The truth is, most of the fluctuations in your bodyweight, at least in the short term (day to day; week to week), are a result of fluid dynamics. Your body is 65-90% water depending on your body composition, and as such where that water goes can really affect your bodyweight! You’ve probably noticed that if you weigh yourself before bed, then again in the morning, you will typically weigh in excess of a pound less after a night of sleep. Is this because you were a fat burning machine while sleeping? Not likely…
Your body is an incredible factory that runs all day, and all night. Do you burn energy while sleeping? Not a lot, but yes you do - and the byproducts of cellular respiration include carbon dioxide (exhaled), and H2O. H2O (water) is a molecule that is used for numerous chemical reactions in the body. “Dehydration” reactions occur when two molecules form in the body, and H2O is released as a byproduct. Where does that water go? Well, quite a bit of water escapes your body during the night through exhalation, and evaporation as part of the natural process of body temperature regulation. Yes, it’s true, you can lose several pounds of water overnight, especially the larger or more metabolically active you are.
Sometimes you “retain” water in the intracellular space, or even in the bowel. This can certainly add to your normal bodyweight.
If you are particularly active, especially in endurance sports or if you perform intense exercise at least 1 hour in length, you can deplete your muscle tissue of a macromolecule called Glycogen. Glycogen is basically stored complex sugars in the muscle tissue that is used to fuel that muscle’s activity. The average amount of whole-body glycogen is 300-400g (for women and men respectively), with some people having as much as 1kg. As much as 65% of glycogen is water, so when you exercise and use up glycogen, you are actually dehydrating. Even higher levels of whole-body glycogen are possible by following a carb-depletion and carb-loading protocol. This possible consequence being that the more fit your are, the more glycogen/weight you can store. This may be partially why weight-loss can slow as a training regimen progresses, as you may be getting better at storing glycogen. Then again, if training frequently but without eating 300-400g of carbohydrate per day to reload glycogen stores, this is not likely the case.
Here's where you really see these fluid dynamics at work: fasting, and low-carb diets. Now to be clear, if you start any diet that creates a calorie deficit (whether explicitly stated or disguised as magic) you will typically begin to lose muscle glycogen if the deficit is large enough (and remember a deficit is the negative balance between energy required and energy spent - in other words a combination of diet induced and activity induced deficit). So in the honeymoon phase of any diet (first month) you could reasonably expect up to 10lbs or more of weight loss. That sounds pretty great right?! Sure it is! And the amazing thing is that pretty well any diet can do that in month one. But unfortunately that's the easy weight loss - the low hanging fruit as we sometimes call it. After that initial loss, reality sets in and true rate of fat loss is revealed as something closer to 0.5% - 1% of your bodyweight per week (e.g. 0.5%-1% x 140lb female = 0.7lbs - 1.4lbs per week). That's only 2.8lbs - 5.6lbs per month. Now let's be clear - 5.6lbs loss in a month for a 140lb female is awesome! For larger individuals their weight-loss will be greater (because they have a higher proportion of adipose tissue (fat mass) to lose, and for smaller, leaner individuals that weight loss will be on the lower side.
More aggressive weight-loss efforts like crash diets, tea "detoxes", and other get-thin-quick schemes can result in much faster weight-loss. Unfortunately, the faster the weight-loss the higher the proportion of lean mass that makes up that weight (especially in the absence of resistance training). This is problematic because even though total body-weight decreases (for a time) body-fat percentage remains unchanged, or perhaps worsens if this cycle is repeated.
The challenge that the average dieter faces after month one is that not only does weight loss slow, thus necessitating a greater deficit, but it can stall and perhaps even increase as a consequence of the fluid dynamics we just discussed. After a month of "clean eating" (i.e. low-sodium, low-carb, and high-protein - makes you urinate more) you are typically glycogen and water depleted. If you have a cheat day or meal, or perhaps have a moment of weakness and scarf down a bag of Doritos for example, you start to drive carbohydrates into your muscle, along with it's associated water component, and the extra sodium will drive water into your extracellular space and lower bowel. This sudden influx can really be seen on the scale, and have a negative effect on your mental fortitude. Frankly, it can be pretty defeating, and unsurprisingly this is where most people fall of the wagon. It is unfortunate because this is where a nutrition coach is worth their weight in gold (OK, perhaps not gold but maybe protein powder?). Not only is a qualified coach educated in the science of weight loss, but many are uniquely equipped to identify and address the mental struggle with weight loss, and work with clients to create practical strategies for staying on track, staying focused, and breaking through barriers.
Coach Solly is a Precision Nutrition Certified Level 1 coach who sees nutrition and weight-loss from dual perspectives: not only has he lost 60lbs and kept it off for 15 years, but he has also coached athletes to Pro level physique status, in addition to stepping on a bodybuilding stage himself.