Until you have spoken to a Kink Aware Professional (KAP) about it in detail...

NEVER, ever, under any circumstance, let a diabetic restrict your freedom of movement (bondage, cages, etc.) or engage in play that doesn't automatically return to a safe state if they pass out, leave, etc... and do NOT engage in breath control.

If you take no other piece of advice, please take that one to heart.

That's what in other settings is known as "default to safe" or "fail open", which is to say that if the diabetic passes out, it is imperative that you be free to move about. Don't rely on being able to undo knots or "wiggling" your way out of a restraint. Don't enter cages or wear handcuffs behind your back. Don't do suspension play. Don't ever try breath control. Don't do things that cause a buildup of fear (i.e. panic response) if it isn't stopped manually, such as perforated bags over the head, etc. Don't use TENS units to the point where the cramps prevent you from moving freely enough to turn them off.

The two main reasons for this are simple enough: (a) if the diabetic passes out, you will need to contact the EMS personell via 911, 112 or whatever your local equivalent is, and will probably need to deal with the emergency in the mean time, and (b) if you are in a dangerous situation that requires the diabetic to be in control and able to abort, then that danger will become reality if they pass out.

While insulin pumps, proper care and planning, and so forth, can all serve to alleviate this risk, you should NOT rely on this until you have discussed the matter in-depth with a doctor who has access to the diabetic's medical history. The ideal doctor is one who is a listed Kink Aware Professional (link; click it), as they will be sensitive to (and familiar with) both sides of the issue.

Don't be afraid to discuss this with the diabetic's doctor ... they've heard "worse". All the time.

In short, the bottom/sub/slave does not want to watch their Top/Dom pass out and slowly die on the floor without being able to do anything, and the Top/Dom would not want their bottom/sub/slave to be traumatized, injured or killed because they couldn't escape a dangerous situation.

...On to the more superfluous advice...

Now, that probably sounded a bit like a rant or an overreaction, but it really isn't. Which is not to say that you can't play safely, just that the advice I'm adding here isn't an adequate substitute for qualified advice from someone who knows the diabetic in question. In that regard, this advice is superfluous, but I realize many just won't talk to a doctor, no matter how many people tell them to, so this is a "best effort" at addressing the issue...

Make sure the diabetic eats regularly and properly. Eating habits for a session should be like for a workout.

Measure the blood sugar level before and after every session to get an impression of how much it changes, and to make sure it is within the usual range before starting out. Also, since getting dizzy or wired during a session isn't all that uncommon, even for people who aren't diabetic, the body signals could get confusing, so the levels should be checked again if anything feels "off". Note that the partner should also know how to use the blood sugar meter, as some have already said.

Use safewords.

If the diabetic starts to show signs of blood sugar issues, the partner should stop the play if the diabetic fails to.

Keeping a sugar boost around is a good idea, as most have said. Both something that is a bit longer-lasting, and something for emergencies. The advice given by earthycouple about gel icing (just make sure it's not sugar-free!) sounds like a very good idea. What I've heard suggested, is to place it between the gums and the cheek, for faster absorption.

Basically, if there's a sugar drop, the boost will be useful. If there's too much blood sugar, the small boost will not significantly worsen things. Of course, it is better to rely on the blood sugar meter than to rely on this approach (which is usually recommended for when you don't have a clue as to whether the blood sugar is too low or too high).

If anyone drops unconscious, an ambulance should be called for anyway.

Also bear in mind that many diabetics have problems with the blood circulation in their extremities. Bondage, clamps and other things that can adversely affect blood flow should only be used with great care.

There are other auxillary problems that often pop up along with diabetes, as well, and learning how to deal with those as well is a Good Thing(tm). For instance, kidney function may be altered to the point where WS must be a rare and well-planned affair. And neuralgic pains might need dealing with, etc.

As for buggyboo, I don't think s/he got the idea, so I'll rephrase it:

An unconscious person should never be given anything to swallow, or that they might accidentally swallow. However, putting some gel between their gums and cheeks, or pouring a small amoung of sugar under their tongue, is a different matter.

Don't administer insulin yourself. Instead, make note of the blood glucose levels, and tell this to the people at the emergency medical services (EMS; US- 911, elsewhere- 112, usually). Also, tell them how long the person has been out, and inform them that you have the insulin kit available. If they think you should use it, they will tell you how much and when. The only time you should consider doing it yourself, is when there's no phone available to contact the EMS with, and I really can't offer any advice about that.

There's an endless number of physical and mental health complications that are entirely compatible with BDSM of (virtually) any kind, as long as the people involved take the time to acquire the skills required to deal with it in a responsible manner.

Hope some of this answers the question...

And, did I mention talking to the doctor about this?
Because you should talk to a doctor about it.
P.S.: Talk to the doctor.

Best wishes,
Aswad.
- who thinks the doctor is your friend on this.