I just wrapped up another great seminar down at Crossfit Ad Finem. Great crew of people and a lot of good questions. One of the discussions that we had was centered around fructose… whether its evil, neutral, beneficial, etc. As always, the answer really depends on who we are talking about and what form the fructose is coming in.
At this point, I think I can skip over why high fructose corn syrup is a pretty terrible thing for you to be eating if you’re looking to improve body composition, health and even performance. If you want to read about it, check out this post:
So I think its pretty clear that processed sources of fructose, especially high fructose corn syrup are worth avoiding.
But what about fructose from other sources like fruit? Well for a long time I was one of those people that thought you should limit your fruit intake, especially with something like bananas which are pretty big fructose bombs. My thinking was that for the body to use the fructose as fuel, it had to be converted into other forms by the liver. Since the liver preferentially stores fructose over glucose, and fructose cannot be used/stored by muscle tissue, foods with higher levels of fructose would be inefficient energy sources for human performance, especially in post workout conditions.
However, I’ve recently changed that viewpoint to “it depends on who you are.” A healthy person with good body composition and a healthy liver can almost certainly deal with fructose from fruit based sources. This was recently shown in a nice study that fructose consumption resulted in conversion to 50% glucose, 25% lactate, 15% to glycogen and only about 2-3% ends up as fat. Assuming you’re not going to be using fructose as a workout recovery fuel, that actually looks like a reasonable result.
What the study didn’t address and what I couldn’t find elsewhere in the scientific literature is whether or not those numbers change in a person that has metabolic disease or a less than perfectly functioning liver… and lets be clear, there are a LOT of people with less than perfect liver function. What we do know though is that higher levels of fructose consumption correlate well with liver dysfunction.
As far as improving performance and/or body composition goes, most of the top trainers our there are saying, “If you are lean and workout consistently, then a higher level of fructose consumption is safe and healthy… But if you’re overweight and looking to make body composition changes, then you need to limit your fructose intake to about an apple a day.” In other words, you need to earn your carbs with hard work.
Lastly, here is an EXTREMELY comprehensive list of foods fructose values. I think many of you would be surprised by what foods are close to the top of this list: