Testosterone production starts with the hypothalamus…
The hypothalamus is the master gland in the brain that is responsible for the production of many of the body’s essential hormones, which also regulates temperature, thirst, hunger, libido and moods.
The hypothalamus needs to be stimulated in order for it to release hormones that will increase testosterone.
Leptin and insulin are the two hormones responsible for the stimulation of testosterone production in the hypothalamus.
Through the hypothalamic–pituitary–gonadal axis, testosterone is increased. As testosterone levels go up, the hypothalamus inhibits the release of GnRH. That’s part of the negative feedback loop. As testosterone levels go up, leptin levels drop (4, 5, 6).
Insulin decreases testosterone, but significantly increases luteinizing hormone (LH), as well as inhibits the concentration of SHBG (7).
Insulin promotes GnRH secretion by the hypothalamus and stimulates gonadotropin secretion from the pituitary gland, and testosterone secretion from the Leydig cells (8).
And insulin is elevated after eating a carbohydrate rich meal.
So how does fasting or non fasting impact this process?
But it isn’t fasting alone that lowers gonadotropin release, but also an energy deficit. If you start to run out of fuel during the fast, your T will go down. But eating a large meal before the fast, will ensure LH secretion stays normal during the fast (11).
Refeeding however increases testosterone after fasting, but is meal size dependent. This study was done on monkeys however, but the more food they ate after the fast (they fasted 24 hours), the more their LH increased. So a pretty large meal after the fast resulted in a huge LH overshoot above baseline. (12)
I think that a shorter fast however (12-16 hours) would not have the same effect, but rather minor.
In this study, men skipped a meal in the evening and their testosterone was significantly lower the following morning than those who didn’t skip the evening meal. But after the refeed in the morning, their testosterone increased back to normal – didn’t overshoot though. Skipping the meal put them in a caloric deficit and their testosterone decreased, so it might not have been entirely from the fast alone.
Positive effect of fasting: Quote from the study: “Short-term fasting increased the GnRH-elicited LH response by 67% in the non-obese group (LH incremental areas 2147 +/- 304 vs 3581 +/- 256, p less than 0.01), and the corresponding testosterone response by 180%.”
So short term fasting increased the pituitaries sensitivity to GnRH and resulted in a 67% greater LH release for the same amount of GnRH. Short term fasting also increased the testes’ sensitivity to LH and resulted in an 180% greater response to the LH.
I have to add however that fasting does increases testosterone urinary excretion quite significantly (13).
So fasting and being in a caloric deficit both impact testosterone negatively independently.
Eating lowers testosterone.
One of the reasons testosterone levels are lower after a meal, is because there is a greater uptake of testosterone in the androgen receptors (14).
Because insulin increases free testosterone by lowering SHBG, there will then be more anabolic reactions as a result.
Eating a big meal can suppress testosterone levels for over 8 hours.
- Lowers testosterone during the fast
- Increases urinary testosterone excretion
- Greater gonadotropin sensitivity
- Greater increase in cortisol when training fasted
- Lowers SHBG
- Constant GnRH signalling for more testosterone production
- Constant androgen receptor interaction
- Less urinary testosterone excretion
- Little to no cortisol response if training in a fed state
- Fasting increases the sensitivity and responsiveness to gonadotropins as well as increasing androgen receptor sensitivity to androgens itself
- Fasting improves leptin and insulin sensitivity
- Eating a huge meal post-workout will result in a huge increase in insulin, leptin and LH
- Which could skyrocket testosterone production as well as free testosterone
- Super sensitive androgen receptors soak up all the abnormally high free testosterone for crazy anabolism.
- Eating a small calorie surplus for the day, will ensure testosterone production stays optimal
- Just make sure the post-workout meal consists of high GI, low fiber carbohydrates with lots of protein
- For the last meal of the day, before the fast starts, eat moderate to low GI carbohydrates with higher fiber content, to minimize the testosterone excretion due to the fast
But keep in mind, basal testosterone levels will be the same whether you do intermittent fasting or not. It’s about consuming adequate calories (maintenance or a small surplus, with proper macro-nutrients, with >100% of your micro-nutrients) for the day, and the timing thereof is not as important.
Your body can be just as insulin and leptin sensitive whether you fast or not. It’s about eating healthy, being lean (low bodyfat) and being active.