Why Your Weight Fluctuates and Mine Does Too
Weighing yourself can be a tricky business. Whether you’re someone who steps on a scale every day, or you hop on once in a blue moon just to see, the number may not be what you expect.
But why? And when is a “surprise” number a problem?
The truth is, every “body” is a different weight all the time. Your body weight HAS to fluctuate - all day, every day - even if you don’t feel bigger or smaller, and even if your weight doesn’t change much over time. It’s because your body is constantly in a state of change. Every time you eat something, exhale, poop, pee, take a shower or even...do nothing...your body is either adding mass or reducing it, often at the same time.
Here is why:
1) WATER - much of the reason your body weight moves around all day is because of water. There is a lot of water in your body, up to 80% of your entire mass. Water is in your skin, it’s in your organs, it’s in your blood, it’s in your other bodily fluids, and it’s also in your muscle cells. You spend a lot of time shedding water from your body - peeing, sweating and breathing are the biggest outlets. But you also spend a lot of time adding water to your body, mostly through drinking liquids, but also through food and even a little bit through bathing. There are many factors that govern how your body stores and expels water, but there are two key ones: Water balance and muscle mass.
Water balance is the homeostatic state of hydration that your body is used to. It’s based largely on how much water you usually drink and how much salt you eat. Water balance is a changeable quality, but in general the more water you drink, the more your body is used to flushing water, and the more vascillating your body weight will be based on water content.
Muscle Mass also dictates how much your body weight may fluctuate from water, because muscles are a major storing house for water. Just like someone who drinks a lot of water, someone who carries a lot of muscle mass on his/her body (meaning someone who lifts heavy weights!) is likely to have more body weight fluctuation based on water. Think of your muscles as shapely sponges that can suck up and store water. An interesting note here - body FAT does not behave this way. Much the way oil (fat) and water don’t mix, fat cells don’t host water the way muscles do.
2) GUT CONTENTS - the other biggie that changes your body weight all day everyday is the contents of your digestive tract. Think about it - if an apple weighs 5oz and you eat it, your body (at least momentarily) now weighs 5oz more. The water in that apple becomes part of the water in your body, and the sugars in the apple get pulled out and thrown into your bloodstream. But the fiber in the apple is going to work its way through your guts and ultimately come out through your rear end as poop. Unless you haven’t eaten in days, your gut heft represents the last day or so of food. The more fibrous your diet, the more the contents of your gut are liable to influence fluctuations in your body weight.
3) MUSCLE TISSUE GROWTH/ATROPHY - while water and gut contents represent shorter term fluctuations in body weight, it is possible that a change on the scale reflects actual change in your body’s stored muscle and stored fat. Muscle mass is a very hard-won tissue. It takes a lot of work to create it, so the rate of gain is slow. Someone who trains very hard and isn’t on steroids can at BEST add 1 pound of muscle a week, even if they’re eating like crazy. A more reasonable rate for someone starting resistance training is 1/4 to 1/2 pound per week.
Guess what’s easier than adding muscle? Losing it! Since muscle maintenance requires a consistent sufficient stimulus (frequent hard training), if you stop resistance training you can expect to lose muscle over time - even if you are still eating as much food. That muscle loss can be at the same rate or faster than muscle gain.
4) BODY FAT COMBUSTION/STORAGE - this is generally the statistic most people are interested in. Has the scale change meant I’ve gained or lost body fat? The answer is Maybe, but let’s look at the numbers. One pound of stored body fat provides about 3,500 calories of energy. In other words, that’s how much energy your body is stowing away for famine times when you add a pound of fat to your frame. In order to have stored that amount of fat, your body must have taken in about 3,500 extra calories. That would be a LOT of extra food in just one day, or even three days. But divide 3,500 by 7 (days) and you can see how an extra 500 calories a day for a week could result in a pound of gained body fat. If that person is also doing hard resistance training, some of that extra food will go toward muscle growth, but sadly not much of it, since your body can only build muscle so quickly. Anything extra that outpaces your body’s ability to thicken muscle tissue is gonna get dumped into your almost infinitely-flexible fat cells.
Beware that while a caloric surplus (extra food) will likely add tissue at a ratio that is more fatty than muscly, a caloric deficit is likely to take away muscle and fat at a more equal rate. This depends on a LOT of things, namely how hard you are training during your caloric deficit, how much protein you’re eating, and your hormonal profile. But in general: it’s easy for your body to add fat, it’s harder for your body to add muscle; it’s easy for your body to lose muscle, it’s harder for your body to lose fat.
Your Ideal Range
While I’m not personally a fan of daily or even weekly weighing, I don’t think that your body weight should be a complete surprise to you. There is a range of weight fluctuation that lies within the spectrum of “maintaining” your body composition, and there are trends that indicate actual CHANGE to body composition (adding/losing muscle, adding/losing fat). Understanding your own ideal body weight range - and when you’re actually shifting out of that range - is a good idea.
THE most important element here is timeframe. The following image shows the four factors for weight fluctuation (water in BLUE, gut content in GREEN, body fat in YELLOW, and muscle in BLACK) against a RED average line. Water and gut content can fluctuate daily, while body fat change happens more over weeks, and muscle tissue change happens over multiple weeks.
Here are some very general ranges for different bodies that indicate a maintenance range (no real change in muscle/fat) of fluctuation due to mostly water and gut content, versus adding or subtracting tissue over time. Notice the potential difference between someone who trains (lifts heavy weights regularly and therefore carries muscle mass) and someone who doesn't, even if they weight the same:
Avg 200 lb man who trains can fluctuate ~6 pounds in either direction with no real tissue change
Avg 175 lb man who trains can fluctuate ~5 pounds...
Avg 240+ lb man who trains can fluctuate ~7 pounds...
Avg 200 lb man who doesn't train can fluctuate ~ 4 pounds in either direction with no real tissue change
Avg 175 lb man who doesn't train can fluctuate ~ 3 pounds...
Avg 240+ lb man who doesn't train can fluctuate ~5 pounds...
Avg 150 lb woman who trains can fluctuate ~ 4 pounds in either direction with no real tissue change
Avg 125 lb woman who trains can fluctuate ~ 2.5 pounds...
Avg 185+ lb woman who trains can fluctuate ~ 5 pounds...
Avg 150 lb woman who doesn't train can fluctuate ~ 2.5 pounds in either direction with no real tissue change
Avg 125 lb woman who doesn't train can fluctuate ~ 1.5 pounds...
Avg 185+ lb woman who doesn't train can fluctuate ~ 3.5 pounds...
Again, these are just approximations, but they're good reference points for someone trying to figure out whether or not their weight change means they're actually gaining/losing fat and muscle. And given the science you've read above, it should be obvious why someone who has a bad stomach bug for a few days (vomiting, diarrhea) might suddenly "lose" a bunch of weight but gain it right back once they're able to hold onto water and food again. It should also show how athletes who compete in sports that require weight class categories are able to manipulate water and/or gut content to weigh in lighter on their contest days but not compromise muscle mass.
Finally, don’t let the scale dictate how you feel about your physique or whether or not you’ve made progress. Body weight matters, for sure, but when you lift weights hard and consistently over time, you fundamentally change the way your body stores muscle, fat, and water, and a greater range of fluctuation on the scale is to be expected. Pay attention to trends over time (weeks and months, not days), and if you’re really unsure, come see me in-person for caliper and circumference measurements to confirm your suspicions.