The concept of weight in medicine and especially in the diet and weight loss industries is very confusing. So much attention is given to the number on the scale and so little to the meaning of that data point in metabolism and health.
Shifting the focus away from weight and to changes in daily routine around food and activity is much more effective for long-term change. If all importance is placed on the number on the scale, success is marked solely by continued downward changes. Any leeway based fluid shifts, metabolic changes and the many other things that affect weight is nonexistent: it is simply a failure. However, if success relates to consistent lifestyle changes, which also are a better marker of health, the person can embrace the positive, and weight changes will follow as one of several key markers.
There are three ways to understand weight as a valuable source of data: the current weight, the local weight range and the set point. Each reflects very different information of varying usefulness. Understanding the nuances of body weight also makes clear the limited value of these data for health.
It's most clear to start with set point, the most longitudinal information, and proceed to the more specific. The set point is a wide range of weight, typically about 15% of total body weight, that anyone can shift within quite naturally. The body is comfortable and not in danger anywhere in this range. Any pressure to go above or below this range leads to a strong metabolic response to attempt to stay within this range. The brain and hormonal system has determined that this range is ideal for health and will therefore protect the range for survival. If enough pressure through starvation or overeating persists, the range can shift down or up over a period of months to years. Then the new range becomes the norm.
The local weight range is a variation of about 2-5 lbs that the body can vary day to day. This weight change is almost completely due to fluid shifts from retained water or dehydration. Fluid shifts can be significant. One salty meal may increase weight the next day by up to 5 lbs. Monitoring weight too often simply reflects these fluid shifts. Body mass changes rarely constitute more than a pound per week and typically much less. Very fast weight loss on diets is almost exclusively water loss.
Any specific data point of weight has very little medical value. This number will rest in the current local range and will be up or down based on the current fluid state of one's body.
Weight data only has value longitudinally. This information over a period of weeks to months will clarify the general set point and range for someone and further history can clarify how long it has been set. Recent eating history and weight change can give a clinician an idea of where the person's weight lies in that range. Longer term history will dictate a plan for lifestyle improvement and how health and then weight may change over time.
A true shift in the concept of weight loss needs to reflect the limited utility of weight data and take attention off of the number on the scale and instead to sustained lifestyle changes.