When insulin is released in the blood stream, it causes the cells to pick up the glucose, which will cause the blood glucose levels to drop down to a level that the body likes. When this happens, it broken down in to ATP or it can be stored as glycogen (for energy when needed).
When you are not able to produce enough insulin, the glucose levels in your body become imbalanced and cause complications that you see in type 1 and type 2 diabetes.
It is important as a fitness trainer that you are aware of what type of diabetes your clients have. If they have type 1 diabetes, then they are insulin dependent, and if they are type 2 diabetes, they have adult onset and it may be controlled with diet (depends on what their physician says).
Regardless which one they have, you should always have candy, juice, snacks on hand in case their sugar levels start to become unstable. In addition, they should also bring their medication with them or have it near by in case they need to resort to it.
Training clients with medical issues can be very rewarding and enjoyable, but you need to make sure you have all aspects covered so your client and you are safe.