Quick take — If you diet or are planning to start a diet, understanding the relationship between bodyweight and glycogen (Glycogen is carbohydrates as stored by your body) depletion is paramount.

Your body stores energy as fat and glycogen. Whereas fat stores can vary dramatically from person to person, your body can only store so much energy as glycogen.

Glycogen requires water to be stored. In the initial stages of diet/caloric restriction and exercise, your body depletes these glycogen stores, reducing your bodyweight from the elimination of both the weight of the stored glycogen and the weight of the water. Note that nowhere in this process is the much-desired loss of fat!

Thus, even as it will feel good to shed 5 - 10 lbs. simply from a few days of exercise mixed with a caloric-restricted diet, the weight loss will be primarily from a reduction in glycogen stores and water. In other words, what you’ll have lost in the beginning is really little more than water weight.

Take heart in understanding the relationship between glycogen stores and bodyweight as an improved understanding will help you set realistic expectations on whatever diet or exercise regiment you are undertaking in 2009.

A deeper dive:

I first learned about the relationship between stored carbohydrates and water retention from Gary Taubes’ Good Calories, Bad Calories. The gist is that for every gram of stored carbohydrate (Stored as glycogen) in your body, there is a set amount of additional water storage that is required.

Taubes had pinned the carb/water storage ratio at two grams of water per one gram of carbohydrate. A random Googled source pins it at 2.7 gram water per gram of glycogen. I found a research paper titled, Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition, which offers the following data on the ratio:

Glycogen is stored in the liver, muscles, and fat cells in hydrated form (three to four parts water) associated with potassium (0.45 mmol K/g glycogen). . . .

Glycogen losses or gains are reported to be associated with an additional three to four parts water, so that as much as 5 kg weight change might not be associated with any fat loss.

Lyle McDonald of Body Recomposition has also weighed in on this subject:

Carbohydrate (stored in your muscles and liver as glycogen) is accompanied by a good bit of water. For every gram of glycogen stored, you store anywhere from 3-4 grams of water with it.

How does this relationship affect bodyweight? In short, diet and exercise will deplete glycogen stores. If your diet is working, the depletion will occur early and have a significant impact on your bodyweight without impacting a permanent change in your body composition.

Let’s take me as an example. I estimate that I have around 155 - 160 pounds of lean tissue. Tack on another 12 - 17 pounds of fat. After a week or two of being on a low-carbohydrate diet that involves intermittent fasting and plenty of exercise (see here), my liver and muscle glycogen stores will be completely depleted. I’ll weigh about 172.

If I go on to eat a bunch of carbohydrates — cookies, pretzels, breads, fruits and other starchy foods (by eating a bunch, I mean consuming something on the order of 1000 grams of carbohydrates over the course of 24 hours, which is about 4000 calories), I will fully replenish my glycogen stores. In the process of replenishment, the 1000 grams of carbohydrates will require anywhere from 3000 to 4000 grams of water for storage! Converting from grams to pounds, the impact on my bodyweight should be an increase of 9 to 11 pounds, taking my weight up to 183*! Of course, the same change would happen in reverse: re-depleting glycogen stores would drop my weigh back to the low 170s.

Mike over at the IF life alluded to this fact in three bullets back on his Trainer Tells All post:

Muscle size is mostly glycogen and water . . . I can go up and down 10lbs in a week easily depending on glycogen and water balance . . . The first big amount of lbs you lose in the first week dieting is mostly water

Mike’s anecdotal experience is explained by the storage ratio between glycogen and water. What it means is that in the early stages of a diet, the magical drop in bodyweight will be mostly water weight.

Another implication of the water/glycogen relationship on bodyweight is that whereas the first 4000 calorie deficit you create will reduce your weight some ten pounds, the next 4000 calorie deficit is likely only going to reduce your bodyweight a paltry two pounds! This is because a pound of fat stores 3500 calories and requires about a pound of water for storage. Thus, the initial weight-loss will seem easy compared to the drudging continued weight-loss when you’re actually burning stored fat.

Failing to understand what is going on with glycogen stores and water retention will set yourself up for a shock when you inevitably “fall off the wagon” – even if the “fall” is only for a day or two of heavy-carb or more “normal” eating.

Understanding the impact of glycogen depletion/repletion on bodyweight is just one more reason why merely weighing yourself on a scale provides a poor indication of your body composition. You’re better served by taking some physical measurements (waist size, for example). Or even better, take some periodic camera phone self-portraits — over time, you should be able to compare them and get a great feel for your progress (or lack thereof).

* I’ve witnessed this fluctuation on numerous occasions over the past year, but I didn’t quite fully understand it until today. You see, I was fully glycogen depleted going into New Year’s Eve. I proceeded to go on a pre-planned “refeed” (that just happpened to coincide with NYE, of course!). The refeed involved eating plenty of pretzels, chips, breads, fruits, cookies, cereal, donuts, etc. Some incredibly unhealthy, albeit tasty, foods. I also drank a good bit of Pinot Noir NYE, which is the opposite of what you should do if you are re-feeding in that your body will be needing water and alcohol will dehydrate you past certain levels of intake. Anyway, after a 24 hour refeed, my bodyweight went from 172 to 184. Hard to believe unless you understand what is going on. And this kind of fluctuation would be entirely disheartening to the ignorant dieter who might feel they just blew their diet in one day! As it is, I expect I’ll be back in the low 170s within five days after I do a fast and get two or three workouts in.

Further reading:

• Michael Van Cise does a deep dive on Taubes’ and water weight