I'm starting to become a fitness geek and recently have been reading everything about how the body works. I've been reading about how the body stores and uses energy and stumbled on the answer to why body weight fluctuates so much.
When starting to diet, most people lose several pounds quickly then level off. The weight lost is often described as water weight. The water weight can be caused by a reduction of the amount of glycogen, an energy reserve, in your body. It can also be caused by less water in your tissue caused by a reduction in the amount of salt you consume--a byproduct of healthier eating.
Both can cause your weight to fluctuate several pounds in a day. It's important to understand how these factors affect your weight so you can focus on what is important: Losing body fat, and not just body weight. Note that there may be other causes for water gain such as medication and medical and conditions and hormones which I won't go into here.
Water Retention from Salt
Water retention from salt is easiest to explain. In summary, your body uses electrical impulses to function. Electrical impulses are made possible by a balance of sodium, potassium and water. If you consume too much salt, the kidneys put more water into your system to keep the sodium in balance. The extra water is absorbed by the body through osmosis and you gain weight. According to Water-Retention.Net, salt based water loss is usually no more than 2 pounds. Read this article, How Salt Affects Your Weight, for more information.
Glycogen and Water Retention
What glycogen does and it's role in water loss is more important to understand and more difficult to explain. In short, carbohydrates are turned into glycogen and stored in the muscles and liver. Think of this as your primary fuel tank. If you run out of glycogen, your body needs an alternative source of carbohydrates (fuel) so it turns protein (muscle) into carbohydrates. If your glycogen levels are full and you eat more, it turns the extra carbohydrates into fat.
When full, the body stores about 400 grams of glycogen worth about 1600 calories of energy (variables include amount of muscle mass and individual physiological differences). Each gram of glycogen needs 3-4 grams of water for storage. That equals 1600 grams of water stored in glycogen, or 64 ounces (half a gallon) weighing 4.15 pounds.
If you cut back on your carbohydrate intake to create a calorie deficit, your body isn't able to replenish the glycogen as it's used. The result is weight loss from the loss of glycogen, not fat. If you have a glycogen deficit then replenish from a large meal, the glycogen is replaced resulting in a weight gain from additional glycogen, not fat. The result is that one 2000 calorie meal can theoretically cause you to put on 4 pounds of weight overnight.
How I Use This Knowledge
I know one pound of fat represents 3500 calories, so I would have had to burn that many fat calories to lose a pound of fat, very unlikely in a day unless I went on an extreme ride (8+ hours). Conversely I'd have to eat over 6000 calories in a day (something like 3 large pizzas) to gain a pound of fat, again very unlikely.
I weigh myself every morning to see how my weight has fluctuated. If I've lost of a pound or more overnight, I know most likely its water so my glycogen stores are down. I don't worry as much about calories for the next day. My "gas tank" is getting empty so I should refill it, without overfilling it (producing fat). If I plan to ride or do something else physical, I'll intentionally eat more (carbo load) to make sure I have energy.
If I've gained a pound or more overnight or haven't exercised for a couple of days, I know to watch my calories more carefully. My "gas tank" is getting full so I don't want to overfill it and add to my fat stores so I'll stay closer to my calorie budget and not worry if I go below it.
This may also explain why many smaller meals are better for you than one or two large meals (although I haven't researched it yet). I think of smaller meals as a just-in-time carbohydrate delivery system for your body which minimizes the risk of overfilling and producing fat, or under filling and creating a situation where your body needs to burn lean muscle mass.
What are your thoughts and experiences?
Some Interesting Articles
Understanding Body Weight and Glycogen Depletion--Blogger Justin Owings describes the concept in more detail than I do.
The Fat Burning Mechanism--Glycogen Depletion--Glycogen depletion from the body builder's perspective.
Don't Burn Lean Muscle Tissue--What happens if you fully deplete your glycogen stores, burning lean muscle, and why I don't want to empty my glycogen stores.
Starvation Response--More on what happens to your body if you don't have enough carbohydrate based energy.
Do you have any interesting links related to this topic?
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