Carb cycling: Good for fat loss or a bad idea?

I was recently asked whether or not I thought carb cycling was a safe, effective way of dropping body fat.  While I think carb cycling can be very effective at dropping excess fat, I think carb cycling as practiced by most people who are practicing it is potentially a bad idea without covering all of your bases.  Let's science...

Carb cycling 101

Carb cycling is exactly what it sounds like, you cycle the amount of carbohydrates you eat each day, normally by eating high carb on workout days and low carb on rest days.  While individual programs vary, most follow a structure similar to the one below:

Taken from: veggiereader.wordpress.com

My primary issue with carb cycling safety is the level of carbohydrates on the low carb days for women, and to a lesser extent men.  Let's take a look at the numbers for a 150lbs woman.  On high carb days, this woman will consume 210g of carbohydrates and on low carb days she will consume 90g of carbohydrates.  This may not seem like a big deal, but when you consider the brain uses 120g of glucose just to keep you operational everyday you can see a woman could easily put herself in to a brain glucose deficit.

A woman who is not ketoadapted(It's impossible to become ketoadapted when you consume a carb level higher than 50g/day in a 1 week period) will burn approximately 1500 calories just to stay alive with 150g coming from carbohydrate.  This is merely to keep her alive, this does not include physical activity which will increase this number considerably unles she is ketoadapted.  So if she stays in bed and doesn't move she is in a 30g glucose deficit.  To make up for that deficit, she will have to call upon cortisol to make glucose from non-carbohydrate sources.  If she does any sort of physical activity, even walking, she will use some of the glucose she made from carbohydrates and the glucose her brain would use later in the day would be used up.  Cortisol could be called in to action to make more glucose, more than likely later on in the day or at night as the brain runs out of energy, especially if intense exercise is part of your weekly plan.  Having cortisol secreted late in the evening or even in the middle of the night is bad for your circadian rhythms and will disrupt sleep.  It would even be possible to go in to a glucose deficit on training days if you train intensely enough and a 200lbs male would be able to achieve a considerable glucose deficit at 180g on low carb days.

So avoid carb cycling?

Since carb cycling is both popular and very effective, throwing it out completely is only a valid option if there is no way around the issue of providing energy to the brain.  Fortunately, the brain can use an alternate fuel source called ketones as energy and can replace up to 90g of it's glucose needs with ketones.  The problem is, the body won't make this amount of ketones unless you maintain a low carbohydrate intake (<50g/day) for at least 4 consecutive days.  However, your liver will make ketones out of medium chain triglycerides(MCTs) or coconut oil regardless of how many grams of carbs you have been eating.  As such, carb cycling is a valid and safe option for fat loss provided you are replacing the glucose your brain needs with ketones from coconut or MCT oil.  And since MCTs are instantly metabolize for energy, you don't need to worry about storing them as fat.  Assuming the brains energy needs are met at the same number of calories from carbohydrates and ketones, 3 tbsp of coconut or MCT oil would be suficient.

Conclusion

Many people have seen the benefits of carb cycling for fat loss.  While carb cycling is certainly an effective way to drop some fat, one must be careful not starve the brain of energy or rely on cortisol to help provide the brain with energy.  By consuming 3 tbsp of coconut or MCT oil on low carb days, you can provide the brain with energy and avoid the primary pitfall of relying on cortisol to produce glucose for the brain with carb cycling.