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What the Number on the Scale Really Means: A Primer on Weight Fluctuations

Understanding weight fluctuations is more complicated than you might think.
Woman Standing on a Scale
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Originally published on Richard "Dick" Talens website. Richard is the co-founder and chief growth officer of Fitocracy, one of the web's most popular fitness tracking sites.  To learn more, check out his site and follow him on Twitter.

There are few morning things that have the power to absolutely dictate my mood for the day. A loss in my fantasy league, for example, will pretty much ensure that I’m scowling, even on the nicest of days. More relevant thing to you, my dear reader, is the number that I see when I step on the scale while on a fat-loss diet.

Fortunately the scale reading is only a number. Like all pieces of data, this number may or may not be an accurate reflection of whether or not you are losing fat. Let’s look at problems with over-relying on your scale weight and how we can better interpret said weight.

Modeling Scale Weight

Let’s say that there were a hypothetical universe where someone’s weight had no variability. In this universe, Joe has 150 lbs of lean mass and 50 lbs of fat mass. That means Joe weighs 200 lbs at 25 percent  body fat.

Now let’s transport Joe to our universe. The one where the scale can be a fickle bitch. How much does Joe weigh? Joe would probably weigh somewhere between 196 and 208 lbs. Why the difference? One’s “scale weight” can be broken down into the following formula:

Scale Weight = True Weight + Weight Variance (AKA weight of the annoying little gremlins that mess with your weight)

True Weight: The weight that you would be in our hypothetical universe above (there are ways to get close to this).

Weight Variance: A value that adds or subtracts from your weight, given the conditions below.

Something interesting that I’ve seen from clients is that the upper and lower limits are asymmetrical. The upper limit of one’s scale weight is about +4 percent of his/her true weight, whereas the lower limit seems to be about -2 percent of his/her scale weight. Hence, why Joe’s scale weight is 196 to 208.

Understanding Variations in Weight

Here are a few things that factor into “weight variance:"

  • Glycogen stores. This amount depends on your current consumption of carbohydrates. For every gram of carbohydrate that your body stores via glycogen, it also stores three grams of water. If you are carbohydrate-depleted, you will be at the lower end of your variance. Conversely, if you consume a crapola of carbohydrates, you will be at the upper end of your variance.
  • Water retention/depletion from sodium. If you suddenly consume more sodium than you are used to, you will likely retain water. Conversely, if you suddenly consume much less sodium, you will release water. Your body adjusts to the new levels accordingly via the hormone aldosterone, so don’t think that you can keep this value low just by cutting sodium out from your diet.
  • Cycle bloat. Women will retain water during their cycle. For this reason, it’s best for women to only compare weight from month-to-month.
  • Dehydration. This obviously comes into play, but we’re going to assume that everyone here is well-hydrated.

Scale Weight Fluctuations

Why does the scale seem so erratic when you are dieting? The foremost reason is that glycogen is a much more volatile substrate than fat. That is, fat loss occurs slowly, while glycogen levels can swing wildly.

Let’s see what happens at both ends of glycogen storage.

The High End: Full Stores (i.e. bloat, often from binge eating)
What happens when people go on a binge? Typically they will retain a ton more glycogen afterwards and see a massive increase in the scale. This is only water weight. Too often, I’ll see people defeated because they “gained all of the weight back.”

One thing that you rarely hear about water bloat is that it makes you look fatter than actual fat. Yes, that means that a person whose true weight is 190 lbs and bloats up to 195 lbs will look fatter than if his/her true weight were 197 lbs.

Try this for yourself. When you are on a diet, take weekly pictures of yourself when you adhere to your nutrition plan. After you’ve lost some weight, take pictures again after eating wildly for a day.

Find the two pictures that match up with the same weight. You’ll notice that you will look fatter in your latter pictures, even if your true weight ls lower.

If you find yourself gaining a ton of weight after a bad day of dieting, remember, this is only temporary. Your true weight hasn’t moved much; it’s still subject to the laws of thermodynamics.

(Funny story: As a test I once consumed 1,200 grams of carbohydrates in one day with only trace dietary fats. Research predicts that almost none of this turned into fat. The next day, I looked like the Michelin man and my “skin” felt hurt and bruised. Yes, my skin. Interpret this as you will.)

The Low End: Carbohydrate Depletion
Those who go on Paleo or ketogenic style diets usually cite the rapid loss of weight at the very start, as well as the rapid influx of weight when they cease their low-carb diet.
This isn’t due to some magical powers from copying the diet of pre-historic man. Rather, this is due to the rapid depletion and replenishment of glycogen.

Similarly, the rapid drop in weight that occurs when one starts a diet can usually be attributed to a drop in carbohydrate intake.

Other reasons: Lyle McDonald talks about “the whoosh effect,” in which scale weight will often lag behind true weight loss. If you haven’t read this article yet, I highly encourage you to do so. I take this one step further by showing that you can use certain measurements to determine an impending whoosh, as you’ll read later.

Clients will also often gain lean mass and/or increased glycogen capacity during a diet, especially with a mild deficit. For that reason, scale weight may remain the same even if fat loss is occurring.

Interpreting the Scale

The true secret to interpreting the scale is building a story. Most people use the scale as a final number, rather than piece together a story using relevant pieces of data. The scale number alone is useless when you need to troubleshoot.

Instead, we can create a powerful story by pairing scale readings with the following data:

  • Waist measurements. This is the most powerful piece of accompanying data. That’s because waist measurements are far more useful at determining overall direction of fat loss. Take measurements at the navel, two inches above, and two inches below. Compare with last week’s measurements and assign the measurement either -1, 0, or +1 if the new measurement decreases, stays the same, or increases respectively. Now add the numbers together to determine overall direction that fat loss/gain is occurring.
  • Strength as determined by PRs. Assuming that you have reasonable programming for a deficit, PRs are a good indicator of how far you are from your caloric deficit in the natural trainee. If your strength is increasing, then you are likely increasing your weight from lean body mass as well.
  • Bloat. This tells you how much variance is going into your measurements. Be keen on noticing whether or not you are holding water in key parts. This will vary from person to person, but it will be areas that seem to swell up after a binge. My face balloons in size for example, but my thighs always look the same.

Remember our hypothetical universe where scale weight is equal to “true” weight? We want to replicate this as much as possible. For this reason, you should not interpret measurements when bloat is high. Either wait for it to go away (if it’s caused by your menstrual cycle) or eat normally for a few days (if it’s from a binge).

After that, use the following chart to interpret your data.

How to Tell if the Scale is Wrong

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