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Eating healthy food doesn't automatically mean your body is absorbing it. I was munching raw spinach for years thinking I was doing my then osteoporotic bones a favour — but only a fraction of that calcium was actually getting in. This episode is about understanding why, and what to do about it. We're diving into Bioavailability.
In This Episode of the Midlife Unfiltered Podcast
We Talk About
* What bioavailability actually means — and why it matters more as we age
* The spinach surprise — oxalates, raw vs cooked, and why only 5–10% of spinach’s calcium is absorbed
* What influences how well you absorb nutrients — food form, preparation, gut health & smart pairing
* The Triage Theory — how your body rations nutrients when stores are low (and why Future You could lose out)
* What changes in midlife — declining oestrogen, stomach acid shifts, and microbiome changes
* Key nutrients to watch — Vitamin D, Iron, B12 and a few others
* How to know if you’re getting enough — the clues to look for in blood tests and biomarkers.
So… What Is Bioavailability?
Simply put, bioavailability is how much of what you eat, drink, or supplement your body can actually absorb and use. Not just what you consume — what actually makes it into your cells and does something useful.
Three things influence it most:
* The form your food takes — the source (plant vs animal), and how it’s prepared
* Your gut environment — especially stomach acid levels, which change with age
* What you pair it with — some nutrients supercharge each other (think Vitamin D + Calcium), while others block each other (oxalates in spinach binding to calcium)
The Triage Theory
Proposed by scientist Bruce Ames, this is a theory that really hits. When your body is running low on micronutrients, it doesn’t share them equally — it prioritises survival today over your health in 20 years - for Future You.
Why We Midlife Women Need to Pay Closer Attention
Your 40s and 50s bring real shifts that affect how efficiently your body processes nutrients — even if your diet hasn’t changed.
Hormones:
As oestrogen declines, so does your body’s ability to convert and absorb key nutrients. Calcium is a good and important example. Lower oestrogen means less conversion and absorption + falling bone density = a double whammy in midlife and beyond.
Stomach Acid:
About 30% of postmenopausal women have sub-optimal stomach acid. You need that acidic environment to convert nutrients. Less acid = less conversion.
Microbiome Shifts:
The diversity of your gut bacteria tends to decline with age — and that affects how efficiently you absorb what you eat.
The B12 Example - B12 relies on stomach acid to convert properly. With lower acid and fewer animal-based sources in many diets, B12 is one to actively watch.
Nutrients Worth Paying Attention To
The BIG 3
Vitamin B12: essential for energy, bone, heart and brain health B12 is converted by stomach acids. From there our small intestine absorbs it. So if acid performance is lacking, so too will be our body’s ability to convert and use it. That coupled with the fact that B12 is harder to get in food sources any way it’s one to watch as we age.
Iron: Very broadly, about 20% of postmenopausal women are iron deficient — even without monthly blood loss. Maryann shares her own experience with iron deficiency causing anxiety and palpitations. Ask your doctor to test not just serum iron, but also ferritin (stored iron) and transferrin saturation (how well it’s transported).
Vitamin. D: is synthesised in our body by sunlight. Food sources that contain it are not so common with fatty fish (D3) leading the charge with high bioavailability. Mushrooms (UV exposed) for plant sources have a relatively good amount of D2 and slightly less bioavailablity than fatty fish.
Others to have on your radar
K2: Directs calcium to where it needs to go — bones and heart. Found in fermented foods. No official RDI has been set yet, but worth knowing about.
Iodine: Found in dairy, seaweed, seafood and iodised salt. Important for thyroid health. Check your country’s RDI (~150 μg/day is common).
Magnesium: Easy to fall short on, even though it’s found in seeds, nuts, legumes, and leafy greens. Around 320 mg/day is often recommended — but check what applies to your country.
💡 Quick Tip
Use the IOF Calcium Calculator (International Osteoporosis Foundation) to understand your calcium sources and how much you’re actually getting. It doesn’t specifically state bioavailability for each source but it does lean into it generally speaking. And Maryann’s advice? Variety is the go — focus on a wide range of sources to help meet your targets.
Where To From Here - Actions To Consider
* Educate yourself on important micronutrients, their sources and bioavailability. eg: Spinach (and other oxalate-rich greens) to improve calcium absorption cook it — or swap in higher-bioavailability options like dairy or Chinese mustard greens (yep, they’re right up their in bioavailablity per serve).
* Pair smartly — don’t just think about the micronutrients in isloation. Many are better absorbed when paired with others. eg: Vitamin D alongside calcium to significantly boost how much you absorb. Small habit, real difference.
* Get blood work done — specifically test Vitamin D, B12, Iron (serum iron, ferritin, and transferrin saturation).
* Diversify your diet — A variety of sources covers more bioavailability gaps.
* Stay curious about RDIs — many haven’t been updated in 20 years, and they vary by country. Check what applies to you and watch for updates.
Please Note:
Everything in this episode is information to help you ask better questions — not medical advice tailored to you. We are all different in how our body converts and absorbs nutrients. Please speak with your GP or a registered dietitian to understand what’s right for your specific situation.
Resources From This Episode
🎙️ Maryann Jacobsen — Certified Dietitian, founder of Midlife Strong on Substack. Subscribe to get her free Biomarker Guide: “The Essential Blood Tests Women Over 40 Need To Track and Optimise Their Health.”
📖 Maryann’s upcoming book — diving deeper into bioavailability and midlife nutrition. Watch this space!
🔗 IOF Calcium Calculator — International Osteoporosis Foundation — understand your calcium sources and calculate your daily intake.
Thank you for tuning in! We hope you picked up a few gems in our conversation.
And if you did, share it on!
Don’t forget to check out Maryann’s Midlife Strong AND stay tuned for when her book comes out next year. It dives deeper into this very topic. Yes!
See you next week!
Anita xx
This is a public episode. If you would like to discuss this with other subscribers or get access to bonus episodes, visit midlifeunfiltered.substack.com
