You should always wait 30 to 45 minutes to eat after you've taken your insulin, but that's not always possible. Try these tips to keep your blood glucose levels in check.
Remember that high-carbohydrate foods such as bread, fruit, and milk will raise blood glucose rapidly, whereas high-protein foods will rise your blood glucose levels much more slowly, or even after the meal. If you must eat soon after you take regular insulin, try eating the foods that won't have much effect on your blood glucose first, such as salads and meat dishes, or sip on glucose-free drinks. Try to hold off on eating starches, such as bread and potatoes, leaving them for last so the regular insulin has more time to work — you can probably manage to wait another 10 minutes at least before having bread and other carbs.
If timing problems are frequent, you might consider changing to rapid-acting insulin, which can be taken up to 15 minutes before the meal if your blood glucose levels are normal.