My lady is a type 2 diabetic, 2 good friends are type 1, and another friend is an ER nurse. Discussing it with them, their consensus is...
**NO** sugar or food of any kind in an enema for a diabetic. That means no regular sodas, alcohol of ANY kind (alcohol converts straight to sugar in the bloodstream). Honey, molasses, refined sugars, agave, fruit juice, milk, etc. are all right out! They're likely to have less impact on blood glucose, but even Splenda and stevia are not recommended. Anything inserted in the rectum is absorbed more quickly than taking it orally and would have a much more noticable impact on a diabetic's blood sugar than simply eating it.
If you want to explore enemas with a diabetic, I'd stick to only saline until you know how their body reacts. Ladydocisin said enemas cause her BG to drop, "sometimes precipitously", while waterbottom and rigid58 have no issues. YMMV.
How much salt and/or baking soda (sodium bicarbonate) goes into a saline enema? I just visited a friend in the hospital and his IV saline drip was marked "0.45% Sodium Chloride." A rule of thumb I've seen over the inter-web is 1/4 to 1/2 tsp per quart. When I accidentally mixed one of my own with 1 tsp per quart, I was thirsty and tasted salt for the next 12 hours... that was too much.
If you're considering administering an enema to someone with known medical issues, discuss it very frankly with them first, including what you do and don't know.