While you must limit your total carbohydrates when using a ketogenic diet, certain carbohydrates are allowed. Most of your carbohydrates should come from fibrous sources, such as vegetables, which do not count toward your total carbohydrate count. Limit your non-fibrous carbohydrates to no more than five percent of your total daily caloric intake. Certain exceptions to this rule do exist. Consult a health care professional before beginning any dietary plan.
Dietary fiber counts as a carbohydrate, but your body has difficulty breaking it down into glycogen, or sugar. Consuming dietary fiber has no effect on your ability to stay in ketosis, and so you do not need to worry about it affecting your diet in any negative way. Vegetables are the most common source of dietary fiber, and should be consumed regularly, for both fiber and vitamins and minerals. Leafy green vegetables are solid choices for most dietary plans.
Simple sugars, many grains, lactose and fructose are all carbohydrates that must be avoided, fructose in particular. Fructose is primarily metabolized by your liver, and the longer your liver is burning sugar, the longer you will be out of ketosis. Milk contains lactose, or milk sugar, and is not allowed, nor are cereals or starches. If you are consuming a 2000 kcal/day diet, your total intake of non-fibrous carbohydrates should not exceed 100 calories.
When dieting, the timing of your carbohydrate intake can be critical. If you are exercising heavily, you are depleting muscle glycogen. Following a workout, this is the ideal time to consume a quickly digesting carbohydrate. This does not mean you can consume pounds of junk food, but quickly digesting sugars immediately after a workout will help restore depleted muscle glycogen without greatly impacting your ketogenic state. Supplementing with dextrose or maltodextrin works well to achieve this, and can help improve exercise performance.
If you exceed your carbohydrate intake, you need to deplete your glycogen levels to re-enter ketosis. This means exercise, and not simply walking on the treadmill. Intense resistance training or intense aerobic activity to deplete both blood and muscle glycogen will help you achieve ketosis far faster than just hoping for the best. The more sugar you consume, the harder it is to get back into ketosis, and the longer it takes you. While any dietary binge is bad, a carbohydrate binge in a ketogenic diet can be disastrous, and must be avoided if you wish to succeed.