It’s not always how much you’re eating.
Most people think nutrition is just about “getting enough.”
But the reality is: what you absorb matters far more than what you eat on paper.
You can eat foods containing iron, B12 or vitamin D — and still be low in them.
Because nutrition isn’t just about quantity. It’s about source quality, bioavailability, digestion, lifestyle, gut health, and what nutrients compete with each other.
Two people can eat the “same” nutrient and absorb wildly different amounts.
This is something we cover regularly in our Nutrition Series at Unit 7 in Liverpool — because understanding it is often the missing piece for women who are training hard, eating well, and still not feeling or performing the way they want to.
The Big Concept: Bioavailability
Bioavailability = how easily your body can absorb and use a nutrient.
Not all vitamins and minerals are absorbed equally.
100mg of iron from spinach is not the same as 100mg of iron from steak. Your body treats them differently.
This is one of the biggest misunderstandings in modern nutrition — and it affects everything from energy and recovery to body composition and weight management.
Plant foods can absolutely be part of a healthy diet. But some nutrients are simply less concentrated, less absorbable, or require larger volumes to meet your requirements.
That doesn’t mean “bad.” It just means awareness matters.
Iron
To understand this better, let’s look at four important parts of the system.
What Iron Actually Does
Iron helps transport oxygen around the body, supports energy production, improves recovery, and maintains performance and endurance.
Low iron can look like: fatigue, poor recovery, dizziness, breathlessness, cold hands and feet, poor training output, and brain fog.
Women, runners, endurance athletes, vegetarians and people with heavy menstrual cycles are particularly at risk.
Heme vs Non-Heme Iron
There are two forms of iron.
Heme iron comes from animal sources — red meat, liver, shellfish, poultry. This is the easiest form for the body to absorb, with absorption rates of roughly 15–35%.
Non-heme iron comes from plant sources — spinach, lentils, beans, tofu, fortified cereals. Absorption rates here drop to roughly 2–20%, and are heavily influenced by fibre, phytates, oxalates, tea, coffee, and gut health.
So even if the food contains iron, your body may struggle to access much of it.
Spinach is a classic example. It’s often held up as an iron hero — but it contains oxalates, which actively reduce iron absorption. The nutrition label looks impressive. The usable amount is much lower.
How To Improve Iron Absorption
Pair iron with vitamin C — it dramatically improves non-heme iron absorption. Think lentils with peppers, spinach with citrus, or beans with tomatoes.
Avoid tea and coffee around iron-rich meals, as tannins reduce absorption.
And don’t blindly supplement — too much iron can be harmful. Get bloods checked if symptoms persist.
Vitamin C
What It Does
Vitamin C supports immune function, collagen production, skin health, wound healing, recovery, and iron absorption. Humans can’t produce it themselves — we have to get it through food.
Best Sources
High quality sources include citrus fruits, kiwi, strawberries, red peppers, broccoli and potatoes.
Interestingly, many vegetables contain more vitamin C than oranges. Red peppers are one of the highest sources gram-for-gram.
Quality Matters
Vitamin C is fragile. Heat, overcooking, long storage, and poor food quality all reduce it. Fresh steamed broccoli retains significantly more than the heavily boiled version.
Most people don’t need mega-dose vitamin C supplements. Your body excretes excess fairly quickly. Consistency with real food matters more than high doses.
Vitamin D
Why It Matters
Vitamin D is technically more like a hormone than a vitamin. It supports immune function, bone health, hormone health, mood, muscle function and recovery.
Low vitamin D is incredibly common in the UK — particularly through autumn, winter and early spring — due to limited sunlight exposure.
Food vs Sunlight
Vitamin D is difficult to obtain through food alone. Sources include oily fish, egg yolks, liver and fortified foods, but sunlight remains the major source. That’s why many people struggle despite “eating healthy.”
Absorb It Better
Vitamin D is fat-soluble, meaning it absorbs better when consumed alongside dietary fat. Taking your supplement with a meal containing healthy fats can meaningfully improve effectiveness.
Common mistakes include taking it inconsistently, taking it on an empty stomach, assuming summer stores last all winter, and never testing levels despite symptoms.
Low vitamin D can contribute to fatigue, low mood, weakened immunity, muscle aches and poorer recovery — all of which affect both fitness progress and how you feel day to day.
B12
What It Does
Vitamin B12 is essential for nervous system health, energy production, red blood cell formation and cognitive function. Deficiency can cause fatigue, numbness, brain fog, weakness and mood changes.
The Important Truth About B12
B12 is naturally found almost exclusively in animal products — meat, fish, eggs and dairy.
Plant foods do not naturally provide reliable active B12. This means vegans and some vegetarians are at significantly higher risk of deficiency.
Some products — nutritional yeast, fortified cereals, plant milks — claim to contain B12, but these are fortified, not naturally rich sources, and absorption varies massively.
This doesn’t mean plant-based diets aren’t possible. It simply means they require more planning, awareness, and often supplementation.
Absorption Problems
Even people eating animal products can become deficient, because B12 absorption depends on stomach acid, intrinsic factor production, gut health and digestive function — and these can all be compromised with age, digestive conditions or long-term antacid use.
The Bigger Picture
Nutrition is not “eat one superfood and become healthy.”
Health is built through consistency, overall diet quality, digestion, lifestyle, sleep, stress, movement and recovery.
You cannot out-supplement poor habits.
The goal isn’t obsession. It’s a mindset shift — from chasing quick fixes to genuinely understanding where nutrients come from, how well they absorb, and what your body realistically needs.
That kind of awareness builds body confidence in a way that no supplement label ever will.
Key Takeaways
Iron — animal iron absorbs far better than plant iron; pair plant sources with vitamin C; athletes and women are higher risk.
Vitamin C — supports immunity, recovery and iron absorption; fresh, minimally processed foods retain more; mega doses are usually unnecessary.
Vitamin D — common deficiency in the UK; sunlight is the main source; best absorbed with dietary fat.
B12 — naturally found mainly in animal products; vegans often require supplementation; absorption issues can occur even with a good diet.
Final Thought
Nutrition shouldn’t be fear-driven or tribal. It’s not “plants bad” or “meat bad.”
It’s understanding the strengths and limitations of different food sources — so you can build a diet that actually supports your body, your performance and your health long term.
Want to understand your own nutrition better? Our Nutrition Practices at Unit 7 in the heart of Liverpool is designed to cut through the noise and give you practical, evidence-based guidance they can actually use — whether you’re training for performance, managing your weight, or just trying to feel better day to day. Drop us a message to find out more or book a free consultation and we’ll get back to you.
