A water whoosh or sudden change in weight when dieting is something that everyone experiences. It is due a change in water balance in the body, not sudden gain or loss of fat. This happens for one of two primary reasons:
This happens generally whenever someone either, a) starts dieting, b) takes a break from dieting, c) has a sudden spike in carb intake.
1g of glycogen holds 3g of water. Our muscles are made up of ~70-80% water which is stored from muscle glycogen. Glycogen comes from the carbs we eat.
So if you eat fewer carbs than normal, which you will when you start dieting, your body (the muscles mainly) will hold less water giving you the false impression that you’ve lost fat if you rely solely on scale weight to gauge progress. (Don’t. I recomment that you track your progress this way.)
If you eat more carbs than normal, when you take a diet break for example, or go out, get drunk and wake up surrounded by pizza boxes that you swear aren’t yours, your body will hold more water giving you the impression that you’ve gained a lot of fat. – This is just a temporary fluctuation in water balance and will come down again when you resume your diet.
This will not be fat gain:
1lb of fat ~= 3200kCal of stored energy. If your maintenance calorie intake is 2500kCal, even if we assume that any excess over regular calorie maintenance is stored perfectly as body fat, then that’s more than 5700kCal you’d have to consume on a single day to gain a pound of body fat. Possible, yes, but not likely if you are eating sensibly.
The simple fact is that though fat loss comes off linearly with a calorie deficit, the way it manifests that in the scale weight is often not. This is because water retention often masks it, and we get stalls and whooshes. We can do nothing about this but be patient and wait.
Here is a good example of this happening to a client for several weeks.
It has been hypothesised by Lyle McDonald that this is due to the fat cells filling up with water as they empty. This is especially annoying because the changes do not show in the stomach measurements either. I’ve seen this many, many times over the years with clients, and while I have seen it happen to people without reason, those with a high amount of stress in their lives tend to experience this more often.
Fat loss will not start or stop suddenly unless calorie balance has changed. The reduction in total daily energy needs (or TDEE) when dieting is a gradual thing (NEAT↓, BMR↓, TEF↓). Metabolic adaptation is not a sudden switch. So, if people have been consistent with their diet, weight loss has been at ~1lb a week for a while (for example) and the scale *suddenly* stops moving, you have a good idea that there is some water retention going on.
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