The drugs work. That part is settled \u2014 people lose weight on semaglutide and tirzepatide that they could not lose any other way, and for a lot of them it is the first thing that has ever worked.<\/p>\r\n\r\n\r\n\r\n
The question nobody answers properly is what the weight is made of.<\/p>\r\n\r\n\r\n\r\n
Both of the big trials measured body composition in a subset of participants with DXA scans, which is the part of the results that does not make headlines.<\/p>\r\n\r\n\r\n\r\n
In the STEP 1<\/strong> substudy of semaglutide 2.4 mg, about 40% of the weight lost was lean soft tissue<\/strong> \u2014 95 people scanned.<\/p>\r\n\r\n\r\n\r\n In the SURMOUNT-1<\/strong> substudy of tirzepatide, it was about 26%<\/strong> \u2014 124 people scanned.<\/p>\r\n\r\n\r\n\r\n So somewhere between a quarter and two-fifths of what came off was not fat. On 30 lb of weight loss that is roughly 8 to 12 lb of lean tissue, depending on which drug and which person.<\/p>\r\n\r\n\r\n\r\n Those are small subsets of large trials, and the two numbers are not directly comparable \u2014 different populations, different durations, different amounts of total weight lost. But the size of the effect is not in dispute. It is large enough to matter.<\/p>\r\n\r\n\r\n\r\n This is where most articles on this subject go wrong, in both directions.<\/p>\r\n\r\n\r\n\r\n DXA does not measure muscle. It measures lean soft tissue<\/strong>, which is everything that is not fat and not bone: muscle, yes, but also water, glycogen, the blood volume that supports a larger body, and the organs themselves. All of those legitimately get smaller when a body gets smaller.<\/p>\r\n\r\n\r\n\r\n So the 40% figure is not 40% muscle. Some of it is water you were carrying. Some of it is glycogen that left with the carbohydrates. Some of it is the extra tissue that existed to service extra body mass and is not needed at the new size.<\/p>\r\n\r\n\r\n\r\n Some lean mass loss is normal and appropriate.<\/strong> A person who loses 50 lb and keeps every gram of lean tissue has done something strange, not something admirable.<\/p>\r\n\r\n\r\n\r\n The problem is not that the number is above zero. The problem is the proportion \u2014 and the proportion in those trials is higher than it needs to be, because nothing in the trials was designed to protect it.<\/p>\r\n\r\n\r\n\r\n Three things stack up, and they compound each other.<\/p>\r\n\r\n\r\n\r\n An energy deficit alone will do it.<\/strong> This is not special to GLP-1s. Any sizeable calorie deficit costs lean tissue unless something actively protects it. Bariatric surgery patients, people on very low calorie diets, anyone losing weight fast \u2014 same pattern.<\/p>\r\n\r\n\r\n\r\n The appetite suppression works on protein too.<\/strong> This is the part that is specific to these drugs, and it is underrated. The medication does not selectively reduce your appetite for the foods you were overeating. It reduces appetite, full stop. Protein is the most satiating macronutrient and usually the hardest to finish when you are full early \u2014 so protein intake tends to fall further, proportionally, than anything else. People on a GLP-1 frequently end up eating well under the protein a person not<\/em> losing weight would need, at exactly the moment they need more of it.<\/p>\r\n\r\n\r\n\r\n There is no signal to keep it.<\/strong> Muscle is expensive. A body that is short of energy and is never asked to produce force will shed it, because there is no reason not to. Nothing about taking a weekly injection tells your body that the muscle is still required.<\/p>\r\n\r\n\r\n\r\n That last one is the one you can do something about, and it is why the answer is not a different drug or a different diet. If you want to keep it, something has to ask for it. A training program built for a GLP-1 appetite<\/a> is a different thing from a normal training program, and that difference is the subject of the next post in this series.<\/p>\r\n\r\n\r\n\r\n Three reasons, in rough order of how much they should worry you.<\/p>\r\n\r\n\r\n\r\n Function.<\/strong> Lean mass is how you get off the floor, carry things, and stay upright when something unexpected happens. This matters more the older you are, and GLP-1s are widely prescribed to people in their fifties and sixties, where losing 10 lb of lean tissue is not cosmetic.<\/p>\r\n\r\n\r\n\r\n What you weigh afterwards is made of something.<\/strong> If you lose 30 lb with 10 lb of lean tissue in it, and later regain 15 lb, the regain is predominantly fat. Do that twice and the same number on the scale describes a substantially different body. This is the mechanism behind “I weigh what I did five years ago but I look worse”.<\/p>\r\n\r\n\r\n\r\n Resting metabolism.<\/strong> Lean tissue is metabolically active, so less of it means a lower daily energy expenditure, which makes maintenance harder at the very point you stop the drug. This effect is real but it is routinely overstated in fitness marketing \u2014 it is a meaningful contributor, not the whole story of weight regain.<\/p>\r\n\r\n\r\n\r\n It does not mean the drugs are bad, and it does not mean you should stop taking one. For many people the metabolic benefit of losing a large amount of fat dwarfs the cost of the lean tissue that came with it. That is a conversation for you and whoever prescribed it, and nothing on this page is medical advice.<\/p>\r\n\r\n\r\n\r\n It means the lean tissue is a variable you can influence, and that almost nobody is told so.<\/p>\r\n\r\n\r\n\r\n The next two posts are about doing that: what actually protects muscle on a GLP-1<\/a>, and how to tell whether it is working<\/a>\r\nwithout a DXA scanner.<\/p>\r\n\r\n\r\n\r\n“Lean mass” is not “muscle”<\/h2>\r\n\r\n\r\n\r\n
Why it happens<\/h2>\r\n\r\n\r\n\r\n
Why it matters past the mirror<\/h2>\r\n\r\n\r\n\r\n
What this does not mean<\/h2>\r\n\r\n\r\n\r\n