Diabetes entered my life in 2007. My doctor said that while he was happy that I exercised regularly, he wished I would do something else other than boxing.
Diabetes and boxing don't mix as far as most doctors are concerned. The first thing they will usually point out is that people with the condition take longer to heal from injuries. The body can't use glucose or sugar the way it once did, so things like cuts become slower to heal. A minor nick while threading a needle wasn't a big deal back in the day. Now I have to carefully monitor it in order to make sure it doesn't get infected as well as checking to make sure it's healing okay.
You can imagine a doctor's concern because of all the injuries - black eyes, bloody noses, etc. - that could happen during boxing. Personally, I don't believe anyone should be discouraged from training, sparring or competing just because of diabetes. But precautions should always be taken. If a boxer has trained well, and is alert in the ring, they can cut down on how many punches they take.
When an injury happens, take care of it on the spot. If it appears to get worse after a few days, see a doctor immediately.
There's no rule that says a sparring session has to be all out each time. I'm aware there are some gyms that set aside times for hard sparring sessions. Some even have special names for such times such as "War Night". But people who have this condition can always say to a sparring partner and the coach, "Look, I just want to go light this time around." Those who aren't interested in sparring and/or competing can skip sparring altogether and just do the workout and get the same good benefits.
Boxers who have this condition should make sure that the coach knows that. Keep glucose pills, extra insulin, or other medical supplies needed in the gym bag and/or in the locker just in case. I would also suggest getting and wearing a medical ID bracelet or necklace.
The carbohydrate content has to be watched. Doctors and nurses may make it sound easy to change one's eating habits to only take in so many carbs at day. Believe me, it isn't, especially if one was used to eating whatever one wanted to have. Sugar can be found in most foods. It may be under different names - corn syrup, fructose, sucrose, glucose, etc. - but it's all sugar just the same. One way to cut down on the sugar is to substitute things. Using a sugar substitute instead of real sugar is one way. But investigate the substitutes as well. Everything marked "sugar free" may not be exactly that.
Another way to keep taking in the sugar is to think about the complications that could arise. My maternal grandmother lost a leg, and lost toes on her remaining foot before she passed away. There's also things like lack of bladder control, nerve damage, and blindness. Sobering to think about indeed. But people can live a long time with diabetes and avoid these things if they take care of it properly.
Depending on the situation, doctors may prescribe insulin, pills, or both to control the condition. Taking meds doesn't have to be a problem with boxing if planning is done ahead of time.
Unfortunately, as I found, diabetes often goes hand in hand with high blood pressure.