Although it may sound obvious, vitamins, minerals and other food nutrients can only provide health benefits if they are absorbed from your intestines and presented to your cells in a form they can use or store. The proportion of a nutrient (or drug) that enters your circulation unchanged, and remains free to produce an effect, is known as its bioavailability.
If a soluble substance is injected directly into the circulation it could have a bioavailability as high as 100%. If the same substance is taken by mouth, however, it’s likely to have a bioavailability that is significantly lower due to the fact that it must interact with stomach acid, digestive enzymes, other dietary ingredients and your intestinal lining cells before it can enter the circulation.
How absorption occurs
Once ingested, nutrients may enter the body through a variety of unregulated processes, such as passively diffusing into intestinal lining cells, following the movement of water to squeeze between intestinal cells, or via a process called cell drinking (pinocytosis) in which part of an intestinal lining cell encloses and engulfs a small droplet of fluid.
Some nutrients, especially minerals, are also absorbed by a regulated process called active transport, which varies depending on the amount of the nutrient is already present in your body and whether intakes are low or demand is high.
As a result, the proportion of an ingested nutrient that remains bioavailable varies from person to person, and changes over time – especially for nutrients such as iron and zinc, whose uptake is tightly regulated according to whether your body levels are low or replete.
The absorption of many minerals (eg chromium) is inefficient and highly variable, while for others (eg selenium) efficiency can be as high as 98% from some food sources (eg selenium-yeast).
Minerals can interfere with absorption of each other, too, by competing for the same uptake mechanisms. Iron interferes with zinc absorption, for example, while copper uptake is impaired by the presence of zinc, iron and molybdenum.
How stomach acid affects bioavailability
The absorption of many minerals depends on the presence of good levels of stomach acidity (pH) which dissolves mineral salts to release electrolytes such as calcium, iron, magnesium, zinc and phosphate which can be absorbed further down the gut within the small intestines.
The stomach lining becomes less active with age and reduced production of stomach acid (hypochlorhydria) or even a total lack of acid production (achlorhydria) can result from inflammation of the stomach lining (atrophic gastritis). Atrophic gastritis affects as many as 19% of people in their 50s, 24% in their 60s, 32% of those in their 70s and 40% of those aged 80 plus – an overall prevalence of 31.5% of those aged 60 or more.
This is why multivitamins aimed at people aged 50+ or 70+ have different, usually increased, levels of certain micronutrients.
|Centrum Advance 50+ is aimed at the over 50s.
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Lack of stomach acid can lead to a number of nutritional deficiencies, especially of folic acid, vitamin B12, carotenoids, iron, zinc and calcium. As many as 60% of people aged over 75 years are deficient in vitamin B12 – mostly due to atrophic gastritis – which can result in pernicious anaemia. One way round this is to use sublingual lozenges.
Taking antacids and acid suppressing drugs for indigestion will lower the level of acidity in your stomach and affect the solubility of minerals such as calcium, iron, magnesium, zinc and phosphate, reducing their uptake further down the gut.
How mineral solubility affects calcium bioavailability
The solubility of calcium salts was traditionally thought to affect how well they were absorbed. Yet salts with relatively good solubility (eg calcium citrate) are not necessarily better absorbed that salts with low solubility (eg calcium carbonate). Most calcium salts have fractional calcium absorption values similar that of milk, with the exception of calcium citrate malate, from which absorption is slightly higher. Each 300mg calcium obtained from supplements provides around the same amount of absorbable calcium as a 240 ml glass of milk.
|Calcium citrate malate is one of the most bioavailable calcium salts. Each tablet provdies 140mg calcium. Dose: one capsule three times a day or as directed.
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Bioavailability of plant versus dairy sources of calcium
The bioavailability of calcium in brassica vegetables is significantly higher than from dairy products – around 61% of the calcium found in broccoli, 49% in kale, and 53% in Bok choy is absorbable, for example, compared with only 32% of the calcium in milk or cheese. It’s possible that some plants may contain calcium absorption enhances that have not yet been identified.
Bioavailability can depend on specific absorption factors
The presence of vitamin D is needed for absorption of calcium and phosphorus from the small intestines. Vitamin D works by switching on cell pumps that extract calcium and phosphate from the gut contents and allows them to cross the intestinal lining cells to reach the circulation.
Similarly, a carrier protein called intrinsic factor is needed to absorb vitamin B12 in the small intestines. Intrinsic factor is made in the stomach by the same parietal cells that secrete hydrochloric acid, so when these cells are lost due to atrophic gastritis, the ability to absorb vitamin B12 is also compromised.
Production of intrinsic factor can also fall in people with autoimmune conditions such as type 1 diabetes or thyroid disease. As many as 60% of people aged over 75 years are deficient in vitamin B12 – mostly due to atrophic gastritis – which can result in pernicious anaemia. One way round this is to use sublingual lozenges.
Interestingly, absorption of vitamin B12 (attached to intrinsic factor) also requires calcium, and taking calcium supplements may improve vitamin B12 absorption in people taking the diabetes drug, metformin.
Taking sublingual vitamin B lozenges or an oral spray will by-pass this effect.
|BetterYou Pure Energy Vitamin B12 Boost Oral Spray provides methyl cobalamin in a tasty spray for absorbtion through the mouth lining. ALso provides chromium and green tea extract. Dose: 2 to 4 sprays.
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How dietary forms affect bioavailability
The chemical form in which minerals are presented to intestinal lining cells affects their uptake. For example, most dietary iron is in the form of inorganic ferrous iron (Fe2+) or ferric iron (Fe3+) which have different uptake mechanisms that are relative inefficient. Vitamin C (ascorbic acid) converts ferrous iron to ferric iron which is better absorbed due to its higher solubility at acid pH. Iron that is already bound to porphyrin (haem iron) is absorbed via another, specific haem receptor, which is two to three times more efficient, making it more bioavailable
How previous intakes can affect bioavailability
The absorption of some nutrients, especially metal ions, depends on your previous intakes and how well your tissues are saturated – this is an important protective mechanism to guard against absorbing toxic amounts. Iron, for example, is initially stored within intestinal lining cells bound to a protein called ferritin, and is only released for further absorption into the circulation, when needed. Once the ferritin within an intestinal lining cell is fully saturated with iron, it does not absorb any more iron from food within the gut. Iron can only leave the intestinal lining cells and enter the circulation by binding to a blood protein called transferrin. If circulating transferrin saturation is high, iron remains within the lining cells and is shed along with the intestinal cell after a life span of three days.
Males absorb an average of 6% total dietary iron, while women of child-bearing age absorb around 13% due to their lower body iron stores (associated with menstrual losses). As absorption is inversely related to body iron stores, those with iron deficiency absorb more. This mechanism ensures helps to prevent absorption of excess iron which can be harmful.
Similarly, movement of zinc depends on a protein, metallothionein, which ‘traps’ zinc within intestinal lining cells when body stores are high.
How fibre can affect bioavailability
Some types of fibre, especially bran and phytate (found in unleavened wheat bread) bind minerals so they remain unabsorbed. These fibres can reduce the absorption of iron by as much as 65%, as well as reducing the bioavailability of zinc, calcium and manganese. This problem does not occur when eating leavened (yeast-raised) bread, however, as yeast enzymes break down phytate so that mineral-binding does not occur.
If your diet includes unleavened breads (eg matzo, tortilla, chapatti, paratha, pitta) it’s best to leave at least four hours between eating these and taking your multivitamins.
If you are following a high-fibre diet, ensure you obtain enough calcium from milk and dairy products, broccoli, nuts, seeds and pulses.
How tea and coffee can affect bioavailability
Tea and coffee contain polyphenols (eg tannins, chlorogenic acid) that bind minerals to reduce their absorption. In fact, coffee can reduce iron absorption by 60% to 80% if drunk within an hour of a meal. These beverages also reduce the bioavailability of other minerals such as zinc, magnesium and calcium.
Similarly, free fatty acids in fats and oils can form insoluble soaps with calcium and magnesium to impair their absorption.
In general, wash down supplements (and drugs) with water or orange juice, and just a few bites of food unless otherwise instructed (eg fat soluble supplements such as co-enzyme Q10 and vitamin E are best taken with food). Avoid drinking tea or coffee within two hours of a mineral-containing supplement.
How grapefruit juice affects bioavailability
Grapefruit juice massively increases the absorption and bioavailability of some prescribed drugs and herbal medicines. This interaction was discovered by accident when researchers investigated how alcohol affected the absorption of blood pressure medications – they used grapefruit juice as a mixer to disguise the taste and found that greatly increased the absorption and blood levels of the test medications.
Grapefruit contains substances (eg naringenin, furanocoumarins) that block the production of an intestinal wall enzyme (CYP3A4) which normally inactivates many drugs before they are absorbed. Blocking this enzyme allows greater amounts of these drugs to enter the circulation. Taking one particular statin, lovastatin, with a glass of grapefruit juice was found to produce the same blood levels of the drug as when taking 12 tablets with water! This effect is irreversible and lasts for at least 24 hours, so that drinking grapefruit juice regularly, or eating fresh fruit segments, can potentially lead to symptoms of overdose.
The list of drugs known to be affected by grapefruit is lengthening all the time. Always check the information leaflet supplied with your medication.
How pepper affects bioavailability
Black and white peppercorns contain an alkaloid, piperine, which increases the bioavailability of some nutrients in foods by up to forty-fold. Piperine affects at least four different uptake mechanisms in the intestines, including the enzymes that convert insoluble ingredients to more soluble forms, their attachment to transporter proteins, and the effectiveness of pumps that move substances in or out of cells against concentration gradients.
In the case of turmeric, the presence of piperine can boost absorption of the active ingredients, curcumin, by as much as 2000%.
Eating black pepper also increases the absorption of numerous drugs, including some commonly used anticoagulants and medicines used to treat diabetes, epilepsy, infections and asthma. Having higher than expected levels of these drugs in the circulation greatly increases the risk of side effects.
How antibiotics can affect bioavailability
If you are taking long-term antibiotics (eg for acne) you will benefit from increasing your intake of vitamin K (found in cauliflower, broccoli, dark green leafy vegetables and fermented foods such as soybean natto). Antibiotics interfere with the action of vitamin K in the body, and also kill probiotic bacteria in the large intestine that produce vitamin K so that levels (needed for healthy blood clotting) are reduced. Tetracycline antibiotics also bind to calcium found in dairy products. This can decrease the absorption of both the antibiotic and calcium.
Taking a probiotic may help to improve bioavailability, as well as converting some food ingredients such as isoflavones and lignans to more active forms.
|Bio-Kult Advanced multistrain probiotic formula contains 14 live bacterial strains proven to survive stomach acidity to reach the large bowel.
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How gut health affects bioavailability
Optimal absorption of nutrients and medicines requires a healthy gut lining. Malabsorption can occur when the bowel wall is affected by conditions such as Crohn’s disease, ulcerative colitis, or coeliac disease. This can reduce uptake through active transport mechanisms, or increase passive diffusion by disrupting the integrity of the intestinal lining.
Illnesses that speed up the movement of food through the gut (eg gastroenteritis, irritable bowel syndrome), or taking laxatives, can reduce nutrient absorption by decreasing the time for nutrients to become absorbed. Excessive use of laxatives can deplete levels of many vitamins and minerals and can also lead to dehydration. In contrast, slowed movement of gut contents (eg due to constipation) may increase absorption due to prolonged contact between the nutrients and intestinal lining cells.