• Marie Otsuka

How do "good" vs "bad" fats affect men's fertility?


In 1967, America’s Sugar Research Foundation paid three Harvard researchers $6,500 (approximately $50,000 today) to ignore research that linked sugar to heart disease, and to push the blame onto fat instead. Over the next few decades, people were told that fat was bad for them, and that they should cut fat out of their diets as much as possible.

As time went by, the narrative changed to accommodate unsaturated fats, while still emphasizing the dangers of consuming trans fats and saturated fats. The idea was that unsaturated fats were “good fats” that we should consume more of, while trans fats and saturated fats were “bad fats” that we should steer clear of.

But it doesn’t stop there: now, new research by the University of South Australia and Flinders University shows that when it comes to men’s fertility, there’s no distinction between “good” vs “bad” fats. More specifically, researchers have found that a diet high in any type of fat negatively impacts testosterone production over as little as five hours.

Is unsaturated fat healthy?

Today, unsaturated fat is popularly considered to be a component of a healthy diet, including the Mediterranean diet. Unsaturated fat is also commonly described as “good” fat, as opposed to saturated fat or trans fat, which is deemed “bad fat”.

But is unsaturated fat healthy or beneficial in the context of fertility? According to this 2019 study, this might not be the case.

In the study, researchers asked overweight or obese fertile men to consume different types of meals to identify whether their diets affected the fundamental male sex hormone, testosterone. The different meals included:

- Meals of polyunsaturated fats (PUFA)

- Meals of monounsaturated fats (MUFA)

- Meals of refined carbohydrates (CHO)

- Meals of whey and egg albumin

- Mixed meals of PUFA and CHO, PUFA and egg albumin, and CHO and egg albumin

After analysing the results, they found that fat significantly reduced testosterone production to a similar degree, regardless of whether the fat was present in PUFA or MUFA forms, or ingested in a mixed meal of PUFA and CHO.

What is polyunsaturated fat? What is monounsaturated fat?

For the uninitiated, unsaturated fats are fats with molecules that contain less than the maximum amount of hydrogen. Polyunsaturated fat is typically found in salmon, vegetable oils, and some nuts and seeds, and monounsaturated fats are found in red meat, whole milk products, nuts, and certain high fat fruits (including olives and avocados).

With this new study showing that unsaturated fats reduce testosterone production, men who are trying to improve their fertility should cut back on foods rich in these fats, even if this might feel counter-intuitive.

What foods should men take to improve testosterone production?

In the study, researchers found that consuming albumen (the protein in egg whites) increases testosterone levels in men. For participants who consumed albumen together with saturated fats, the albumen also helped to “ameliorate” the effect of the bad fats on testosterone levels. Bearing this in mind, men trying to improve their fertility should add more egg whites to their everyday diet.

At the same time, these men should avoid the Keto diet, which is a low-carb, high-fat diet that puts your body into a metabolic state called ketosis. Those who abide by the Keto diet consume plenty of fat, and as we’ve seen in this study, that’s not a good idea for men who are trying to boost their fertility!


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