The FatWatch Weight Log

You should weigh yourself every day

If you’re trying to manage your weight, you need to measure it, and that usually means stepping on a scale. The question is, how often to do it? Once a week? Every day? Several times a day?

To answer the question, let’s consider why you are weighing yourself in the first place. You are probably trying to manipulate your body in one of three ways: lose fat, gain fat, or maintain the current amount of fat. No matter the scenario, you need feedback to know whether or not you are getting closer to your goal. That knowledge can be used to motivate you and to adjust your diet and exercise plans.

Your weight is a statistic, and stepping on a scale is a form of sampling. Any statistician will tell you, generally, the more samples you have, the more accurate a picture you can create. More samples are better. But if that’s true, why do people recommend only weighing once a week?

Once a day?

If a scale tells you that your weight has increased by a pound since yesterday, that means you are carrying a pound more “stuff” in your body, but that stuff is not necessarily fat. In fact, it’s much more likely to be water. Water comprises about 75% of your body weight, and you are constantly adding to it when you eat or drink, and losing it when you sweat, exhale, or, ahem, make water. Approximately 10 pounds of water passes through your body each day.

By comparison, your fat stores change agonizingly slowly. For example, if your metabolic rate is 2,000 calories per day, and you ate nothing at all (I do not recommend this), your body would only need to burn half a pound of fat to keep running. Probably even less, since the act of starving will slow your metabolism. (Starvation also results in embarrassing diseases like scurvy.)

So, if you weigh yourself every day, you will see a lot of change, but almost all of that will be due to water. Your water level is noise that is masking the change in your fat. If you let your emotions follow these day-to-day swings, your mood will be riding a roller coaster.

Once a week?

Your body doesn’t accumulate water like it accumulates fat. The amount of water in your body will rise and fall but generally hover around the same level. Over a longer period of time, changes in fat have a greater influence over your weight than changes in water. Thus, weighing yourself less often results in a less bumpy ride.

This seems like a good argument for weighing only once a week, right? Well, the problem is that a week still isn’t a long enough time. Unless you are burning or gaining at a fast rate, the scale measurement will still be dominated by changes in water. You could burn off a pound of fat in a week but the scale might say you weigh the same as you did the week before. Is your diet working? You might think it’s not and be tempted to give up.

Mathematics to the rescue

So, we’ve decided that we’d prefer to weigh-in once a day, because that provides more feedback, but the noise from your changing water level makes that feedback hard to interpret. We can correct for that problem by weighing less often, at the cost of less feedback.

It seems hopeless, but what if there were another way to reduce the noise?

It just so happens that the same statisticians who recommended taking more samples also have a method for dealing with noise. If a data set yields a very bumpy line, statisticians compute a moving average to find a smoother line through the data. The average makes it easier to see the trend through noise. Using this technique on your weight, you could effectively eliminate the influence of water, and get a better picture of how your fat is changing, on a daily basis!

It may sound super complicated, but it’s not so bad. If you’re using pen and paper, you can compute a moving average fairly easily using this method. On the first day, your weight and the average are the same. On every day after that:

1. Take today’s weight and multiply it by 0.1. (e.g., 185 * 0.1 = 18.5)
2. Take yesterday’s average and multiply it by 0.9. (e.g., 187.2 * 0.9 = 168.5)
3. Add the two numbers to get today’s average. (e.g., 18.5 + 168.5 = 187.0)

Now, instead of your weight, you can follow the average. If today’s weight is higher than yesterday’s weight, but below today’s average, that means there is no need to despair! You are still making progress.

If the prospect of doing all these calculations seems a little bit tedious, I recommend having a computer do the work for you. If you have an iPhone or iPod touch, you can enter your weight into FatWatch, which will not only do the math, but also make some nice charts and do additional analysis, such as figuring your daily calorie surplus/deficit and helping you convert that number into useful units, like minutes of running or chocolate kisses.

If you don’t have an iPhone, there are other tools available. On the web, you can use The Hacker’s Diet Online or PhysicsDiet.com to compute a trend line and also make a chart. If you know of a good app for other mobile platforms, like Android, I’d like to hear about them.

I hope I’ve convinced you that the right way to track your weight is to measure every day and then be smart about it.