Exercising whilst underweight and/or under-fuelled.

What’s the harm?



When I was stuck in my punitive, compulsive exercise regime in the midst of my battle with Anorexia, I was convinced my workouts were doing ‘something’.

Losing fat, gaining muscle, maintaining or reducing weight…any/all of it, I didn’t really know but I thought exercise was one of the biggest players in my self-destructive mission.

In the years since, my perspective and understanding have seen a huge shift, as I’ve learnt just how counterintuitive exercise is when you’re body is under-fuelled and/or under-weight, and just how little influence exercise has in our body based make up.

We’re often told ‘more is better’, but this just isn’t true, especially if you’re working with an already struggling human based system. To reap any benefits of exercise, mental or physical, you have to appreciate the impact the multiple demands have on your body. It’s only by using up nutrients and breaking down muscle tissue that we are able to rebuild ourselves, and become stronger. Without the correct nutrition, frequent replenishment, ample rest, and appropriate stress management, exercise will not serve you in the way you may think (or be told).

Not only that, research suggests that overall, exercise has very little impact on your weight. Our bodies are wired by evolution to survive and reproduce. As a result, they fight hard to keep our daily energy intake and expenditure within a narrow, unique range, regardless of our lifestyle.

So, what does this mean?!

This means we don’t need to spend hours at the gym, we don’t need to push our bodies to extremes, and we don’t need to compulsively exercise. Doing so, means we’re often fighting against the natural, dynamic, and VERY complex internal system which has more of a say over any of this than you’ve probably been led to believe, and that your lifestyle choices ever will.

So what was happening to me (and now maybe to you) when I was exercising whilst underweight and under-fuelled?



  • Often, I became injured. Why? Because my body didn’t have the ability to withstand high levels of activity due to a reduction in muscle mass and strength. My joints were unsupported and weak, and exercising on top of this only made them weaker. 
  • I developed Osteoporosis, and was highly prone to stress fractures from even low impact exercise such as walking. Lots of the research and self-help information you’ll read about Osteoporosis tells you to engage in strength based exercise. This is true, and it’s something I do a lot of for that very reason. But this does NOT apply to everyone. (There is a huge lack of research and support for those who develop Osteoporosis – often at a young age – as a result of disordered eating). Exercising (in whatever form) whilst underweight will be counter productive to gaining greater bone density, and you will continue to put yourself at risk of either developing the disease in the first place, or making it worse by reducing your bone density further. It’s only through regaining weight in the first instance that will then allow you to reap the benefits of resistance focused exercise for improving bone density. 

  • Both my cardiovascular system and circulatory system (amongst others) were placed under extreme stress. My heart muscle had shrunk, and my altered blood chemicals (due to poor nutrition, and constant restriction) meant I had low blood pressure, altered heart rhythms, dizziness and I often felt faint. Your brilliant body is resilient, but it can’t do something on nothing. Looking back now, I really believe I was so lucky it didn’t just stop altogether.

A recent (highly enjoyable!) sugary doughnut beach trip!

  • I believed if I gained weight (and didn’t exercise), all I would gain is fat tissue. I feared this so much. In fact, perhaps it would have helped me to know that our body is wired to rebuild in a way that aligns with our genetically determined composition; which (of course!) includes rebuilding muscle tissue. We need this for everyday tasks, so it’s only right our body will aim to rebuild it, regardless of whether we exercise or not. Our bodies know what to do. They know what they need. Sometimes, our minds need to allow them to do it.

  • I was likely experiencing elements of RED-S (Relative Energy Deficiency in Sport). This can affect anyone (whether or not you identify yourself as engaging in ‘sport’ or not) and it can occur when someone isn’t eating enough to match their activity (‘activity’ includes fitness, but also the everyday functional activity we do in day to day life e.g commuting, unloading the dishwasher etc). Your body tries to preserve anything it can, and essentially begins to shut down. There are multiple signs and symptoms of it (and I could write another blog posts on just this..!..maybe I will actually?!?!) but I would recommend learning more about it through Train Brave; a campaign dedicated to raising awareness of of RED-S.
  • I experienced NO enjoyment from my exercise. How could I possibly enjoy it when all I could think about was my rumbling tummy, and my over-active thoughts? I didn’t allow time for rest, ignoring or perhaps not understanding that it takes approximately 48 hours for our muscles to recover, repair and replenish post exercise. Not only that, our bodies need adequate nutritional intake to support this process. Only by giving my body time, love, and fuel would I begin to enjoy my exercise. 

I know it’s easy to read this, and I know the hard part comes in taking action and making changes if/when we need to (I know…because I did this!) However, when it comes to exercising with a reduced weight, knowledge really is power. The facts are clear, our bodies don’t like it and they certainly don’t benefit from it.

Read and consume, read and consume, read and consume…repetition and re-wiring our thought patterns can have a huge impact on changing habits. 

Research suggests that psychological approaches are highly recommended to challenge these thought patterns and behaviours, in a bid to reduce exercising whilst under-weight (or ideally cease it completely until it is safer to do so). Understanding the health benefits of moderating exercise, while acknowledging the serious consequences of doing so at a low weight is a good place to start. 

This isn’t a straightforward ride. Allow yourself grace to take forward and backward steps towards change and try to surround yourself with positive influences (both online and in real life) . They can have a huge impact on recovery. They were EVERYTHING for me. 

People used to always say to me “Think of your body like a car, it can’t run on empty, you have to keep stopping to fill it with petrol”. My reply was mostly “F*ck OFF I’m a human not a car”…but actually it’s an alriiiiiiiiiiiiiight analogy y’know. 

Exercise brings us no joy, no benefits and no favours if we’re doing it in an under fuelled, under nourished and un-happy body. 

…and I guess my car DID break down ONE TIME when I left it too long without petrol…

By Leah

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