Dr. Mike Israetel’s recently made an appearance on The Jack Neil podcast, where he talked about the biological relationship between weight loss treatments like GLP-1s and hormone levels.
When discussing the impact of GLP-1 medications like semaglutide and tirzepatide on testosterone levels, Dr. Israetel explained that the relationship is practically guaranteed.
“As you cut your calories substantially and you generate a huge calorie sink, your testosterone levels will fall. As your body fat drops, your testosterone levels fall. As your fatigue from training that hard and dieting rise, your testosterone levels will fall. So, it’s three independent pathways of lower testosterone levels from one dieting process that lasts 12 to 16 weeks,” he stated.
The professor emphasized that this isn’t a minor effect but rather a convergence of three separate mechanisms all pushing testosterone downward simultaneously.
The caloric deficit, the reduction in body fat percentage, and the accumulated fatigue from training while under-eating all contribute to hormonal suppression.
Despite the concerning impact on testosterone, Dr. Israetel offered reassurance about muscle preservation.
“Even if your testosterone falls, generally it doesn’t mean you lose a lot of muscle mass. And if you’re still diligently training and you’re losing like a pound or a pound and a half a week instead of like three pounds a week on retatrutide every week, you probably will lose a very meaningless amount of muscle,” he explained.
His research review revealed something remarkable: “Within about a year and a half that people take these modern anorectic dr**s, simaglutide, tirzepatide, retatrutide, and all the other ones, they typically lose an amount of muscle just because they’re losing so much weight, some muscle, some fat. That is a little bit less muscle lost than the average person gains with about two to three days of 30 or 40 minutes at a time of resistance training for the same amount of time.”
For those concerned about maintaining muscle while using GLP-1s, Dr. Israetel outlined a clear strategy.
“For most people if they start on tirzepatide, get a good diet going high protein and they start lifting weights a few times a week, together they will lose pure body fat by the pounds and pounds and on net lose no muscle mass at all,” he said.
However, he acknowledged the practical challenges: “Your motivation from a pharmacological perspective will be lower for sure, especially towards the end of that process because your testosterone is down.”
Yet he identified a powerful counterbalance to this hormonal dip. He stated, “Because you look better every week, it does magical things to your motivation. When you look at your body and it’s changing weekly into something more and more like you want, dude, there’s like a freight train effect there.”
For those using testosterone replacement therapy alongside GLP-1s, the picture changes dramatically.
“If you have testosterone replacement therapy, it won’t fall at all. And then super magical stuff happens,” Dr. Israetel noted. This combination allows users to bypass the hormonal suppression that typically accompanies aggressive fat loss.
Dr. Israetel expressed enthusiasm about future pharma developments. He discussed retatrutide as the next generation of weight loss treatment, noting that it “boosts your metabolic rate” and “reduces your food intake, but it burns more calories on top, so you lose weight like way faster.”
He also mentioned that because it constantly liberates energy, “you feel mentally way more alert and awake and physically way more able as if you’re not dieting at all.”
