This past weekend I did a century and had a bonk experience. I forgot my pack with my fuels, so I overdid the lunch with foods at a restaurant all at once (food choices were “heavy” – mashed potatoes, broccoli rice casserole, mac and cheese, and salisbury steak – not the best options). There were 60 miles to go. At mile 90 I lost energy and went from 16-18 mph to about 12-14. I felt like I had enough to calories to get me there, but apparently not. Do you think this was from not refueling every hour, or did I just not fuel enough period?


Mostly, it’s a matter of too much intake all at once vs. smaller, usable amounts throughout your ride (which is what you normally do and what you intended to do).  The difference is: when you eat a lot at once, your body has to divert more blood, energy, and oxygen to your gut to digest it – less to your legs/muscles.  You may not have felt it this time, but that’s why athletes often feel like they are dragging immediately after eating too much; then, once digestion and metabolism of the food source happens, they get more energy for a short time period.  Once the food has become usable energy in your bloodstream (blood sugar), some will be used, but the rest has to be stored because your body simply cannot “use” it all at once for energy.  A rise in blood sugar leads to a rise in insulin (hormone).  Insulin will cause you to store it – stored as glycogen (good), others as fat (okay, but an inefficient energy source anytime soon).  Then, after all the insulin is pumped out to deal with too-much-blood-sugar-at-once, you’ll usually experience a low blood sugar, and since you didn’t have another fuel source ready for immediate energy, you bonked.  Very different than small amounts, that are able to be used consistently throughout a ride.  This is also why it’s not a good idea when athletes save all their fuel for the 1/2 way point or the summit of their climb and then eat a bunch at once (I’ve done a fair amount of mountaineering and see this ALL the time – very little fuel on the way up, then a big fat lunch on the summit).  So again, it’s not about total calories at one time.  It’s about the amount of carbs your body is able to use per hour – if not used, then stored.  This amount is generally believed to be 40-60 grams per hour.  On longer rides like this, you can use other sources of slower carbs, protein, and fat in small amounts throughout the ride because you do have time to digest them.  Keep in mind, liquid and simple carbs can hit your bloodstream as soon as 10-15 minutes, but proteins, fats, and fiber can take hours.  Overloading at once will leave you with delayed digestion, risk of cramps, a “peak” of blood sugar followed by too much insulin in system, then a low and no new fuel source.  Bonk.

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