If you’re a runner, iron deficiency can quietly wreck your performance before you even realise what is going on. One week your runs feel fine, the next your pace drops, your legs feel heavy and recovery takes far longer than usual.

I’ve been recommended to take an iron supplement and opted for Thorne Advanced Iron Complex. Here’s my honest, unsponsored review.

What is iron deficiency and why does it matter for runners?

Iron deficiency is one of the most common reasons runners experience unexplained drops in performance. It often shows up before anything is clinically obvious as a sudden loss of endurance, slower paces at easy effort and heavier legs even on short runs.

Iron is essential for producing haemoglobin, which carries oxygen to working muscles. When levels are low, oxygen delivery becomes less efficient which directly affects stamina, recovery and training consistency.

For runners, this often means sessions that should feel easy suddenly feel harder, heart rate drifts higher than usual and recovery between runs takes longer.

It is particularly common in female runners, endurance athletes, those who don’t eat iron-rich foods such as meat, and anyone with higher training loads or reduced nutrient absorption.

Common signs of iron deficiency in runners

In active people iron deficiency is often performance based before it becomes clinical.

Unexplained drop in pace or endurance
Heavier legs during easy runs
Increased perceived effort at normal training intensity
Slower recovery between sessions
Shortness of breath earlier than usual
Persistent fatigue despite adequate rest
Reduced race or interval performance

These symptoms are often linked to reduced oxygen transport in the blood due to insufficient iron levels. Many runners mistake this for overtraining or lack of fitness rather than a nutritional issue.

Why iron supplements can be difficult to tolerate

While iron supplementation is commonly recommended for low iron levels, not all forms are well tolerated.

Traditional iron forms such as ferrous sulfate are known to cause digestive side effects in some people, including nausea, constipation, diarrhoea and dark stools. As a result, consistency can become an issue, which limits effectiveness over time.

This is where formulation quality becomes important, particularly for anyone who has previously struggled with standard iron supplements.

What makes Thorne Advanced Iron Complex different?

Thorne Advanced Iron Complex is designed with both absorption and tolerance in mind, which is particularly important for runners who need consistent support across training cycles.

It uses chelated iron (Ferrochel®), which is generally better absorbed and gentler on digestion compared to traditional forms. This makes it easier to maintain during heavy training blocks when gut sensitivity can already be an issue.

It also includes vitamin C, which helps enhance iron absorption, alongside the active forms of key B vitamins involved in red blood cell production:

Vitamin B6 (pyridoxal 5’-phosphate)
Vitamin B12 (methylcobalamin)
Folate (L-5-methyltetrahydrofolate)

These nutrients work together because iron alone is not enough to support healthy red blood cell formation. B vitamin deficiencies can also contribute to fatigue, low mood, memory issues and general cognitive sluggishness.

Key benefits of Thorne Advanced Iron Complex

This formula is designed as a comprehensive approach to iron support rather than a standalone iron supplement.

It provides:

36 mg of elemental iron per capsule (as Ferrochel® chelate)
Vitamin C to enhance iron uptake
Active forms of B6, B12 and folate for red blood cell support
Reduced likelihood of common iron supplement digestive side effects
Third-party testing for quality and contaminant safety

This makes it particularly relevant for people who need a more complete nutritional approach rather than iron alone.

Who is Thorne Advanced Iron Complex suitable for?

This supplement is commonly used by individuals who:

Need nutritional support for iron-deficiency anaemia
Experience low energy linked to low iron intake
Have difficulty tolerating standard iron supplements
Have increased iron needs due to life stage or health status

Because iron absorption varies significantly between individuals, a well-formulated supplement can make a meaningful difference in both consistency and results.

Where to buy Thorne Advanced Iron Complex in the UK?

You can easily order it, alongside the rest of Thorne’s supplement range on Healf. RRP: £20.49 for 60 capsules.

My personal review of Thorne Advanced Iron Complex

I’ve never been a big supplement person, but iron has been one of the few things I’ve actually needed to pay attention to.

I don’t eat meat so I’ve always had to be quite intentional about iron in my diet. I was never officially iron deficient, but I was constantly sitting at the lower end of the normal range. My doctor strongly recommended supplementing, especially as I run regularly and that combination can make it harder to keep iron levels stable, particularly for women.

Before Thorne, I tried a couple of different iron supplements and honestly never really stuck with them. One caused digestive discomfort that made me stop within days, and another was in gummy form which was easier to take but I never felt convinced I was actually getting much out of it.

When I came across Thorne Advanced Iron Complex, what stood out was how “proper” it felt. Not in a marketing way, but in a formulation way. The combination of chelated iron with vitamin C and active B vitamins just made sense and it felt like it was designed for absorption rather than just ticking an iron box.

I started taking it and actually stuck with it, which says a lot in itself. I take one capsule in the morning on an empty stomach and it has been surprisingly easy to keep consistent. No stomach discomfort, no nausea and none of the issues I had experienced with other iron supplements.

It also just fits into my routine without effort. One capsule, easy to swallow and done before I even think about breakfast.

I didn’t expect much beyond “hopefully my levels stay steady” but it has genuinely been the first iron supplement I haven’t had to overthink or quit halfway through. It just quietly does what it is supposed to do, which is exactly what I needed.