The role of estrogen in weight gain in men is highlighted in a recent study which shows that adding an aromatase inhibitor to a weight-loss intervention increased weight loss by a factor of almost three.

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The important role of estrogen in fat gain is underappreciated and under-investigated. Hopped beer, for instance, is a highly estrogenic substance.

Estrogen and Fat Loss

A study from 2020 has shown that the effectiveness of a weight loss regimen for obese men can be dramatically improved with the addition of an aromatase inhibitor.

The study began from the premise that obese men show an increased expression (i.e. production) of the aromatase enzyme, which is responsible for converting androgens (i.e. male hormones, including testosterone) into estrogen. As a result, the prevalence of hypogonadism – dangerously low testosterone – in obese men ranges from 29.3% to 78.8%. The researchers note that administration of exogenous testosterone to obese men is therefore likely to be ineffective, because the testosterone will simply be converted into estrogen by the excess aromatase; in fact, it may even be counterproductive.

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They decided, therefore, to see whether an aromatase inhibitor would have positive effects instead, principally by improving the subjects’ hormonal profile.

Twenty-three obese men (BMI≥35 kg/m2), aged 35–65 years old, were randomized and selected either for weight loss (diet and exercise) with anastrozole (an aromatase inhibitor) at 1 mg daily or weight loss and a placebo . The experiment took place over a period of 6 months. 

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Mean serum Total Testosterone (A), Estradiol (B), FSH (C), and LH (D) levels at baseline, 3 and 6 months for placebo and weight loss (PBO+WL) and aromatase inhibitor and weight loss (AI+WL)

The results clearly show that the aromatase inhibitor significantly improved the hormonal profiles of the subjects who took it: testosterone, follicle stimulating hormone and lutenizing hormone all increased significantly, while estradiol decreased significantly.

But the most shocking results by far – because studies had already shown that aromatase inhibitors improve male hormonal profiles, including testosterone levels (for instance) – were that those who took the aromatase inhibitor lost 2.6x as much weight and 2.3x as much fat as those administered the placebo.

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Absolute change (adjusted for baseline value) of body composition parameters at 6 months. (A) Weight Change; (B) Total Fat Change; (C) Body Fat Change; (D) Total Lean Change; (E) Truncal Fat Change; (F) Appendicular Lean Change.


The study clearly suggests that obesity creates a vicious cycle, if you will, in which fat tissue generates more estrogen, which then makes it harder to lose weight / easier to gain weight. We’ve already discussed this in our article on xenoestrogens, industrial chemicals that mimic the effects of estrogen in the human body.

Strangely, the study’s authors state estrogen causes fat loss and that what the aromatase inhibitor does is increase estrogen sensitivity; obese people, they claim, are resistant to estrogen in the way that they are also usually resistant to insulin. Many would dispute that the aromatase inhibitor works in this way. 

However the aromatase inhibitor works its magic, there can be no disputing that these are stunning results, and clearly suggest that aromatase inhibitors could be a powerful tool in the fight against obesity.

Aromatase inhibitors have long been used in bodybuilding and strength competition to mitigate the side effects of taking steroids and exogenous testosterone, which will aromatise, producing unsightly symptoms such as gynecomastia (development of breast tissue). As the video above shows, however, the use of aromatase inhibitors like anastrazole comes with potentially serious side effects, especially when used in conjunction with steroids. In the weight-loss study, no adverse effects were reported.

Instead of using aromatase inhibiting drugs, you can turn to natural aromatase-inhibiting foods, herbs and substances, such cruciferous vegetables, thyme and vitamin D. Later this week, we will discuss some of these foods, herbs and substances in more detail.

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