Fat Adaptation

Some thoughts.


Any fledged member of the endurance community will no doubt, at some point, have been privy to discussions of “fat adaptation”. It’s a controversial topic, owing largely to an internet-wide barrage of anecdotes and pseudo-science. While there is a rational discussion somewhere amidst the rhetoric and tribalism, it has been largely obscured.

Before I go any further, I would just clarify my own position: if you’re diabetic, epileptic, planning on crossing the arctic or taking on any other ultra-endurance challenge, then “fat-adaptation” might be of benefit to you. In all other cases, it just doesn’t really make sense.

There’s a reason the best distance runners of our species consume a high carbohydrate diet1 and there’s a reason why the best species of distance runners rely (metabolically) on carbohydrates12. If you want to run far and you want to run fast, you need carbs. Why?

Carbohydrates yield ~10% more energy per litre of oxygen consumed compared to fats11. And endurance athletes deal in the currency of oxygen consumption, so the implications of this are significant. For example…

Imagine we have two twins – Twin 1 (T1) and Twin 2 (T2). So committed to science were their parents that the two were raised in total parallel, and thus identical in every sense. And serendipitously, the two also conform perfectly to mean population characteristics (because it’s my story and that makes life easier).

The parallelism ends, however, when the two decide to run a marathon. T1 decides to prepare for the marathon while following a conventional, high-carbohydrate diet while T2, having perused some vegan/barefoot running forums, decides that a high-fat diet is the way to go.

Both T1 and T2 arrive on the start line with identical VO2max values (60 ml/kg/min), and both will maintain 75% of their maximal oxygen uptake for the duration of the race10 (i.e. 45ml/kg/min). T1, having gorged on carbohydrates before the race, runs at an RER of ~0.933 (~79% carbohydrate metabolism)13. T2 on the other hand, having assiduously abstained from carbohydrates, runs at an RER of 0.7414 (~87% fat metabolism; i.e. 1.4g/min)13. Assuming the energy cost of running is 1 kcal/kg/min for these athletes6,9 – and the course is perfectly flat and windless – the total energy cost of the 42.2 km run is ~2,950 kcals. Given this information and a bit of number crunching, we can estimate the finish time of our glycogen-loaded and “fat adapted” runners: 3:09:31 and 3:18:17, respectively.

Granted, the above is an imperfect representation relying on a lot of assumptions, many of which are inadequate (e.g. assuming a constant physiological state throughout the race). Nonetheless, it illustrates that carbohydrates are fundamentally a more economical energy source than fats. For a given level of oxygen consumption, oxidising carbohydrates means you’ll run faster, while also affording greater scope for changes in intensity 8,16.

Carbohydrates may be superior in terms of rate, but their issue is one of capacity. A typical trained athlete (70kg body mass, ~16kg leg muscle mass) would normally store ~310g of glycogen in his legs5. Assuming this is completely oxidized, it represents an energy reserve of ~1,200 kcals (40% of the total energy cost of a marathon). Factoring in typical liver glycogen content15, this figure is increased to ~1,500 kcals (50%). Following a successful carbohydrate loading protocol, however, muscle glycogen content can increase to upwards of 550g3,5 — representing ~2,200 kcals, or ~2,800 kcals including a maximally glycogen-loaded liver. While this could cover the energetic costs of a marathon PB, perhaps assisted by some supplemental carbohydrate intake4,7, it is insignificant compared to the capacity of fat stores. At a lean 10% body fat, a runner would have the energy potential to run some 21 marathons. Even at a critically low 2% body fat they would still have the energetic means to run 4 marathons. The difference is simply in the speed at which they could run them.

On a similar note, and in the spirit of being contrarian, I would cast some doubt on the obsession of endurance athletes with “sparing” muscle glycogen. While this makes total sense if you’re on a lab ergometer and want to keep pedaling at, say, 250 Watts for as long you can, it is slightly less applicable where actual race performance is concerned. Given that glycogen is one of our most powerful energy resources2, you can not have achieved your full potential if you finish a race with ample glycogen. I’ll finish with this: while “LCHF”, “keto-adaptation” (etc.) are contentious issues, nobody is going to run a sub-2 hour marathon sans carbohydrate. If you’re a competitive athlete, the debate ends there.


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