In Canada, blood glucose level is measured in mmol/L. Normal values for blood glucose is between 4.0 mmol/L and 8.0 mmol/L.
Blood glucose less than 4.0 is called hypoglycemia. It is caused by too much insulin (or not enough food/sugar). Hypoglycemia is very dangerous. If the diabetic doesn't get any sugar asap he/she can become unconscious and even die.
Blood glucose greater than 8.0 is called hyperglycemia. It is caused by too little insulin (or too much food/sugar). If not treated, hyperglycemia can lead to ketoacidosis (diabetic coma). Frequent hyperglycemic episodes over time are what cause long term complications such as heart disease, blindness, kidney failure and amputation.
So, life for someone living with Type 1 Diabetes is a 24/7 balancing act between food/sugar and insulin to avoid hypoglycemia and hyperglycemia.