I have been testing out a carb cycling diet and am currently into week 6 of a 12-week diet. Four days of the week I eat 2000 calories, with most coming from protein and fat. Three days of the week I lift heavy weights, consume 3000 calories, and drastically increase my carbohydrate consumption while lowering fat intake. The low carbohydrate days are said to take advantage of the fat-burning effects resulting from keeping insulin levels low. However, low insulin and a lack of carbohydrates can impair muscle recovery and growth, as well as hormone function, so on training days the high carbohydrate intake helps refill muscle glycogen, aid in recovery, and stimulate hormone production. Fat intake is lowered on training days for two reasons, one is simply to make room for more carbohydrates, but also since humans typically don’t convert excess carbohydrates into fat in any significant amount, the fat gain is minimized even though calories are typically over maintenance levels.
Has it worked magic? Well, no. Does it work to an extent? Yes. I can’t say I’ve gained much in the way of muscle but my strength has maintained and I’m leaner than I’ve ever been (~12% body fat as of this writing). It is one of those things where “your mileage WILL vary.” For example, having an ectomorphic build & doing a strength-based routine (as opposed to a hypertrophy-based or bodybuilder style routine) limits how much muscle I put on in general, let alone on a diet. These are two limiting factors that can’t necessarily be blamed on the diet itself. Next, finding the sweet spot for caloric intake is a little tricky and my current calorie levels favor fat loss over strength gain. Were I to increase my calories a bit I’d likely see the slightly slower fat loss but also more muscle growth. That’s a trade-off I’m currently willing to take.
Some important things to consider with a diet like this is that most of the advantages from cycling carbs only exist for people with lower body fat levels and good insulin sensitivity (two things that tend to go hand in hand). People with high body fat levels do just fine with a standard diet, not to mention it can unnecessarily complicate food choices and habit-forming. And it’s also only really useful for people doing a weight training routine of some sort; absent that there’s not much advantage to spiking carbohydrates drastically.
When it comes to implementation, it certainly helps to have a familiarity with tracking macronutrient consumption. Eating a low carb high-fat diet one day, and a high carb low-fat diet the next takes a little logistical planning. Like most diets though, once a few staple meals are established, it becomes second nature. And that actually brings me to my favorite part of this style of dieting, namely that I have a wider variety of food choices than most diets. The high carb high-calorie days especially allow me to eat foods most people wouldn’t think are diet-friendly, which is very satisfying psychologically & makes dieting an almost effortless process.
After a couple of days of dieting, the metabolism slows down, hunger increases, and more and more muscle mass is sacrificed by the body for energy. Sadly, the human body is very efficient at adapting to new conditions. In short, thyroid hormone T3 levels drop by 30%; conversion from T4 to T3 in the liver is being slowed down, the half-life of cortisol increases and the production of Insulin-like growth factor (IGF-1) is down.
Your muscles are so low on glycogen that they become resistant to growth despite training. In fact, you are probably losing muscle. At this point, we need to talk about leptin for a second since it is also an important player in the diet scenario. Normally, it is a messenger hormone that inhibits your appetite to prevent you from overeating and gaining weight.
Now when you are dieting, the opposite holds true. With the reduced calories, leptin levels drop and appetite goes up. This means that a person who lowers his body fat is at an immediate disadvantage: his metabolism is automatically slowed down by as much as 30% within days while suffering from hunger pangs.
So during a diet, all of a dieters nightmares come together: higher protein turnover combined with a lower level of T3, IGF-1 (insulin-like growth factor, one of the strongest muscle building hormones), leptin, and testosterone.
Why is that a nightmare? Because not only will you not lose any more fat, you will actually look worse than before. The loss of muscle will create a skinny fat version of yourself, the type you can see on most treadmills in the country.
Increasing the calories for a short period of time reverses the process described above. Testosterone, IGF-1, and leptin levels are brought up; the production of cortisol is slowed down; muscle loss is stopped, even reversed. As a result, the rate of metabolism increases, which then sets the stage for further fat loss.