Over the last month, I've been saying I need to look into what influences the body fat percentages and muscle percentages that my scale gives me. Back on March 12th I did a quick search, which told me they fluctuate over the course of the day (unfortunately I didn't put the link to that information in that post). So I need to start from scratch and see what I can find!
The Basics
My search first brought me to the Men's Health article "What Your Body Composition Metrics Actually Say About Your Health." That article looked at a number of other body composition statistics on top of fat and muscle percentage. But I don't think my scale's numbers line up with all of them (for example, my bone percentage keeps popping up at around 11%, but the average seems to be more in the 3-5% range, so I'm not sure how or what exactly my scale is measuring).
According to that article, which is quoting numbers from Withings, a smart scale manufacturer, normal fat mass ranges are:
- 22-33% for women and 8-20% for men, ages 20-39
- 24-34% for women and 11-22% for men, ages 40-59
- 25-36% for women and 13-25% for men, ages 60-79
- 63-75.5% for women and 75-89% for men, ages 20-39
- 62-73.5% for women and 73-86% for men, ages 40-59
- 60-72.5% for women and 70-84% for men, ages 60-79
This was the point where I thought to go looking for my scale's user manual (that links to the online .pdf). It only lists the body fat percentages and hydration percentages (I'm ignoring this measurement because I always weight myself when I get up in the morning before I've had anything to drink, so of course this number will be low at that time). The manual's fat percentages are a little more stringent, but along the same lines as those given by Withings.
The Men's Health article also explains the benefits of healthy fat and weight percentages (which include fall prevention, illness prevention, and a faster metabolic rate in the case of higher muscle percentages).
I also found an interesting article by Healthline on muscle mass percentage, which says that there isn't a lot of good data on muscle mass percentage. At the time of last medical review (July 8, 2019), that article states that only one study from 2000 has credible data. This article gives even more stringent percentages (the ranges are only about 3 percentage points, instead of the approximately 10 given by Men's Health).
The Healthline article then talks about how to go about measuring both body fat percentage and muscle mass percentage. The short answer on how to do it is that it's very difficult to do so without having the right equipment. It talks about body fat scales, because you can estimate your muscle mass by subtracting your body fat percentage from 100, but also cautions that they give estimates only. Healthline then talks about the benefits of higher muscle mass, the downsides to high fat percentage, and explains how to start building more muscle mass, no matter your age.
Why Do These Percentages Fluctuate?
So with all those basics out of the way, what makes these numbers change? Azcentral says the main reason is because body weight scales rely on how much water is in your body to take their readings. So having a drink, or even having a bath or shower before jumping on the scale can make a difference in the reading (and I know I have jumped on the scale after having a shower in the morning - I didn't think it made a huge difference because my weight didn't really change. But now I know it throws off the fat percentage and thereby muscle percentage calculations!) Hormonal changes can also influence the readings. And this is all in addition to the normal variations that do happen from day to day. Azcentral cautions that you should use these scales to track your overall progress; don't get hung up on the individual numbers.
To second this, I found an article on Women's Health that talks about the normal weight fluctuations that everyone experiences from day to day. Just be careful if you see more than a 5lb change that doesn't reverse itself in a few days, or really rapid weight changes. Both of these can signal that something is wrong and needs addressing.
Finally, I found a discussion on the Fitbit community that talks about the fluctuations in these numbers. Their basic advice is that when you lose weight, especially at the beginning of your weight loss journey, you lose water first. That in turn will make your body fat percentage higher. So even if you're losing weight, your body fat percentage will increase, particularly in the beginning. In that discussion, user MatthewFitbit says "In short, fat mass decreases more slowly than other types of mass in your body, which may result in a higher body fat percentage. [. . .] On a longer timeframe, you should see both of these values start to decrease."
