High Fat Diets: a look at a recent review paper

High Fat Diets: a look at a recent review paper

Howdy Icon Athletes! I regularly get questions on high fat diets, and so I try to regularly provide information based on sound scientific evidence to help athletes come to the best conclusion for themselves. I first and foremost understand that every individual is different and will accordingly respond differently to different dietary approaches. There is no ‘one size fits all.’ That being said, there is a solid body of scientific evidence that provides a basis for best practices and can help guide our nutrition information in a way that can then be individualized.

The high fat, which, if taken to the extreme is a ketogenic diet (yes, the keto diet is an extreme approach), has become more popular in the past few years. However, high fat diets have been under scrutiny for many, many years. There are several claims for a ketogenic diet and athletes may adopt this dietary pattern for any number of these reasons. The most commonly cited motivators include:

  • Sparing of glycogen. In either endurance exercise, or high intensity exercise that is done of longer duration, glycogen (the muscle’s storage form of carbohydrate) can become the limiting factor for performance. The perverbial experience of ‘bonking’ during a workout is most commonly the results of depleted glycogen stores. The body can only store a certain amount of glycogen (~300-350 g in normal muscles), and so once the muscles have used up these stores then the athlete feels totally depleted. High fat diets train the body to use fat as an energy source, and since our bodies have almost limitless fat, the athlete doesn’t experience that ‘bonking’ effect.
  • Weight loss. High fat, and particularly ketogenic diets turn our bodies in a ‘fat burning machine’ and thus facilitate weight loss.
  • Management of hunger and fullness – athletes on the ketogenic diet specifically (but usually not on low carbohydrate diets otherwise) state that once their bodies have switched over to ketosis, they don’t get hunger pains and feel great all of the time.

Do these claims stack up? Yes and no. Here’s what we do know about high fat diets:

  • Limiting carbohydrate availability during training consistently does upregulate enzymes and the machinery needed to burn fat at higher exercise intensities. This makes fat metabolism more efficient.  However, this comes at a cost. This also means that the body downregulates its ability to utilize glycogen as a fuel source.
  • This does not change the fact that the body requires glucose as its primary energy source at high intensities. So, while perhaps adapting to greater fat oxidation means you can use fat as a primary energy source at higher intensities (let’s say you can burn mostly fat at 75% of Vo2 max, instead of just at 65% VO2mx, you will still be limited at the higher intensities (>75%). The body still needs glucose in these situations. And these high intensities are quite common in CrossFit training. Plus, even if you consume some carbohydrate before important events (competitions) in order to hit those higher intensities, if you’ve been training on a high fat diet your body won’t be as effective in utilizing carbohydrate since you have downregulated the machinery need (see bullet point above).
  • Weight loss: there are some studies indicating some individuals do lose weight on high fat diets, particularly ketogenic diets. Not everyone responds in the same way, but there are people who do see success with weight loss.
  • Performance benefits? Well, actually…no.  This was first studied in the 70’s up through the early 2000’s, and then there have been some more studies done recently (please see review paper at the end). Ultimately, there just have not been any studies to suggest there is actually a performance benefit. So, even if you’re burning fat at a higher intensity, this doesn’t translate to better gains in the gym.  I know, bummer.
  • Recovery – I find unless an athlete is working out just 30-60 minutes 4-5 days a week, any training volume greater than this results in poor recovery when following a high fat/keto diet. Depleted glycogen stores increase feelings of fatigue, and so even if the body is more efficient at burning fat, there will always be some glycogen utilization, and without replenishment this results in the athlete feeling tired longer and just overall has a harder time recovering in between workouts.

I realize I can send negative when it comes to high fat, low carbohydrate diets. To be honest, I just don’t see the benefit of them, wither in the gym or as a lifestyle. I find them to be very restrictive. I realize part of my bias comes from being a sports dietitian that specializes in the treatment of eating disorders. But I cannot tell you how many athletes have come to me with guilt and anxiety about food that has stemmed from trying restrictive diets such as these. I also worry about their sustainability, which is always my number 1 question when someone comes to me telling me they want to try a new diet (whether it is Paleo, keto, zone, or even a smoothie diet) – my response will always be – is it sustainable? Is this something you can do 7 days a week, 365 days a year? What about holidays? Social functions? Will this diet work for all that life throws at you?

If it does, and you know you will not necessary get any stronger, faster, etc., but perhaps you’re motivated to shed a few pounds, then give it a try. I do know some athletes that have cut weight and are very ‘strict’ with their (keto) diet, and they make it work. Of course, be sure to have lots of (sugar-free) breath mints, as your breath will chronically stink. Otherwise, if you’re looking to reduce your body fat, AND get stronger, AND find something that works with family life, holidays, etc. then perhaps a more moderate approach will work (such as the foundation of the Icon nutrition program).

I think for most CrossFit athletes, a moderate carbohydrate, moderate protein, and even moderate fat approach works optimally for health and performance gains. We can always tweak the numbers based upon individual needs (and I do this all of the time with Icon athletes that reach out to me!), and I will always recognize that different approaches work for different individuals.

Here’s the recent review paper that consolidates both the historical research done on high fat diets and the more recent publications. The author (Dr. Louise Burke), by the way, is truly one of the world’s leading sports dietitians, is head of the Australian Institute of Sport, and by no means recommends a high carbohydrate diet for all athletes; rather she believes in customization and periodization. She knows her stuff (and the research) and you will find no one more knowledgeable on the topic. In fact, her husband is a leading researcher/physiologist (John Hawley) and himself is the author of many research studies supporting the increase in fat oxidation and physiological adaptations to high fat diets. Yet he also supports how this should actually translate to sports nutrition recommendations, which is an alignment with what his wife has to say (smart husband ;).

Burke review high fat diets

Questions?! Thoughts?! Please check in with me and let me know!


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