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GROWER
by Dr. Lynette Morgan
Q
In what form is magnesium naturally found in food such as sunflower seeds? There are so many
different types of magnesium supplements. What do plants naturally do with magnesium?
What is the bioavailability of the magnesium in food compared to supplements? Can you tell
the difference in foods that have magnesium and the same foods that don’t?
Thanks, Ralph
a T
his is a rather difficult question as the absorption of
magnesium from different magnesium-containing
compounds by humans is not only biochemically
complex but still under investigation as there are many
factors, apart from the type of magnesium supplement,
that will play a role in this. In plants, magnesium is
typically found as part of the chlorophyll molecule which
is why deep green leafy vegetables such as spinach
have high levels of magnesium. However, this magne-
sium is highly water-soluble and can be leached out
into water during cooking or processing, thus reducing
the magnesium content of the food. Magnesium is also
found in high levels in unrefined whole grains, seeds,
and nuts where it plays a role in carbohydrate storage
and energy release. In mung beans, for example, about
90 per cent of the total cytoplasmic magnesium concen-
tration is complexed with adenosine triphosphate (ATP).
Adenosine triphosphate is the main source of energy in
cells but must bind to the magnesium ion in order to be
biologically active, thus magnesium is typically present
in every cell type. In some plants, the concentration of
magnesium differs depending on soil levels — thus
the magnesium content of foods can vary slightly from
source to source. The highest sources of plant-based
magnesium are in seeds such as pumpkin kernels
(307 mg per quarter cup), sunflower seeds (129 mg per
quarter cup), and spinach (83 mg per half cup), with an
adult requiring 300-400 mg per day. Surprisingly, humans
often obtain some magnesium from water supplies,
along with calcium, which are common elements found
in hard water and many mineral or bottled waters.
With supplements there are no “different forms” of
magnesium as magnesium is a chemical element, but
there are different complexes to which the magnesium
may be part of or bounded to. For example, supple-
ments may be supplied as magnesium citrate, chloride,
sulphate, carbonate, gluconate, or oxide as amino acid
complexes and many others. Some of these magnesium
supplement forms are designed to be better tolerated by
the digestive system, thus improving uptake and reduc-
ing side effects, while others provide a more sustained
release of the magnesium over time and prevent the
occurrence of side effects. Any excess of magnesium
taken as supplements is usually just excreted by healthy
humans. Bioavailability is simply a term used to describe
how much of the magnesium is typically absorbed by
humans and how much is excreted by the urinary system;
some magnesium-containing compounds are easier
for the magnesium to be absorbed than others. Studies
have found that magnesium from food sources such as
almonds are just as bioavailable as from soluble magne-
sium supplements. However, while it is unlikely to obtain
excessive levels of magnesium from food sources, this
can occur with supplements, which in turn reduces the
uptake of calcium in the digestive system.
The only way to tell foods that contain magnesium
from those that don’t is to check the food labeling —
magnesium is often listed on packaging, particularly
in foods where magnesium has been added in either
during processing or as nutritional enhancement.
Kind regards, Dr. Lynette Morgan
LM Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University,
New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site
consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and
product development for manufacturers of hydroponic products. Lynette authored five hydroponic technical books.
Visit suntec.co.nz for more information.
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Maximum Yield