Carb Cycling: Key to Fat Loss or Just a Fad?
Presented by Karen Polensky, MS, RD, LDN, CDE
CrossFit 717 Advanced Topics Nutrition Seminar

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Carb Cycling Defined: Alternating periods of low and high carbohydrate intake.

History: Dr. Mauro DiPasquale, an MD and accomplished powerlifter, introduced carb cycling with his book, The Anabolic Diet, back in 1995. Since then it has been a staple for many power athletes and body builders. More recently studies have explored potential benefits for weight management and reduction of chronic disease risk for the general population.

Typical Protocols
• Rotate between high, medium and low carb days – varies depending on goals and workout schedule. Most protocols have you eating higher carb on workout days and lower carb on rest days. Mike Roussell, Matt McGorry
• Reduced calorie, high protein, moderate fat and carb diet with periodic (once every 3 days to 2 weeks) high-carb low-fat “re-feed” day. Layne Norton, Tim Henriques
• Alternating high and low carb days. Chris Powell
• Eat low carb early in the day, work out late afternoon/early evening, and eat most of carbs between workout and bedtime. John Kiefer , Robb Wolf
• 5:2 – 5 “normal” moderate-calorie Mediterranean days and 2 very low calorie/low carb days (500-600 calories) each week. Michelle Harvey, PhD
• Link to T-Nation article summarizing various carb cycling approaches in the bodybuilding/powerlifting world.

The physiology of body fat regulation is very complicated and not completely understood. We’ll explore just a couple of the many hormones affecting this process:

Leptin

• Leptin is secreted by fat cells and binds to receptors in the hypothalamus. Leptin shuts down appetite. More body fat = more leptin which should = reduced appetite. Less body fat = less leptin = increased appetite.
• Leptin levels respond to short-term energy balance. A severe caloric deficit = less leptin = more hunger. Overeating = more leptin = less hunger.
• In a challenging food environment, extreme hunger is what motivates people to put effort into hunting/gathering to ensure survival.
• Many overweight individuals are leptin resistant. They produce large amounts which should inhibit hunger but their bodies are resistant so they still feel hungry all the time. Leptin resistance appears to be tied to insulin resistance.

Insulin – adapted from Colorado State University website:

• Insulin is secreted by the pancreas in a dose-related response to increased glucose (and protein to a lesser extent) levels in the bloodstream.
• Insulin facilitates entry of glucose into muscle, adipose and several other tissues.
• Insulin stimulates the liver to store glucose in the form of glycogen.
• Insulin promotes synthesis of fatty acids in the liver.
• Insulin inhibits breakdown of fat in adipose tissue
• Insulin stimulates the uptake of amino acids by muscle cells.

Clinical Studies

• Multiple studies have demonstrated that weight loss and chronic calorie restriction will reduce daily energy expenditure, leptin levels, T3 and T4 thyroid hormone levels.
• It’s also well established that, in the short-term, leptin levels are reduced by fasting and increased by eating, especially when meal is high in carbohydrate.
• During maintenance of a 10% reduced body weight, circulating T3, T4, and leptin concentrations were decreased. All of these endocrine changes were reversed by administration of “replacement” doses of leptin.
• In a study of obese (BMI >30) police officers, moving majority of carbohydrates to evening (as compared to same calorie-level plan with carbs evenly spread throughout the day) resulted in better weight loss, reduction in abdominal circumference and body fat mass reductions, lower hunger scores, and greater improvements in fasting glucose, average daily insulin concentrations, and homeostasis model assessment for insulin resistance (HOMA(IR)), T-cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, C-reactive protein (CRP), tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6).
• 1 day of high-fat, low-carbohydrate meals produced less spiking of insulin and glucose than low-fat, high-carbohydrate meals, but also resulted in lower circulating leptin concentrations over a 24-hour period. This study suggests that leptin levels are affected by not only fat stores and energy balance, but are also dietary macronutrient content.
• In an 8-week study of 58 men, eating carbohydrates mostly at dinner and protein mostly at lunch (in comparison with carbs at lunch/protein at dinner) within a hypocaloric balanced diet resulted in more diet-induced thermogenesis, better muscle mass retention and lower fasting insulin levels.
• In a small study of 10 lean women, 3 days of high-carb high-calorie (overfeeding) diet increased leptin levels by 28% and 24-hour energy expenditure by 7%. Would be interesting to see what would happen in overweight individuals who had been on a weight loss diet for a while.

My Take on Carb Cycling

• It makes sense to me from an evolutionary and physiological perspective. Our physiology is designed for survival in an unforgiving environment, not to maintain the relatively low body fat levels that most of us would like to obtain to enhance performance and appearance. Alternating periods of feast and famine were the norm (and still are in much of the world) until relatively recently in human history.
• There is some pretty good evidence that adding an occasional high carb meal/day to a lower calorie/carb diet may offset the long-term drop in metabolic rate that we see with reduced calorie intake and body fat loss.
• May not be best approach if you have a history of binge eating or battle cravings when your macronutrient intake fluctuates a lot.
• Those with diabetes, prediabetes, reactive hypoglycemia and thyroid or other endocrine disorders need to be extremely cautious.
• Many of us likely cycle our carbs naturally. You “get strict” for 3 or 4 days then cave in to hunger and fatigue and eat a “cheat” meal and either feel guilty about it or choose really unhealthy options. Following a strict diet is tough. Having built-in and guilt-free “cheat meals/days” may help you stay on track.
• Some of the programs out there are pretty extreme; keep in mind some are geared toward bodybuilders who are dropping to extremely low (and unmaintainable long-term), body fat levels. I don’t recommend this extreme approach.

Should I Carb Cycle?

1. If you are feeling strong, performing well, and maintaining body composition you’re happy with there is no need to change anything. Move along!
2. Start with a good base before delving into carb cycling. Eat quality whole foods. Get rid of most sugar. Cut back on gluten-containing grains. Toss out processed foods, fast food, etc. Crank up the veggies. Don’t restrict fat too much. Make sure you’re getting enough protein.
3. If you got #2 dialed in AND have been consistent for at least a couple months, preferably longer, but are not seeing results, carb cycling is worth a look.
4. Choose one carb cycling approach and try it for a month.
5. Take a moderate approach. For example, if you’ve been eating low-carb already, try adding 1-2 days a week with an extra 50-100 grams of carb per day or add an extra 25-50 grams of carb from starches to your post-workout meal.
6. Keep carb choices relatively healthy. My recommendations: sweet or regular potatoes and other starchy tubers, rice, quinoa, gluten-free oats, fruit. Ideally you’ll avoid foods that are high carb AND high fat such as ice cream, cake, cookies, chips. At least give yourself a strict limit – one or two servings a week – if having occasional treats keeps you on track long-term.