It's still early days for endurance athletes going low carb/Ketogenic so I feel we just need to be patient. We can of course help speed things up by helping out ourselves :-)

  4. An additional feature of interest is that in the MFO group, the maximum rate of fat utilization was correlated quite strongly with VO2max (r= 0.64) indicating that those with the greatest capacity to metabolise glucose also had the greatest capacity to metabolise fat. This is consistent with the fact mitochondrial oxidative capacity contributes to the utilization of both fat and glucose.

    Furthermore, in both high and low VO2max groups, peak fat utilization was achieve at a mean of 63% of VO2 max, suggesting that despite the higher fat utilization in the high VO2max group, the ratio of energy derived from fat to that from glucose at the pace of maximum fat utilization was similar in both groups.

    Since 63% of VO2 max is not far from the typical oxidative rate at marathon pace, both high and low VO2mx groups in the MFO sample would have been expected to run out of glucose at about the same distance in a marathon. So the unsurprising conclusion is that increased VO2max will get a marathon runner no further before hitting the wall, though the athlete with higher VO2max is likely to reach that point in a faster time (assuming similar capacity to metabolise lactate).

    This reinforces the accepted fact that marathon runners need to find a way to increase fat utilization capacity relative to glucose utilization. A ketogenic diet is perhaps one way to do this, but I would like reassurance that this could be achieved without damaging the capacity to utilise glucose.

    1. Dang your observational skills! I had been planning to dedicate a whole blog post to this observation made in the MFO study - that Fatmax was observed to broadly proportional to VO2Max, and the % of VO2Max at which Fatmax occurred was consistent between low and high VO2Max groups.

      One thing studies often do is look for averages rather than look specifically for variations, I feel for understanding what effects training and diet have on fat utilization, lactate threshold and VO2Max requires one to look at the variations between individuals. The MFO study does mention that they saw a large variation in Fatmax - 20 to 40% of total energy usage at Fatmax, and also speculate briefly that diet may be one factor in this but as it wasn't the main focus of the study it doesn't attempt to go more deeply into this topic.

      From Caroline and Peter's results with low carb and Ketognic diets respectively and the massive increase in fat utilization that we see over the MFO study I can only conclude that looking such studies have limited value in judging an individual's potential performance.

      On the theme of individual variation, when I did Kielder Marathon back in October my resting HR was 45, Maximum HR up around 189, and my average HR in the race was 169, which is 86% of my heart rate reserve and over 30bpm above where 63% of VO2 max is likely to be! This is also a higher average HR than Caroline's max HR of 168, and I'm 17 years older (43 vs 26). These differences will be down to genetics rather than training or diet.

      As to the effect of boost fat utilization via dietary changes might have on maximum VO2max and ability to utilize glucose, I suspect it'll only the ability to utilize glucose when anaerobic that will be impacted. I also suspect that a runner trained in low carb state is likely to develop their aerobic capacity more strongly and if they go back into on more carb rich diet they'll be able burn more glucose aerobically and up their VO2Max - a bit like the way altitude training can work.

  5. I agree that the low carb diet has intriguing possibilities, though I am still cautious. Another issue that I touched on in my first comment is that training in a low carb state is likely to result in higher adrenalin levels leading to a catabolic state in which muscle might be damaged. As an elderly runner, I am very cautious about encouraging an unnecessary catabolic state.

    Your high HR during the Kielder marathon is impressive, and is likely to be due in part to training. As I understand it, the main point of the sub-LT runs in the Hadd training program is to elevate the HR at which LT occurs. This makes it possible to run the marathon at a higher HR. This effect can also be achieved by other types of training session, including tempo runs and intervals . One of the impressive things about Paula Radcliffe was her steady increase in LT over a period of about 10 years.
    Conversely, it is possible that Caroline’s low HRmax might be due not only to her genes but also her training and perhaps also to her diet. I find that after long periods of mainly slow running, my HRmax decreases appreciably (typically by around 5 bpm). Did Peter Attia’s report whether or not his HRmax decreased following his ketogenic diet?

    1. From books/online resources I have read about low carb diets and performance, the issue of higher adrenalin levels has not been mentioned. Lower muscle fatigue and quicker recovery times are cited of one of the key advantages of using a low carb diet when training so I believe there once adapted to fat burning one doesn't suffer from raised adrenalin levels. My guess is that adrenalin is used by the body for short term needs when blood sugar is low and insulin levels are still high enough to suppress fat metabolism. Insulin is required a low levels to prevent your fat stores from releasing fat into the blood, if little carbohydrate is consumed then insulin levels can drop lower and allow greater release of fat for metabolism without needing adrenalin to be raised.

      Once one if fully fat adapted then the need to radically alter the carb/fat metabolism balance in your body when glycogen levels get low is greatly diminished so the need for increasing adrenalin levels during long periods of exercise is diminished. This year I've reduced my carb intact whilst increasing fat intact and found that my HR drift during long runs is now much less than last year. At least this was the case back in February prior to getting injured and ill. The lower HR drift may well be partly attributed to my shift in diet.

      Even with my shift in diet I still can reach a high HR when the whim takes me, so far I don't believe my max HR has changed much since last year. I wouldn't classify my diet as low carb quite yet though, moderate carb now, rather than high carb previously. The main thing I've cut out is wheat based foods so far, but even here I haven't cut them out completely - if my girls bake a cake then I still enjoy helping eat them :-)

      My diet does now contain more healthy fats - coconut oil, olive oil, rapeseed oil, butter, cream, cheese, home made yogurt, and now avoid vegtable oils/sunflower oils. We are also experimenting with home made fermented veg. Inspiration for these changes come from reading the book Perfect Health Diet ( While it's not specifically a low carb diet - just not a high carb diet that most westerners have adopted, the book provides well researched and carefully constructed diet so I'd love to see what your thoughts on it.

      The other key books I've read on the subject is "The Art and Science of Low Carbohydrate Performance", and am currently working my way through "The Art and Science of Low Carbohydrate Performance". These are very much tailored to selling the ideal of going ketogenic to improve performance and fix metabolic ill health respectively. I'd recommend both of these books too, but the am find the later book a bit laboured in terms of trying to justify the approach and fighting against existing high carb dogma's.

  6. Great post. I'll be honest and say I don't understand the majority of it...but I want to learn. I'm working my way through a collection of books on the matter.

    1. Thanks Debs. Well done for your fabulous result in the Thames 100 :-)

      If there are bits that are confusing/need explaining better just let me know, communication skills isn't my strong point.

      Is there any chance that you've done a similar fitness test to the one that Caroline did? It would be great to analyse your results as you are an excellent example of runner that excels at maintaining pace over ultra-distances, despite rather modest PB's at shorter distances.

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