Thank you for your reply. This mod has great potential, allowing players to change character physiques at a cost without using an editor. However, I have a suggestion to introduce a mechanism to control muscle growth, rather than only being able to reduce it by avoiding certain activities. I noticed that most fitness equipment is focused on muscle building, but there are too few options for keeping colonists at a standard physique with a value of 25. I suggest that endurance exercises, like running or cycling, could reduce both muscle and fat, while yoga and balance-focused equipment could help keep the physique consistently at a value of 25.

25 is not standard. 15~35 (or whatever your setting is) is. There is no need to be fixated on the number 25, as it's the range that matters.
I have tried to set up the value so that pawns that don't do strength workout would have to do excessive hard works to have their muscle mass go over 35, so they will most likely stay "Standard physique" if they don't work out.
I think losing muscle for doing cardio/balance workout doesn't make sense. You get toned when you run. But if you want to change how that works, you do you. You can change the muscle gain/lose of workout machines by following the below instruction.

1. Go to your mod folder where Rimbody is installed.
2. In rimbody mod folder, go to Mods/Get Rimped/Patches
3. Open Rimbody_GetRimped.xml
4. Find the machine you want to change
5. Change the <strength> and <cardio> value as you see fit. setting strength of Treadmill and ExcerciseBike to -0.5 should do what you want.

Unfortunately, running strength factor is hard coded in the dll, so it can't be adjusted. But I will add a setting value for hauling factor when I have the time so that you can set up to make hauling don't give any muscle growth at all.

hi amazing mod, although i find the fat calculation weird. from what ive seen pawns get fat if they sit around or in bed for long periods of time, but that just makes every psycaster fat for example, which is kinda weird? i mean irl movement doesnt burn that many calories, you get fat irl mainly by eating too much (calorie surplus). i have an idea which could maybe work better if you had time to implement it and if you would even want to consider it: all foods could have certain amount of calories (the categorization could be simple smth like meat from small creatures, birds etc would have less calories than from a cow lets say, and vegetables would have much less) and if pawns go above the surplus they get certain amount of mass (fat + muscle), based on how much they worked out that day (you already have the yellow bar implemented) would influence how much of fat and how much muscle they get specifically from the mass they got from the surplus. if they are below maintenance calorie intake they lose fat and muscle (more fat than muscle). "food" bar in the needs section would be more like a if they are full or not more than a calorie thing. anyway the mod is already amazing at it is so thanks for the work

The current system works more or less as you've described, but with a few extra steps. Let me explain the logic behind the current system.
The game already has something like calories, referred to as "nutrition." When a pawn eats food, the food need (food bar) increases based on the nutrition value.

-Fat gain is based on this food need, more specifically, integral of the food need raised by the intake of nutrition.
-Fat loss is determined by other factors such as your current body fat, age, current activities and other variables.

Combining these two, you essentially get the system you described. You gain calorie from eating food and the surplus value compared to the maintainance makes you fat (your food need rise based on the nutrition value of food, and you gain fat when the integral of this food need goes above fat loss)

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I think the key point of your suggestion—having a 'calorie' value assigned to all food, distinct from 'nutrition'—is to make it so that meals with vegetables and meals with meat (for example) will have different effects on fat gain, even though both satisfy a pawn’s food need completely.

From an implementation perspective, this would cause significant compatibility issues if done thoroughly, and would have little to no impact if done broadly, while also putting a strain on performance. It would also require players to micromanage the ingredients they use to prepare meals.

From a realism standpoint, in real life, people get hungry when their blood sugar levels drop, and blood sugar is definitely related to calorie intake, which in turn affects fat gain. So, having a 'calorie' value related to fat gain that isn’t directly linked to food needs (how full they are) seems unrealistic to me.

Also, I believe people gain weight in real life if they sit around or lay in bed a long period of time as long as they have the same food intake. The difference is that real people usually require less food when they are inactive, while Rimworld doesn’t account for this in its food needs calculations.

I hope I didn’t overwhelm you with this wall of text! The mechanisms behind fat and muscle gain and loss are something I’ve thought about quite a lot, so I wanted to take the opportunity to explain the logic behind the current implementation. I really appreciate your insight and the ideas you’ve shared. I still wonder from time to time how the system could be improved, so I enjoy hearing various perspectives. Thanks again!

thanks for the reply, now i understand how you implemented it (or i at least think so). the calculation seems really thought out and the work i appriecieate the work you have put in. what ive meant though with my original comment was that i have a phytokin (from ve series) that is a psycaster and sits in the sun and gets fat very easily if i let her with a normal same diet as others. the way the phytokin work is their food need is frozen if they sit outside when its sunny and i have her meditating outside all the time which is convenient, but she gets fat from it xDDDD i understand if you dont want to make an exception to such a niche thing, either way thanks for responding in such a detailed manner

I see. Yes, races that freeze hunger via hediffs or forceful harmony patch have been one of the main source of light incompatibility. I thought about taking into account the hunger rate multiplier when calculating the fat gain. However, since GetHungerRateFactor is quite costly (it loops through all the hediffs a pawn has), I didn't want to sacrifice the performance. Thanks for bringing them to my attention though. I will try to think of a way to integrate them better.