I just read yet another reputable Registered Dietician suggest that we need to take in 250-350 calories or more per hour when training and racing. Really? Does cramming in more calories than your body can tolerate really work? In my experience with athletes, especially triathletes and endurance athletes, I can say no. 
Cramming in calories only guarantees GI distress and the potential to either throw up on the run, or walk until the stomach settles down.
The body can only process about 200-250 calories per hour. It is easier to get these calories in on the bike portion of a triathlon, but even harder if you are running. You cannot take in as many calories as you are expending. You cannot cram calories in hoping to get that food digested. I know Mark Allen suggests 400+ calories per hour and to train your body to take it in. Hmm, every triathlete I have worked with has had a terrible run when trying to cram in calories that high. They get off the bike, and all of a sudden when running, their stomach is sloshing, feeling very uncomfortable, and they resort to walking instead of running. Not what I would call an ideal race scenario.
With every athlete, when we have lowered their caloric intake, and optimized the hydration portion with the right balance of electrolytes, the athlete has performed better! 
When I work with athletes, cramming in calories never works. I worked with RAAM athlete several years ago, who stopped for a hoagie about mile 65-85. Each time he ate one of those, he basically bonked by mile 125-135. His body just could not process the food. And the only way to recover from too many calories is to either puke it up, or stop your activity. Once we changed his overall eating to help balance blood sugar and get the body into fat burning mode,we then created smaller meals and optimized the caloric needs on the bike. My RAAM athlete was able to complete his 450+ mile brevets and qualify for RAAM a year later.
The overall goal when I work with my athletes is not to cram in calories, but to find out how few calories they need per hour. You can always add more calories as needed, but once you get to too many, it is really hard to recover from it. The harder we exercise, the higher intensity, the more hills, the less calories we can cram in. We work more on balancing the blood sugars in every day eating, so the body burns fat for energy, not sugars. Then when training and racing, your body is already burning your own fat stores for calories. It does not need additional sugars from outside sources (glycogen stores can be depleted very quickly, so you want your body to burn fats, not glycogen and sugar). Then you do not need as many calories per hour. I know personally that I take in about 70-150 calories per hour now where in my younger days, I could take in more. My focus now is more on electrolyte replacement vs fueling. You will dehydrate before you run out of fuel….provided your body is in fat burning mode from your overall diet.
If you are struggling with your longer runs or running off the bike, you may want to consider taking in fewer calories. If you are at 400 now, try 300. Then 200-250. This should also be a combination of what is in your bottles, gels and solid foods (including bars, dates, potatoes or other real food options). Maybe you want to try mixing half the calories in your bottle so you can keep eating some solid foods. There is no right way for everyone. You do want to practice different combinations to see what works best for you. But cramming in the calories doesn’t really work for anyone.
Joanna is a sports nutrition coaching, helping athletes perform there best using real food as often as possible. As an athlete herself, she has gone thru the mistakes so you do not have to. If you want to perform better, consider a Race Day Nutrition Strategy Session for your next race, or the Race Day Package which includes 4 sessions to help with everyday nutrition as well as pre and race day issues. We can guarantee your success.