Most people think of calcium and phosphorus as the most
important dietary components for bone health. But magnesium also plays
an important role. It is involved in hundreds of biochemical reactions,
many of which help keep not only bones strong, but the heart rhythm
healthy and the nervous system functioning smoothly.
A study by ARS physiologist
Henry C. Lukaski and nutritionist Forrest H. Nielsen reveals important
findings on the effects of depleted body magnesium levels on energy
metabolism. Lukaski is assistant director of ARS's Grand Forks Human
Nutrition Research Center, Grand Forks, North Dakota. He and Nielsen,
with the center's clinical nutrition support staff, showed that inadequate
magnesium is associated with a need for increased oxygen during exercise.
They found that during moderate activity, those with low magnesium levels
in muscle are likely to use more energyand therefore to tire more
quicklythan those with adequate levels.
The study's first phase provided 10 postmenopausal women
with a controlled diet adequate in magnesium for 35 days. In the next
phase, a low-magnesium diet provided less than half the recommended
daily intake for 93 days. The last phase provided a diet adequate in
magnesium for 49 days. The volunteers were subjected to exercise tests
at the end of each dietary phase, along with biochemical and physiological
After consuming the low-magnesium diet, volunteers showed
a significant overall loss of magnesium. They had lowered muscle levels
of magnesium, and their red blood cells were at the low end of the normal
The data shows that during the low-magnesium-status phase,
the volunteers used more oxygen during physical activity, and their
heart rates increased by about 10 beats per minute. "When the volunteers
were low in magnesium, they needed more energy and more oxygen to do
low-level activities than when they were in adequate-magnesium status,"
The study was published in the May 2002 issue of the Journal
The body stores about half its magnesium inside the cells
of tissues and organs. The other half is combined with calcium and phosphorus
inside bones. A tiny amountjust 1 percentof the body's magnesium
circulates within the blood at a constant level.
These findings are consistent with other studies showing
that too little magnesium makes the body work harder. "The effects
are likely to occur in individuals with low magnesium, regardless of
whether the person is athletic or sedentary," says Lukaski. "That
means that athletes wouldn't be able to work or train as long as they
would if they had better magnesium levels. People need to eat adequate
magnesium to make sure their hearts and muscles are healthy enough to
meet the demands of daily living."
The Recommended Dietary Allowance (RDA) for magnesium,
based on the current Dietary Reference Intakes (DRIs), for women over
age 31 is 320 milligrams (mg) daily and for men over age 31 is 420 mg
daily. The DRIs are a set of categories of nutrient requirements by
age and/or gender groupings that are released as an authoritative, ongoing
series by the National Academy of Sciences. Starting in 1941, RDAs were
updated periodically as a separate group to reflect current scientific
knowledge. Now, RDAs appear as one of several categories that together
make up the more detailed DRIs. ARS provides key data for use by experts
who help establish the DRIs.
Another study at the Grand Forks center, headed by physician
Leslie M. Klevay, with biochemist David B. Milne, found an association
between ordinary diets low in magnesium and irregular heartbeats. Klevay
heads the center's Trace Elements and Cardiovascular Health Laboratory.
The study provided 22 postmenopausal women, aged 47 to 78, with a diet
of conventional foods containing either less than half or more than
the RDA for magnesium of 320 mg daily. All volunteers ate both controlled
dietseach for 81 daysthroughout the study, though they were
not told which diet was given first.
After each 81-day diet phase, magnesium concentrations
were measured through blood tests. The scientists also wanted to evaluate
the characteristics of electrical impulses that provide pumping instructions
to the heart. Electrocardiograms were taken at the end of each phase
to measure volunteers' heart rhythms.
When the women ate a diet containing just 40 percent of
the RDA for magnesium, they showed signs of magnesium depletion in their
blood serum, red blood cells, and urine. At the same time, they experienced
small increases in heartbeats that arise in abnormal areas within the
heart, which are called ectopic heartbeats. They indicate increased
"These changes indicate that 130 mg of magnesium
is too little and that the RDA may be correct," says Klevay. "People
who live in soft-water areas, who use diuretics, or who are predisposed
to magnesium loss or ectopic beats may require more dietary magnesium
than would others."
Perhaps not surprisingly, people who live in areas where
the water is hardand therefore contains more minerals, including
magnesiumhave been found to have a reduced risk for heart disease.
Magnesium is sometimes provided intravenously to people shortly after
they've had a heart attack, though at least one study showed no evidence
of a "survival benefit" from the practice.
Klevay's study was published in the March 2002 issue of
the American Journal of Clinical Nutrition.
Data from the nationwide food consumption survey, the
USDA-ARS Continuing Survey of Food Intakes by Individuals (CSFII 1994-96),
indicate that only about 24 percent of women between ages 40 and 69
were meeting the 1989 RDA for magnesium. The current RDA-DRI for magnesium
is 40 mg higher than the 1989 RDA for that age group.
In addition, the ARS Community Nutrition Research Group
(CNRG) reports data from its online Community Nutrition Map (CNMap)
indicating that only about 32 percent of the U.S. population met the
RDA-DRI for magnesium. The CNRG is one of six laboratories in the Beltsville
(Maryland) Human Nutrition Research Center. The mapping project combines
a collection of food-and-nutrition-consumption indicators (obtained
from several sources, including CSFII 1994-96) with Geographic Information
System software. This data provides a snapshot of the percentage of
people meeting the requirements for different nutrients. CNMap can be
accessed at www.barc.usda.gov/bhnrc/cnrg/cnmapfr.htm.By
Rosalie Marion Bliss,
Agricultural Research Service Information Staff.
This research is part of Human Nutrition, an ARS National
Program (#107) described on the World Wide Web at www.nps.ars.usda.gov.
C. Lukaski, Forrest
H. Nielsen, and Leslie
M. Klevay are with the USDA-ARS Grand
Forks Human Nutrition Research Center, 2420 2nd Ave., Grand Forks,
ND 58202; phone (701) 795-8429 [Lukaski], (701) 795-8455 [Nielsen],
fax (701) 795-8230.
"Lack Energy? Maybe It's Your Magnesium Level" was
published in the May
2004 issue of Agricultural Research magazine.