Science Friday: High Fat Tapers

We all know about carbo-loading. A couple days before the marathon, we suppress our tapering woes with heaps of carbohydrate goodness. There’s a lot of research behind it and so inevitably the Italian restaurants on marathon weekend are teeming with runners.

But what about a few days before the carb-fest begins? I’ve heard that the week before is a good time to make sure you’re eating healthy fats. What’s the logic behind this?

The idea is to get the body to oxidize (i.e. break down and use for energy) fat more than glycogen (that precious molecule we so carefully try to stock up and save). This can be achieved to some extent with training; the more we train to become endurance athletes the more we teach our bodies to use fat. But can we help out by eating a high fat diet?

There’s a lot of research into this idea; the problem is not all the studies agree. Some use long periods of fat-loading (seven weeks!), some only a few days. Some use different percentages of calories from fat, some look at endurance, others at speed... it’s hard to know what to think. 

I looked up the article that’s often cited for the recommendations I’ve heard. In this study, the researchers had well-trained cyclists eat 10 days of a high fat diet (>65% of calories from fat, see below for an example day), followed by 3 days of typical carbo-loading (>65% calories from carbohydrates). They compared this to a separate trial where the same subjects ate their normal diet followed by 3 carbohydrate filled days. They had the subjects bike for 150 minutes before completing a 20 km time trial. The high fat diet increased fat oxidation and decreased oxidation of muscle glycogen, as the researchers hoped. Moreover, when the cyclists had eaten the high fat diet, their times for the time trial were slightly (but significantly!) faster.

A fatty day!

So this study suggests 10 days of high fat diet followed by the typical carbo-load helps improve performance. The inclusion of the carbo-load is important; studies that don’t include it often don’t see any benefit from the high fat diet. It seems like without the carb-fest, the fat-fest won’t work. You always need to eat that pasta.

Now available in
stores research labs.

One thing that jumped out at me was that they also gave the subjects easily digestible fat solutions just before the time trial and a combination of digestible fat and carbohydrates during the trial. I picture a fatty version of GU. The authors say this was done to maximize potential fat and carbohydrate oxidation during exercise, but they don’t have any controls to see if this is necessary to reap the benefits of the high fat diet. I've never eaten any fat during a race, so I wonder the real world relevance of this.

I’m fine with eating some healthy fats (bring on the guacamole!), but I don’t know about 65% of my diet from fat. That seems like a lot of fat at a very delicate time in the lead up the marathon. It’s certainly an interesting idea, and I’ll keep a look out for more studies. In the meantime, has anyone tried this drastic of a pre-race diet?

Dream big,

Teal