That's because changes in your eating pattern - and weight loss itself -- affect your blood sugar level, so you'll probably need changes in your diabetes medicines.
"Diabetes is about balance - balancing food, activity, insulin, pills every single day," says Larry C. Deeb, MD, a diabetes specialist in Tallahassee, Fla. and president-elect of the American Diabetes Association.
Low blood sugar (hypoglycemia). Blood sugar levels below 70 occur when your insulin level is higher than your body needs. This is common when people lose weight because cutting calories affects blood sugar levels.
If you don't reduce your insulin dosage to compensate for the calorie shift, you'll risk low blood sugar, whose early warning stages include:
High blood sugar (hyperglycemia). Blood sugar levels above 240 can develop when your insulin level is too low to control blood sugar. This leads to ketosis, a condition in which you're unable to use glucose for energy, so your body switches to burning fat instead.
In ketosis, fat is converted to ketones which get into your blood and your urine. Glucose also builds in your blood and spills into your urine -- pulling water from your body and causing dehydration, a potentially life-threatening condition.
"Ketosis decreases oxygen delivery to the tissues, which puts stress on eyes, kidneys, heart, liver," says Christine Gerbstadt, MD, MPH, RD, LDN, a spokeswoman for the American Dietetic Association and a diabetes specialist in Philadelphia.
That's why a low-carb, high-protein diet, like Atkins, for example, is not really safe for people with diabetes, says Gerbstadt. "Diabetics need to try to stick with a more balanced diet so your body can handle nutrients without going into ketosis."
If your blood sugar level is under control, hyperglycemia or ketosis isn't a risk, Deeb explains.
Controlling Your Blood Sugar
Before starting a diet, talk to your doctor and a dietitian about blood sugar management, Deeb advises.
"You'll need to know how to alter your insulin and medication based on what you're eating and whether you're exercising more. That's the safest way to lose weight," he tells WebMD.
Regularly testing your blood glucose levels is critical, adds Gerbstadt. Check it every morning when you wake up and shortly before you eat a meal, she advises.
A red flag: If you're fasting blood sugar in the morning is below 60 or 70 consistently, call your doctor or diabetes educator. That means you need to cut back on medication or insulin, Gerbstadt advises.