The majority of people on a diet will tell you that they’re dieting because they want to lose weight . If you ask them why they want to lose weight, you’ll hear things like “I don’t want to have a heart attack at 50” or “I have some extra flab” or “I want to get rid of my spare tire” or “I want a six pack.” So people want to be healthy, or they want to look good naked, or both. Importantly, though, none of these goals have much to do with weight per se. What people are actually concerned with is excess fat. And we generally know this, of course. We know that when we say somebody is overweight we really mean the person is overfat. Yet, in day-to-day practice, we don’t use the word “overfat,” we keep saying “overweight.” And as a result, we keep confusing weight and fat, and more importantly, we use the wrong metrics to keep track of our dieting success.
Ask anybody on a diet how they’re keeping track of their progress, and the answer you’ll hear most frequently is going to be “I weigh myself regularly.” There we have a major problem. People want to lose fat, but they keep track of their progress using a metric that inherently cannot distinguish between desirable (i.e., lean) and undesirable (i.e., fat) mass. As a consequence, they’re setting themselves up for failure. They may indeed lose weight but this weight loss may not be accompanied by the desired improvements in physique and health .
The flawed metric of weight leads to such terrible TV shows as “The biggest loser,” which promote awful dieting advice. If you make it your goal to lose as much weight as possible in as little time as possible, you’ll inevitably burn off a lot of muscle mass. And that’s terribly sad, in my mind. The one thing every obese person has going for them is that they have a substantial amount of muscle under their layer of fat, from all the extra weight they’re carrying around all the time. Strip away the fat and keep the muscle, and you’ll have a strong, healthy person. But burn the muscle alongside the fat, and you’ll end up with a skinny-fat person. This person would now have to embark on a process of regaining (lean) mass, which will be psychologically terrifying and will usually not happen.
At this point, I should probably demonstrate that there is indeed a substantive difference between losing fat and losing weight. Enter one of my favorite pictures on the internet. (Warning: mildly NSFW, woman in underwear.) It shows how the same person can look visibly slimmer and more toned after gaining a substantial amount of weight, if that weight comes in the form of lean mass not fat. The reason for this dramatic difference is that muscle is substantially more dense than fat. You need a lot of muscle to fill a decent volume of space, and that muscle is going to weigh.
If weight is a bad metric for fat-loss progress, how can you know whether you’re on track with your diet? Let’s first go over all the other commonly used methods that also don’t work:
Assessing body-fat bia bio-electrical impedance analysis (BIA). Many scales these days have BIA built in as well. When you step on the scales, they show you not only how much you weigh but also what your body-fat percentage is. Only that this number can be quite inaccurate. BIA results depend on numerous factors, not the least of which is your hydration status which may change daily. You can find a more detailed discussion of this issue here. The one-sentence summary is that you shouldn’t track your fat-loss progress with a BIA device, and in particular not with regular scales that have BIA built in.
Looking at your abs in the mirror. This doesn’t work because it’s easy to trick yourself into thinking you’re doing great when actually you’re not. Even more likely is the opposite, though: You may think nothing much is happening when actually you have changed a lot. Fundamentally, this method is not sufficiently quantitative. You can never really be sure whether things have actually changed and by how much.
Pinching your belly fat. This method is also surprisingly unreliable, in particular when you’re getting lean. As you’re leaning out, your skin gets looser and you can pinch from a larger area, seemingly being able to pinch more at a time. This effect is particularly pronounced when you’re pinching from top and bottom. Try pinching your belly fat right underneath your navel, either from top and bottom or from left and right. Do you see the difference? Also, sometimes we can have a substantial layer of fat but can’t pinch it. For example, try to pinch the fat on top of your thigh. Can you do that? Chances are, if you can you’re pretty lean. For most people, the fat on the thighs doesn’t really separate from the muscles, and as they lean out it separates more easily.
One method that works reasonably well is dual-energy X-ray absorptiometry (DEXA), basically a low-dose X ray. It’s the same method that is used to measure bone density. While it’s a great way to keep track of your body composition over time, it’s also cumbersome, expensive, requires professional equipment, and exposes your body to unnecessary X rays. Not something you want to do once a week .
The other method that works is super cheap, easy, and reliable: get a $5 tape measure and measure your waist circumference (just at the height of the belly button). That measurement should go down over time if you’re progressing successfully. How do you know you’re not losing lean mass at the same time? Your strength should stay the same or go up. If you’re increasingly lifting heavier weights while your waistline is slimming, you can be pretty certain that everything is on track. And if you happen to keep track of your weight at the same time, you may notice that your weight is going up even as your waist circumference is going down.
So in the future, don’t plan to lose weight, plan to lose fat. And lose that fat while keeping as much of your weight as you can, maybe even increase your weight by a few pounds!
 As incredulous as it may sound, there are indeed some people who are on a diet to gain weight. There are entire online communities dedicated to gaining weight.
 It’s important to realize that people can weigh little and nevertheless have poor metabolic health. See e.g. this article.
 It probably also isn't sensitive enough to measure changes week by week. But doing a DEXA scan once a year might not be a bad idea.