Last week, I wrote about RED-S and what it is. If you missed that blog, click here. As I mentioned, RED-S was introduced in 2014 to replaced the Female Athlete Triad (FAT). Today, I’m going to talk about FAT and how it differs from RED-S.

The Female Athlete Triad was originally introduced in 1992 after seminal work in the 1980s by Barbara Drinkwater showing reduced bone mineral contents in athletes with missing periods and improved bone mineral density when periods were regained. When FAT was first introduced, it involved three components: disordered eating, hypothalamic amenorrhea, and osteoporosis. It was later updated to show that each component compromises a spectrum of interrelated conditions (see diagram below). Another change with this update was that disordered eating was replaced with energy availability (read my blog last week to learn for more about energy availability). This is because low energy availability is not always the result of disordered eating. Sometimes undereating can occur inadvertently as athletes are often unaware of how many calories they require.

The other component of FAT includes menstrual function, which lies on a spectrum of eumenorrhea to subclinical menstrual disorders to functional hypothalamic amenorrhea. Eumenorrhea refers to regular periods occurring every 21-35 days. Functional hypothalamic amenorrhea means the absence of a monthly period. It can be either primary, which means no period by 15 years of age, or secondary, which means the absence of 3 consecutive cycles. Menstrual dysfunction also refers to oligomenorrhea, which is cycle lengths greater than 45 days, anovulatory cycles, which is absence of ovulation, or luteal phase dysfunction, which is shortened luteal phase. All of these can be the result of under-fuelling, or low energy availability.

The final component of FAT, bone health, lies on a spectrum of optimal bone health to low bone mineral density to osteoporosis. This means that an athlete has weak bones, and greatly increases the risk of bone fractures. The weakened bones can be a result of low estrogen levels caused by menstrual dysfunction. This is because estrogen helps to build strong bones. However, under-fuelling, independent of estrogen levels, can have negative outcomes on bone health.

Relative-Energy Deficiency in Sport (RED-S) was introduced to replace FAT as a more encompassing term to highlight negative outcomes of low energy availability beyond the outcomes on bone health and menstrual function. Furthermore, low energy availability impacts not only females, but also males, making RED-S a more inclusive term. Next week, I will be highlighting RED-S in male athletes. Sign up below to get it right to your mailbox.

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Categories: Megan Kuikman

Megan Kuikman

Hello! I’m Megan Kuikman. I’m a Registered Dietitian with specialized training in sports nutrition. My goal is to help athletes and active individuals achieve a healthy attitude towards health, training, and food. I empower athletes to fuel properly for training in order to restore their health and enhance performance. You can get in touch with me at:


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