Monday, 5 September 2016

Can I run as fast on fat as on carbs?

I was watching the diamond league athletics on the telly last night and wondering, could I still run 400 meters in under a minute? When I was a boy I had a PB of 55 seconds. And 4 years ago pre-LCHF I clocked 62 seconds after an ill-advised bet with with my son; could I still get anywhere close to that?

The stock answer to the question "Can I run as fast on fat as on carbs" tends to be that the fastest runners are all fuelled primarily by glycogen, not fat. This may be true, particularly as they all tend to be young and insulin-sensitive; they are suited to running fast on carbs. Most of us are neither of these.

But where is the line between carb-speed and fat-speed? According to Tim Noakes this line is probably only a few minutes back from the leaders of a Marathon, or a few seconds for a 10K.

And so the number of people for who the stock answer applies is actually tiny. Look at the details of elite low-carb runners and you will see that they run sub-2:30 marathons and 30 minute 10ks. How many people do you know who can do that?

What I hope I have proved on this blog is that for everybody else it is possible to train ourselves to run just as fat on fat as we could on carbs. See this post for lots more detail on how we can train our bodies to run fast on fat.

Perhaps there is a competitive advantage to the shorter, faster races from burning carbs, but again, how many people does this apply to? Unless you are a track running member of an athletics club or still at school then it is very unlikely that you will ever run a fast 400 meter or mile race again (I'd put the parents race on sports day in a separate category here!). In which case this advantage is of no consequence, you could run just as fast on fat as you could on carbs.

And so the big question should really be, why would I want to run on fat?

Well my answer to this question comes in two parts:
  • As a runner I am much happier and feel more consistently energised when burning fat. I also don't have to carry anything on long runs which is rather nice.
  • As a human I am convinced that the older I get the more suseptible I will become to insulin resistance and the risk of obesity or diabetes is just too great.
And as for the standing quarter in under a minute. I am confident that I'll do this down at the track one evening this autumn.


  1. Agree with everything you have said here. I have been on a Ketogenic Diet since Jan'16 and training for a Marathon since April. I am loving both the diet and training, although I am currently injured haha.

  2. Hi, Malcolm.
    Great blog! really helpful.
    Can I ask you something?
    I´m 38 years old, been running for 8 years and have run 7 marathons (PB 3:35, last july)
    I have high blood cholesterol and I don´t want to take statins for the rest of my life, so I decided to try LCHF for the 1st time.
    It´s been 10 days of LCHF and I´m really struggling with my running. Before LCHF, my easy pace was 5:15/km and I could run 15-20 km @ 4:50/km if I had to. Now, I can barely run for more than 50 minutes and my pace is 5:25-5:40/km at best.
    I bought Tim Noake´s book and I´m following his diet suggestions thoroughly. Can´t complain about being hungry or any mood swings, but the slow running is troubling me.
    - How long did it take you to run at your former pace after switching to LCHF?
    - How you lost weight after LCHF? I haven´t, yet
    Thanks and sorry for my English (not my 1st language)

    1. I don´t count calories, but I think my diet is in the very very low carb range (no potatoes, sugar, sweets, fruits, rice, pasta, chocolate or cheating whatsoever)

    2. Hi Felipe
      It can take up to 3 weeks for your body to adapt enough for running to start to feel easier again, and anywhere between a month and two before you are back at the same level of fitness. This took me less but I know others who look longer. It should be viewed as a long term commitment!
      Keep at it, and Good Luck!

    3. Oh, and make sure you and taking on enoug fat! You need energy.

  3. I received the latest post from Malcolm Kendrick today and he gives a timely description of how we can perform perfectly well on a diet of zero carbs, Perhaps this helps explain how I managed to get a PB racing up the local hill yesterday!