Our bodies primarily run on two sources of fuel, carbohydrates and fats.

Carbohydrates provide us with readily available energy and are used as our default fuel.  Carbohydrates fuel our workouts and allow for muscle growth.  In addition, carbohydrates cause a spike in insulin levels which in turn promotes fat storage.   Carbohydrates are stored as glycogen by the liver and in the muscles.  When your carbohydrate intake is high, these stores fill up.  In this case, carbohydrates are converted to fat and stored by the body as potential energy.

The ability for the body to store excess energy is endless.  Your body will simply continue to store fats in the fat cells if there is an excess of energy intake.   Fat cells release leptin.  As was stated in a previous article, leptin is a hormone that regulates energy expenditure and appetite.  When circulating levels of leptin are high, your appetite will decrease and your energy expenditure will go up.   Conversely, when leptin levels are low (as with a high carb, low fat diet), your body increase your appetite and decrease your resting metabolic rate to store energy.

The caveat to the above statement is that in people with significant obesity, their leptin sensitivity decreases.  Because circulating leptin levels are directly proportional to the amount of fat or adipose tissue present, obese individuals have consistently high circulating leptin levels.  The constant elevation of leptin results in decreased sensitivity to leptin, or leptin resistance.  The result is that despite high levels of fat, these individuals are still hungry and continue to store fat.

Fat metabolism is turned on by the body when glycogen stores get low or circulating levels of leptin decrease.  Compared to carbohydrates, fat provides the body with a more sustainable form of energy.  About 100 grams of glycogen is stored in the liver and the rest is stored in the muscles.  These glycogen stores in the liver can be depleted in just one day of fasting.  When glycogen levels drop, it takes different individuals varying amounts of time before their body can efficiently metabolize fat to makes ketone bodies.  Ketone bodies can then be used as a form of energy.

The theory behind carb cycling is to provide the body with the beneficial effects of carbohydrate intake (i.e. muscle growth, fuel workouts, gut health) without the drawback of increased fat storage due to elevated levels of circulating insulin.

By eating a low carb diet, insulin sensitivity is increased and glucagon (a hormone which increases the synthesis of glucose from glycogen) production is increased as well. The ultimate result is more efficient fat burning for energy.  In theory, a prolonged low carb diet can lead to decrease in thyroid hormone production, elevated cortisol levels (a stress hormone), loss of periods (amenorrhea), bowel dysfunction and immune dysfunction.   According to Paul Jaminet, PhD, low carb diets can also cause the body to decreases production of certain proteins and molecules resulting in symptoms of dry eyes, dry mouth, and decreased healing times in superficial wounds.  Persistent low carb diets also can stress the liver.  When needed, the liver synthesizes glycogen from proteins (or fats) in a process called gluconeogenesis.

Carb cycling is used to allow the beneficial effects of low carb dieting, but to offset or prevent the above mentioned potential side effects.  In individuals consuming a consistently low carb diet, the body becomes very efficient at burning ketone bodies(fat).   Often these individuals reach a plateau or stall with their weight loss attempts.  By adding in a higher carb meal or “cheat meal” this essentially jump starts your metabolism and up-regulates the fat burning process through the effect of leptins.

It is important to note that using complex carbs such as sweet potatoes, brown rice, slow cooked oatmeal, yams, etc. is more beneficial than using simple carbs such as breads, sugary foods, candies etc during your high carbohydrate days.  This is because complex carbohydrates are digested more slowly and do not cause as much of an insulin spike.  Complex carbohydrates that are not processed also contain vital vitamins, minerals and nutrients your body requires. In addition, it is felt that the undigestible (resistant) starches found in certain plant-based complex carbohydrates may have a beneficial effect on gut flora and increase mucin (one of the main parts of mucous, which helps to lubricate/ moisten body surfaces) production to offset symptoms of dry eyes or dry mouth.

The timing of carb cycling is very important.  For example, eating a meal high in carbohydrates before bedtime will promote fat storage due in increased insulin levels and decreased energy demand.   At this time, research indicates that the best time to increase carbohydrate intake is after a heavy training, lifting or sprinting day when the glycogen stores are depleted.   The carbohydrate influx will be used by the body to refuel the glycogen stores instead of stored as fat.   This increase of carbohydrate intake will also raise the leptin levels transiently.  The rise in leptin levels will result in a decrease in hunger cravings and prevent down-regulation of hormones which would otherwise decrease the overall metabolism.

The reason for fat cycling or decreasing fat intake on the high carb days is to allow an individual to maintain a fairly consistent calorie intake.  On the lower carb days, high quality fats can be eaten to increase calorie intake and stimulate satiety.  On higher carb days, fats should be limited so that overall calorie intake for the day remains constant.  In addition, the combination of high carb and high fat intake can result in an unfavorable changes in the type of fats found in the blood stream.

In summary, carb cycling is a way to allow an individual the benefits of eating a low carb diet, without the potential drawbacks of a persistently low carb diet.  Intermittent carbohydrate “refeeding” helps to prevent “stalls” in weight loss, refuel glycogen stores in the liver and muscles, regulate leptin levels and prevent possible thyroid or immune consequences of continuous low carb diets.   If one suffers from symptoms of worsening hypothyroidism or adrenal fatigue, it may indicate that more complex carbs should be added into the diet.  In those individuals with significant obesity, carb cycling is often not as effective in “jump starting” the metabolism due to persistently high levels of leptin, or leptin/insulin resistance.   To help decrease insulin resistance and increase leptin sensitivity, a low carb diet coupled with adequate sleep, routine exercising and stress relief is recommended.