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There could be two reasons:

1. Testosterone release

As far as I understand, when one works out, it provokes a testosterone release that promotes the repair of microtears in the muscles involved in the workout. But testosterone also promotes a systemic anabolism, making virtually all tissues grow, including the intestines. Hell, even some bones, like the jaw, grow under the influence of testosterone!

2. Increased food intake

One key factor for muscle grow is the availability of protein in the organism. Bodybuilders often eat a lot more than the general population so they are sure all energy requirements for the workout are met, and all protein and aminoacid requirements for the muscle repair are also met. I guess that eating a lot could have the same effect on the intestines as a workout on a skeletal muscle, making them grow to handle more work.

So when my weight increases, is that in part due to the size of my guts ?

Edit: I recently read about "GH gut", which would be a side-effect of GH abuse. This question is not about it, nor is it about any other substance abuse.

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Forgive my ignorance, but even if it did cause your intestines to grow, what perceived benefit/risk is that? 9 times out of 10, when your weight increases it is either due to muscle mass or fat mass. These are the two most dynamically changing systems in your body. –  Berin Loritsch Mar 12 '12 at 17:58
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It may not be as dramatic pound-for-pound, but I think one of the major benefits of strength training is that all of the body's systems grow to support the additional strain being placed on the body, including internal organ support. If I remember correctly, this was a claim made in Body By Science and some of the old-time body builders (I'm thinking Joe Weider, but not sure... I can't back this up, hence a comment). –  Greg Mar 12 '12 at 18:52
    
You seem to answer your own question: "But testosterone also promotes a systemic anabolism, making virtually all tissues grow, including the intestines." –  user3085 Mar 12 '12 at 19:34
    
Berin, I'm in an off-season-like phase, gaining a lot of weight. For curiosity's sake, I'd like to be able to know how much real skeletal muscle I put on. That would be weight gain - fat (measurable) - guts (how much?) - other negligible stuff. Others may include bones, for instance. –  Gabriel Mar 13 '12 at 12:42
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1 Answer 1

As your weight increases, the weight of your small intestine increases too, about 1-2% per every pound of gained weight.

Digestive system is energetically expensive to maintain. Therefore, the growth of guts should be balanced by maximizing the capacity to extract nutrients and energy from the diet, and minimizing the energetic cost of maintaining longer guts. The more food, and the more difficult it is to digest, the longer the guts.

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1-2% of the body mass increase is due to the guts OR the guts enlargment may increase the body mass by 1-2% ? There's an order of magnitude between the former and the latter (: –  Gabriel Mar 13 '12 at 12:37
    
it's a complex relationship. 1-2% of the body mass increase is due to the guts. Enlarged small intestine and new microbial populations can contribute to more efficient absorption of food that might lead to further weight gain. –  Irene Mar 13 '12 at 17:19
    
That would fall in the case 2 then. Really negligible though. Not even visibly noticeable. –  Gabriel Mar 13 '12 at 18:08
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