Researchers recently conducted an 8 week study on resistant trained subjects to answer that question and found some interesting results:
- Testosterone dropped by 20.7%
- IGF-1 dropped by 13%
- Insulin dropped by 36.4%
- T3 dropped by 10.7%
Let’s look at each of these and see just how much the matter:
Q1. The Drop in Testosterone. The first question that would come to my mind is if that drop in testosterone of 21% would be noticed by most men? After all, a healthy young male will experience a drop of at least 30% in his testosterone levels from early morning to late afternoon. My guess is that for the typical younger male, the 20% drop from intermittent fasting would not make much of a difference in how he feels.. If you have nice, high youthful testosterone levels, a 20% drop is not that much.
The case where this may be more important is in men that are already low in testosterone. I am pretty sure that I was low testosterone all my adult life, and I can tell you that fasting of any form was miserable for me. My testosterone was probably in the low 300’s and 20% lower from the low 300’s is pretty ugly.
Q2. The Drop in Hypertrophy Hormones. The next question that would pop into my mind is if I would lose muscle mass. Traditional muscle building theory states that testosterone, IGF-1 and insulin are the three most important hormones for hypertrophy. Wouldn’t a sag in any one of these make it hard to put on mass much less a drop in all three?
Surprisingly, the answer to this question seems to be ‘no.’ For reasons poorly understood, intermittent fasting (with weight / resistance / strength) training is actually a great way to lose fat and preserve muscle according to this study. The authors noted the following for example:
“After 8 weeks, the 2 Way ANOVA (Time * Diet interaction) showed a decrease in fat mass in TRF [time restricted feed or intermittent fasting] compared to ND [normal diet] (p = 0.0448), while fat-free mass, muscle area of the arm and thigh, and maximal strength were maintained in both groups.” 
Men already low in T3 or IGF-1 may struggle with intermittent fasting however. Testing beforehand could be prudent and can be done using these labs (here in the U.S.): Testosterone Labs.
ANTI-AGING COMMMENTS: I would add that all of the drops in the above hormones are generally associated with anti-aging. In particular, lower IGF-1 and insulin levels are all core markers for improved healthspan and potentially lifespan. Lowered T3 has also been informally associated with the same benefits. I would also add that too much testosterone also probably has negative anti-aging properties, due to overactivation of the mTOR pathway. Reasonable doses of testosterone likely improve mortality: Testosterone and Aging.
CONCLUSION: Intermittent fasting using an 8 hour eating window helps to lose weight without losing muscle, even though the traditional muscle building hormones are significantly lowered.. This is very important, because normally, when one loses fat, muscle is also lost as well. The accompanying loss of muscle can make weight maintenance more difficult on an ongoing basis. Intermittent fasting aso results in powerful anti-aging benefits.The wild card in all of this is if the lowered testosterone will negatively impact some men, and, hopefully, more study work will be done in the future on this subject.
WHAT KIND OF INTERMITTENT FASTING? As an fyi, the scientiic name for intermittent fasting is time restricted feeding. This study looked solely at the 16/8 form, i.e. only eating during a certain daily window of 8 hours. Other forms of intermittent fasting include 5/2, where you eat normally 5 days per week and then drop calories very low on the weekend, and lowering calories every other day. It is likely in my opinion that any of these methods that drops caories significantly will also lower testosterone as well.
1) Journal of Translational Medicine, 2016, 14:290, “Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males”