By Zoe Wilson APD

Zoe Wilson

There’s been lots of talk about the High-Fat Low-Carb (HFLC) diet recently, in fact one of the Danny Moore crew was asking me about it the other night at squad. So what is it and is it a good idea for triathletes?

HFLC is not a new concept for athletes, with research into the diet and sporting performance going back about 20 years. The theory goes like this: if you train without eating enough carbohydrates, your body has to turn to the next best fuel option – fat. With more than double the kilojoules in a gram of fat (37kJ) compared to a gram of carbohydrate (16kJ), fat is a space-saving energy storage device.

The premise of the HFLC diet is that after a time without eating enough carbs to fuel exercise, the body becomes better able to utilise these fat stores for energy (‘fat adaption’). Fat storage is unlimited (unlike carbs where you can only store enough for about 24 hours) so if you can utilise fat for energy instead of carbs, you have a much larger amount of fuel at your disposal. Theoretically this means you can ride for much longer before you run out of fuel.

The problem is, converting stored energy in fat to energy your muscles can use when you’re exercising is inefficient. Carbs are much more easily accessible, covering to energy for fuel quickly – exactly what you want as you’re sprinting for the finish line.

Despite the opinions of HFLC advocates, decades of sports nutrition research has consistently found carbohydrates to be crucial to endurance performance. The recommendations still stand that endurance athletes should take on 60g carbs per hour for two to three hour sessions and up to 90g carbs per hour for sessions longer than three hours, regardless of weight or training status.

The pros:
In a perfect world fat adaption will mean you’ll require less food during your sessions and be better at avoiding the dreaded ‘bonk’. Anecdotally, a host of other health benefits have been reported including reduced appetites with reduced body fat and weight, a perception of increased energy throughout the day, improved sleep and reduced cravings for sweet foods, although research is still needed.

The cons (and this is a biggy):
This diet won’t necessarily improve your performance. That’s right… Most studies show that athletes who start a HFLC diet are indeed better at utilising fat for energy, but this doesn’t directly relate to an improvement in their speed or endurance. In fact, it may negatively affect their speed at high intensities so if you try for a sprint finish, you will not have the juice.
Fat adaptation takes time – weeks or months in some cases. So, if you’re going to try this at home definitely don’t try it in the middle of the racing season. Especially for the first week or so, you’ll notice symptoms like headaches, tiredness, crankiness and ‘bonking’ during sessions.

The bottom line…
There is still a lot of research to be done before we can make a definitive decision as to whether or not the HFLC diet is the new frontier of endurance sports nutrition. Becoming more ‘fat adapted’ could potentially be helpful for longer events like long course or IM, but only if you’re happy to stay at a low-moderate intensity. Right now though, for sports performance, the evidence still stacks undeniably in the high-carb corner.

If your event is reliant on high intensity (i.e sprint and olympic), the HFLC diet may actually hinder performance. And whatever you do, don’t try this now – you will ruin your performance for the next 3 months which is no good if you want to be racing over summer. I also believe that any diet that tells you to cut out whole food groups will never be balanced and shouldn’t be the answer.
Instead, you can have the best of both worlds by matching your carbohydrates to your training and competition needs. Introducing some longer, slower sessions where you ‘train low’ with help with fat adaption but including carbs for high intensity sessions will help you train and compete hard. My best advice is if you’d like to try this, don’t go it alone – a sports dietitian can help you to find the best diet for you.

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