Underweight = being at a weight that is lower than the weight that your body would be if you did not suppress it with restrictive eating or exercise.
So, taken from my definition of what “underweight” means above, it stands to reason that a person can be underweight at any size. Yes, you can be underweight in a larger body just in the same way you can be underweight in a smaller body.
If your natural unsuppressed bodyweight is in the range of a BMI of 22, then you are underweight if you are BMI 19.
If your natural unsuppressed bodyweight is in the range of a BMI of 30, then you are underweight if you are BMI 26.
If you are underweight, you are likely in a state of malnutrition. Or, you are certainly at risk for becoming so.
Egregiously, not many people seem to be able to grasp this concept of what underweight actually means on an individual basis. Terrifyingly, many medical doctors frequently practice size discrimination by instructing people in lager bodies to artificially suppress their bodyweight via restriction (and/or exercise). Yes, this means that medical professionals actively promote ill health when they do not follow a HAES approach to evaluating their patients health.
The assumption that people in smaller bodies are healthy, and people in larger bodies are not is founded on weight bias. This means that many people in larger bodies are dismissed when they go to their doctors with health concerns and told to go away and lose weight — something that may actually increase their risk of ill health rather than decrease it. Likewise, people who are in smaller bodies and underweight are often told that their weight is fine, and are more likely to be given medical treatment to “treat” a symptom that is actually a symptom of malnutrition.
For example, many underweight people in all body sizes are given thyroid medication to treat an out of whack thyroid, when the reason that the thyroid is out of whack is because it is under stress from malnutrition. No medication required! Nutritional rehabilitation required!
You can be underweight at any size. You can have anorexia at any size. You can be in a state of malnutrition at any size.