Ketosis — when your body burns primarily fat for fuels — requires a very low dietary carbohydrate intake. A juice fast requires you to consume practically nothing but carbohydrates. A ketogenic diet, such as Atkins or other forms of low-carbohydrate diets, remains completely incompatible with any juice-based diet. If you wish to lose fat, you must select one or the other. Consult a health care practitioner before beginning any dietary program.
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Your body can run on several types of fuel: amino acids, sugar and fat. When your body burns primarily fat, or free-floating fatty acids called ketones, you are in dietary ketosis. To achieve this, you must severely restrict your intake of all non-fibrous carbohydrates. Exercise will allow you to further deplete your glycogen reserves; over time, your body will shift from running on sugar to running primarily on ketones.
Sugar and Ketosis
If you consume sugar while in ketosis, you will shift back into burning glycogen or sugar. It will take time to achieve ketosis again. The volume of carbohydrates that you consume will have the most bearing on this; however, consuming carbohydrates and fat together is even worse. This is because your body will generate a great deal of insulin in response and you will continue to run on glycogen while your body stores the fat you just ate. While juice contains no fat, it usually contains fructose.
The juice may be predominantly fructose, depending on its type. Fructose is ordinarily a good energy source, largely due to its limited ability to cause a large spike in insulin levels; however, it is far less than ideal if you wish to remain in ketosis. Unlike many other types of sugars, fructose is broken down primarily in the liver. While your body is metabolizing fructose, your ability to burn fat, particularly ketones, is limited. As fructose takes longer than other sugars to metabolize, a cupful of strawberries will keep you out of ketosis for many hours.
Juice fasting is exactly that — consuming nothing but juice and water. While at first this may seem a good way to decrease calories, there are numerous problems with this approach. You are not getting any protein. Your body requires protein for its amino acid content, and when you do not get enough protein, you will break down muscle tissue for essential amino acids. You do not get fat, and fat is required for hormonal production and regulation. Essential fatty acids are required for proper brain function and sexual health. When you neglect to get your essential amino acids and fatty acids, you can expect problems to occur.