Dr Libby: The Nutritional Fallout From High-Intensity Exercise

In this special series, guest writer Dr Libby Weaver shares her health insights. This week, she discusses nutrition and exercise.

We often think of exercise as one of the best things we can do for our health. And in so many ways, it is. Movement supports our cardiovascular system, helps foster a healthy mood and outlook, improves metabolic and bone health and enhances mental clarity. It boosts our confidence, helps us sleep and can even be a form of meditation when approached mindfully.

Yet what we don’t talk about as often is what happens when the training gets intense. When we push our bodies day after day, week after week, without taking a moment to consider whether our nutrition and recovery are keeping pace with our effort. In a world that celebrates themes of more, harder, faster, it’s especially easy to fall into the trap of believing that if some exercise is good, more must be better. And yet, without the right kind of support, more can quietly become too much.

When we train intensely – whether that’s through long-distance running, HIIT, or even back-to-back group fitness classes – we’re asking more from our body. And in return, it asks more from us. More rest. More recovery. And critically, more nourishment. Not just to fuel the movement itself, but to support the complex biochemical processes that underpin performance, resilience and repair.

Because while movement has the power to elevate our health, it can also become a hidden stressor if our nutrient stores are already low, or if we’re not keeping up with the additional demands. Over time, we can begin to see the signs: fatigue that doesn’t resolve with rest, slower recovery, dips in mood or motivation, disrupted sleep, or that frustrating sense that you’re doing all the “right” things but still not feeling your best.

Iron loss

Iron is one of the first nutrients to feel the impact. When you train at a high intensity, your body not only needs more oxygen – it needs more iron to deliver that oxygen to your cells. You also lose iron through sweat, and in certain types of exercise – particularly running – there’s something called footstrike haemolysis, where the repeated impact of your feet hitting the ground breaks down red blood cells. Add in the monthly blood loss many women experience along with some other mechanisms through which iron loss increases with intense exercise, and it’s easy to see why iron deficiency is so common in those who train at a high intensity regularly. And you don’t need to be anaemic to feel it. Even less than ideal iron stores can leave you feeling flat, foggy, low in mood or unable to recover well between sessions. Iron is like oil for your engine. Without enough, things start to feel harder than they need to. When deficient, people have a tendency to say they feel “defeated”.

Cortisol spikes

And then there’s stress – something exercise can both ease and increase, depending on the type, intensity and also what else is going on in your life. When you move your body moderately, especially outdoors or in a way that brings you joy, it can help to regulate part of your nervous system. But when training becomes intense, frequent, or you’re not allowing enough recovery time, it can start to register as a stressor. Your body responds by producing more cortisol, your long-term stress hormone. Higher cortisol levels increase your need for key nutrients like magnesium, zinc, vitamin C and the B vitamins. These are all nutrients we burn through faster under stress, and if we don’t top them back up through food (or, in some cases, supplements), the result is often fatigue, poor sleep, mood changes or a plateau in progress.

Magnesium is a big player here too. It’s involved in hundreds of processes in the body, including muscle contraction, relaxation and recovery, energy production and nervous system regulation. You lose it through sweat and urine – especially during intense training – and the symptoms of low magnesium can be subtle at first: twitchy muscles, broken sleep, increased anxiety or feeling more emotionally reactive. It’s easy to ignore or push through for a while but the symptoms tend to become more and more confronting (in an effort to gain our attention, prompting us to address them) the longer the lack of magnesium persists.

Fuelling the body

Of course, micronutrients don’t work in isolation. We also need to zoom out and look at the bigger picture of how we’re fuelling ourselves. Protein often gets a lot of attention in the training world – and rightly so. It helps rebuild muscle after training, but it also supports immunity, tissue repair, neurotransmitter production and hormone creation. Without enough, recovery slows, strength gains stall, and you may start to feel rundown. Many people, particularly women, underestimate just how much protein their bodies need when they’re training hard.

Then there’s fat – another misunderstood but essential macronutrient. Whole food fats like olive oil, avocado, nuts, seeds, oily fish and fats from grass-fed animals help stabilise energy levels, support hormone production and reduce inflammation. These fats are also crucial for absorbing fat-soluble vitamins like A, D, E and K – each of which plays a role in recovery and overall wellbeing. I still see people trying to eat very low fat when they’re training regularly, and it’s something that can really compromise energy and hormone health over time.

All of this is to say: when you ask more of your body, it needs more from you in return. And that’s not a punishment – it’s an invitation to care more deeply for yourself. Movement is a beautiful, powerful tool for building strength, resilience and vitality – but only when it’s supported with the nourishment and rest your body needs to keep up. Your body is incredibly wise and responsive. It will rise to the challenge of your training if you give it the raw materials it needs to rebuild, repair and thrive. This means not just fuelling your workouts, but fuelling your life – your recovery, your sleep, your hormones, your emotional steadiness.

Dr Libby Weaver PhD is a nutritional biochemist, 13 times bestselling author and international keynote speaker. For more on supporting yourself inside and out visit Drlibby.com

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