I've noticed a few peanut butter cravings after long runs recently.

In terms of energy-to-weight ratio, it would make a lot of sense. Indeed, all commercial gels are based on carbohydrates, which provide 4 KCal/gram. This ratio is fairly constant across the main carbohydrate sources: glucose, fructose, maltodextrin, or potato starch. The only difference is the speed at which the energy is assimilated by the body. So the final energy-to-weight ratio of a gel depends mostly on the amount of other ingredients (mainly water) added to the carbohydrate base. The highest ratio I've found during my tests was 3.75 KCal/gram in the Honey Stinger, thanks to the little bees drying out their product with their wings. Conversely, this means that a SiS Go Gel with a poor ratio of 1.3 KCal/gram is probably composed by 2/3 of water and 1/3 of maltodextrin.

Peanut butter packs 6 KCal/gram, because of its high fat content. Typical composition includes 50% fat (9 KCal/gram), 25% proteins (4 KCal/gram like carbohydrates) and 20% carbohydrates. Peanut butter also contains salt, which might be beneficial during long/hot runs. Because protein and fat are slower to digest than carbs, that might be a suitable option for multi-days runs.

We're still far from the 9 KCal/gram you would get by stuffing your bag with lard or olive oil, but that's a step forward :) .

Ultra energy content