Bioavailability - what it means for your health
Did you know that only 30% of calcium is absorbed from food by the average healthy adult? This is because a number of factors affect how much of each nutrient you absorb. Although our bodies have a pretty good idea of how much we need, there are a few simple strategies you can use to make this process a little more efficient.
WHAT IS BIOAVAILABILITY
The term bioavailability may sound complex, however it’s actually quite simple. As the name states, it’s the amount of ingested nutrient which has been absorbed and retained and is therefore available to our bodies. Although it makes sense to think that we would extract 100% of the nutrients from our food - this is not the case. The amount absorbed depends on a number of factors, which either enhance or hinder nutrient absorption.
FACTORS AFFECTING BIOAVAILABILITY
There are some factors we can control through our diet and others we can’t. Those that can’t be controlled are incurable digestive diseases such as coeliac disease, enzyme deficiencies, like lactose deficiency, and most importantly our body’s actual needs.
Factors we can control include:
o Competitive absorption- These nutrients ‘compete’ with one another to be absorbed when eaten around the same time. This is because some nutrients have positive or negative ‘charges’. These charges can repel each other (a bit like when two corresponding magnet poles are forced together).
o Enhancement of absorption- When eaten around the same time, some nutrients actually ‘enhance’ the absorption of the other.
o Components of food- Although food is mostly broken down before it enters the intestine, some food particles remain larger and more intact than others. These act as a physical barrier or ‘bulk’ in the intestine, which slows and prevents nutrient absorption.
WHAT ARE THE MAIN NUTRIENTS AFFECTED?
The absorption of calcium and iron is affected the most. Along with other functions, calcium is well known for its role in maintaining strong bones and teeth.
Calcium is found highest in dairy products (including milk, yoghurt and cheese), followed by vegetables (greens, spinach, broccoli, green beans).
Iron assists with the transportation of oxygen around our bodies. There are two types of iron in our diet - haem and non-haem. Haem iron comes from meats (including fish and poultry). Non-heme iron comes from plants and legumes (spinach, parsley, tofu and beans) and fortified breads and cereals.
There are also other plant compounds that are affected by cooking and processing methods too, which will be discussed in a later post. Things such as carotenoids in vegetables require fats for their proper absorption in the body as they are fat soluble.
THE ABSORPTION OF CALCIUM
About 30% of calcium is absorbed from our food, however in periods of growth and pregnancy, this can increase up to 75%!
Factors that lower calcium absorption include:
o Iron- Both iron and calcium compete for absorption because they share similar positive charges.
o Fibre- Forms a ‘bulk’ in the intestine, preventing calcium from being absorbed.
o Oxalates (naturally-occurring substances)- Forms strong bonds with calcium in the intestine, preventing it from being absorbed.
o Excess salt, protein and caffeine- When eaten in excess, these components have shown to increase the excretion of calcium.
THE ABSORPTION OF IRON
Haem iron is absorbed better than non haem iron (25% vs 10-15%).
Factors that increase the absorption of non heme iron include:
o Vitamin C- Whilst heme iron is directly absorbed, non heme has to undergo a further step. This step is achieved by vitamin C.
o Pairing with heme iron- Pairing non heme with heme iron sources increases its absorption.
Factors that lower the absorption of iron include:
o Phytates, phenolic compounds, oxalate- These components in food trap iron in the intestine, preventing it from being absorbed.
o Calcium- Like iron, calcium has a similar positive charge and therefore competes with iron to be absorbed.
WHAT STRATEGIES CAN PREVENT THESE INTERACTIONS?
o Separate sources of iron and calcium, especially if you’re taking these as supplements!
o Follow a diet high in fibre (25g/day females, 30g/day males), however avoid exceeding 35g/day.
o Choose leavened varieties of bread (contain yeast) over unleavened bread. The enzymes in yeast have shown to break down bonds formed between phytates and iron and phytates and calcium.
o Include a good source of vitamin C (unsweetened orange juice, capsicum, guava fruit, parsley, chilli) when consuming sources of non heme iron to assist with its absorption.
o Stick to recommended intakes of salt, protein and caffeine. Aim for 460-920mg/day of salt, roughly 0.7-1.0g/kg/day protein and no more than 400mg per day caffeine (about 4 cups of coffee).
SHOULD EVERYONE BE DOING THIS?
If consuming a well-balanced diet, healthy individuals are usually able to absorb enough nutrients from their food. It is however highly recommended that individuals following vegetarian or vegan diets apply these strategies, as iron and calcium intake is generally low. Those with malabsorption conditions like coeliac disease and older individuals would also benefit, as their ability to absorb nutrients is reduced.
Hi fellow food lovers! My name is Paris Owen and I am currently undertaking a Masters in Nutrition and Dietetics at University of Wollongong. I am also a university-qualified Nutritionist, graduating last year with a Bachelor of Science in Nutrition. I work with Body Bloom, a women’s fitness training group, providing specialised sports nutrition advice and meal plans. Bondi Farmers Markets is where you will find me on Saturdays making nutritious goodies with an incredible team at the Inside Out Nutritious Goods stall.
I like to take an evidence-based approach for nutritional recommendations to ensure the best health outcomes for each unique individual. I also like to approach health in a holistic way incorporating physical, mental and spiritual wellbeing. In the future, I will become an Accredited Practicing Dietitian, working in the public health system and eventually, in private practice. I am particularly interested in food allergies and intolerances and sports nutrition, however there are so many other areas I am yet to discover!
Meyer, B 2013, ‘Week 3’, lecture notes, SHS351, University of Wollongong, viewed 9 December 2014.
NHMRC 2014, Sodium, Nutrient Reference Values for Australia and New Zealand, viewed 9 December 2014, < https://www.nrv.gov.au/nutrients/sodium>.
NHMRC 2014, Protein, Nutrient Reference Values for Australia and New Zealand, viewed 9 December 2014, <https://www.nrv.gov.au/nutrients/protein>.
Hobar, C 2014, Osteoporosis and Calcium, eMedicineHealth, viewed 9 December 2014, <http://www.emedicinehealth.com/osteoporosis_and_calcium/page6_em.htm.