It's a question a lot of us seem to ask, and it personally caused me a lot of grief when I'd be very careful the whole week, fast, etc., and then see a HUGE hike in the scale the day after a planned indulgent meal. It made me give up at many points. Here is some info on carbs, water, and glycogen that helped to clear up (for me, anyway!) a lot of the misconceptions about carbs, carbs making me "fat," etc.
I hope it can give others similar piece of mind to know what is going on with their body, and not to panic when the scale seems to surge up after a single carbohydrate-heavy meal.
This is an excerpt from fitness expert Leigh Peele's "Starve Mode," which I highly recommend if you're interested in learning more about metabolism and "starvation mode." She talks a lot about training, but the points are still worth noting nonetheless. It's a bit of a read, but worth it for a little bit of understanding and, hopefully, peace of mind. I put the most important points in boldface font if you are short on time.
When glucose is not stored as fat, it is stored as glycogen. Each gram of glycogen is stored with approximately three grams of water. So, if you eat 120 grams of carbs in a day, you could say you are roughly storing 360 grams of water from those carbs. How you train before carb intake, how depleted you were before dieting down, or utilizing a low carb will vary the level of glycogen you store. Muscle amount is also going to alter storage.
Oftentimes people accuse diets of being muscle-eaters, but this isn't usually the case. While you are freaking out and making false claims about atrophy, you can make muscle magically reappear with carbohydrates in one training session. One of the first things to go when you begin dieting down is glycogen, especially if taking part in an extreme diet or one that is low in carbs (even if high in calories).
The reason carbs are so valuable is that glycogen storage is pulled mainly from carbohydrate intake. Though a small amount can be taken from protein, it is never on a large enough level to maintain adequate or noticeable glycogen retention. That plump look you are going after with your muscles comes from glycogen. However, if you are not lean enough to see this definition pronounced, all you are going to notice is that your fat looks fuller on the days you eat carbs. Cue people saying carbs make them fat. They don't. Your fat makes you fat, and water can look and act like fat. The key is to put the carbs to work by pulling them into the muscles through training. Go for plump, not bloated. Going back to the point of this book, it's all about understanding what goes on in your body, not just trusting what media or simple appearances tell you.
If you have been eating in a deficit for a short time, a sudden increase in carbohydrates is going to result in a large change in glycogen storage. This is normal and the expected increase in bodyweight that will need to take place in order to return to your body to a homeostasis of recovery.
Not eating carbohydrates (starch, sugar-based) for a long period of time and then eating them can cause you to retain water on an excessive level. It is a constant argument source from women to say "I can't eat carbs, when I do they bloat me and make me feel fat!" There are three primary reasons this takes place from carbohydrates alone:
1. They are eating/binging on carbohydrates at a fast or large rate. This will cause an overflow of storage and increase digestive strain. This is also going to increase food and water volume. This is a problem of excessive caloric intake, not just carbohydrate intake. It is extremely easy to overeat carbohydrates after a deficit.
2. They have been on a low carb diet (less than a hundred grams a day) that's very restrictive of starch/sugar-based carbohydrates. During reintroduction at a fast pace, these people can experience digestive dysfunction due to decreased enzyme production and increased insulin-based edema.
3. They have been dieting down and overtraining for a long time and have acheved a high level of caloric metabolic adaptation. While this does not usually include carbohydrate restricion as well, on its own it can have rebound effects.
The best way to avoid rebound from intake of carbohydrates is not going extreme with macronutrient and caloric restriction. Utilizing extreme diets without periods of breaks or utilizing macronutrient-specific restrictive diets can lead to more problems with reintroduction in the future.
Some may argue, "I am not planning on ever reintroducing starch or sugar-based carbs." If that is the case, it is possible you will be met with a long-term water imbalance. Contrary to some popular arguments, carbohydrates have an essential nutrient placement in your daily nutrition, especially in training nutrition. Please investigate the amount of carbohydrates needed for your body and your use of them for dieting down, training, and performance.
This is a key reason why carbs get a witch-hunt. It isn't the glycogen's fault; it's your body fat level or unhealthy balance. You need to learn to love what the carbs can do for you
There can be issues with saving and storing water in fat cells themselves once they have been emptied from stored lipids. When fat leaves a cell it also leaves behind a hole. Because fat cells don't disappear (merely fill or empty), they can interchange with water. If you notice a squishy appearance to your body and a stubbornness to weight drop even when sure of a deficit (and you have been treating your body right), you should come upon a release of that water overtime."