TL;DR: Individual bioimpedance measurements of body fat percentage can vary around ~2% based on hydration levels (based on a Withings Smart Body Analyzer scale). The distribution of water in your body (the time it takes for everything to come into equilibrium after laying down for a long time) also results in a potential shift of approximately 1.5%. Individual measurements based on bioimpedance are not particularly accurate, but on average the reading is correct and long-term trends can be distinguished. A bulk/cut cycle can be an effective strategy for changing the composition of your body, if you proportion the cutting and bulking periods correctly. A high quality scale that measures body fat as well as weight can be an invaluable tool for tracking what works for both weight loss as well as muscle gain, and allows you to adapt and change your routine as you find out what works and what doesn’t.

In health and fitness, talking about weight is often regarded as the main (pardon the expression) elephant in the room. Almost one third of Americans are now considered either obese or morbidly obese, a number that only seems to be getting higher. One of the most helpful decisions someone can do for their health and general well-being is to decide to lose some of the excess baggage they carry around on a daily basis. People at a healthy weight live longer lives1, spend less on medical costs each year2, and (unfairly) are seen as less competent at their jobs3.

One of the best things that someone can do for their waistline is simply know how much they weigh.

No matter how someone decides they are going to lose weight, one of the best things that someone can do for their waistline is simply know how much they weigh4 . This is a tough activity for many people, as when you're struggling with weight the scale can seem less like a helpful tool and more like judge at an unfair trial, laying down adverse rulings without regards to your applied effort or perseverance. I'm lucky in that I'm fairly pragmatic when it comes to the number on the scale, and thus feel no hard feelings towards myself when I have a bad day or week. But for many people, scales are a demotivating force: a reminder of past failures rather than of current successes. So what then makes a scale such an essential tool?

One reason: it allows you to understand your body and your personal “weight rhythm.” The following graph is a snippet of my weight over a period of a month where I was holding steady: in the green is the actual data measuring my weight twice a day, and in the red is measuring about once every week.

On a day-to-day basis, I see fluctuations of up to 3 lbs in any direction, depending on the time of day. The red line is a sampling of about one measurement per week, which while an obvious outlier in the green looks to be a disappointing upward trend in the coarsely sampled red data. By exposing yourself to your weight on a daily basis, you desensitize yourself to the daily ebb and flow of the number on the plate and begin the path of truly understanding your own body.

In a world of vague promises of 6-minute abs and 60-day full body results (guaranteed!), a scale a remarkably simple tool that allows you to cut through the madness and see what actually works.

In a world of vague promises of 6-minute abs and 60-day full body results (guaranteed!), a scale a remarkably simple tool that allows you to cut through the madness and see what actually works. However, a simple scale’s failures quickly become apparent when you goal becomes more complex than losing weight. After defending my thesis in May and starting a desk job in July, I started to see my weight slowly creep up last summer. I decided to take advantage of the “slow bulk” I saw myself inadvertently doing and start working out to try and gain muscle in October. Here is the plot of my weight over that period:

Once I almost hit 180, I was feeling a little inflated and although happy with my strength progress in the gym, somewhat dissatisfied with my current body composition. I decided that I had “slow bulked” enough. I cut down my daily calorie allotment with the goal to lose weight and decided to go "mid-to-low carb", basically turning down opportunities to eat carbs when they came up outside of a meal, unless the desire was strong enough. It wasn't easy, as avoiding sweets and baked goods when offered is harder than I'd rather admit, especially in the middle of the work day. But adopting the attitude of consciously deciding every time that I really wanted that brownie or cookie that I could have it--rather than just eating it by default or out of boredom--made the times I do decide I want it much more enjoyable. You own the cookie, not the other way around. And, thanks to the power of the scale, I know it’s working great:

The scale told me what I did was working, and now I know exactly what I need to do if I want to gain weight at a certain rate, or lose it at a certain rate. However, my goals were never just to lose weight: I started working out back in September with the intent on gaining muscle. Luckily, this scale tells me my body fat percentage, so technically I should be able to tell how much of the weight I gained was muscle vs fat, and how much I manage to keep when as I lose weight. Indeed, I did this at the end of this post, and the results are pretty cool. But first, let’s figure out if we can trust the body fat percentage numbers.

Consumer scales that read body fat percentage all do it through the same method: bio-electrical impedance. Fat is an insulator and your lean body mass conducts electricity, so by measuring the resistance of your body from one foot to another, it uses that information to deduce how much fat is in your body. However, this number is also affected by other variables. Hydration is a big one, where the more hydrated you are the less resistive your body is. This can lead to misestimating your total body fat. In addition, the Withings Smart Body Analyzer Scale (which is where all the data in this post came from) manual states that the liquids in your body take a while to adjust after waking up, temporarily altering the distribution of water in your body and thus creating inaccurate measurements. So how accurate are these scales really?

I decided to see how substantially dehydrating myself affected my body fat percentage measurement.

In the grand tradition of scientists performing experiments on themselves, I decided to see how substantially dehydrating myself affected my body fat percentage measurement. This involved fasting and avoiding all food, water, and other beverages all day long. Including coffee, so this informal study probably wouldn’t have passed an IRB board due to crimes against productivity. Anyway, the plan was to weigh myself several times in a dehydrated state, drink 16oz of water, weigh myself again, drink more water, and continue until my body fat percentage reading leveled off or I suffered acute water poisoning. Once full, I then would measure my body fat percentage over time to see if it’s purely the amount of water in my body that affects the body fat measurement or if it indeed is the distribution of water (which would result in my body fat percentage changing over time, after I had stopped drinking). Finally, after peeing out the water, I would weigh myself and use the difference in weight to calculate the remaining water left in my system and my subsequent body fat percentage.

Here’s what the weight vs time graph looks like, colored by amount of water left in my system and the mean of each measurement indicated by the line:

As you can see, I drank a total of 48 oz of water and gained the expected ~3.1 lbs for that amount. I stopped at 48 oz because, as you will see in the next graph, my body fat percentage did not seem to change much between 16 and 32 oz. I used the bathroom at 130 and 225 minutes, and used the difference in weight to calculate the amount of water remaining in my body. Now let’s look at how my body fat changed over time:

There was a large initial jump with the first and last 16 oz of water, but a slow and steady increase even an hour after I finished drinking. This confirms that it’s not just the presence of water in the body, but rather the distribution of water throughout that affects your body fat percentage reading. This was confirmed after some review of the literature, where it was was suggested that "due to the large cross-sectional surface of the trunk, even fluid intake of up to 2L is shown to be "electrically silent" during the first hour after consumption5,6".

The following is body fat percentage as a function of weight, showing how my weight and supposed body fat weight changed over time (marking the amount of water in my body over the baseline and the time since the first measurement):

Or, in a non-animated format, with total body fat in place of the percentage:

You can see that being dehydrated ends up underestimating the amount of fat on my body. As my body’s water content goes up, so does it’s (supposed) fat content. This is the opposite of what you’d expect if it was purely based on electrical impedance, since the added water should lower my body’s conductivity. This could possibly be explained by noting dehydration also change the your bodies electrolyte balance, which potentially could have increased the impedance measurement. In addition, there's a slight overestimation of bodyfat when I was over-hydrated, but not nearly as large as the effect of dehydration. But the takeaway is less about the individual variation, and more about the major assumption that these scales make: these scales use the electrical measurement assuming your body doesn't deviate from the norm in any major way, other than fat content 7. Any major difference from the standard day-to-day "normal" body will degrade the output from the scale8. Being dehydrated was a major deviation, so it decreased the accuracy of the results. Withings explicitly accounts for this if you happen to be an extremely athletic person, where turning on “Athlete mode” just subtracts 5% from your body fat percentage because that’s how much the reading can be off if you’re much fitter than the average person.