8 Agricultural Research/May 2004

Most peoplethink of calci-um and phos-phorus as themost importantdietary com-ponents forbone health.But magnesi-um also plays

an important role. It is involved in hun-dreds of biochemical reactions, many ofwhich help keep not only bones strong,but the heart rhythm healthy and thenervous system functioning smoothly.

A study by ARS physiologist HenryC. Lukaski and nutritionist Forrest H.Nielsen reveals important findings on theeffects of depleted body magnesiumlevels on energy metabolism. Lukaski isassistant director of ARS’s Grand ForksHuman Nutrition Research Center,Grand Forks, North Dakota. He andNielsen, with the center’s clinical nutri-tion support staff, showed that inade-quate magnesium is associated with aneed for increased oxygen duringexercise. They found that during mod-erate activity, those with low magnesiumlevels in muscle are likely to use moreenergy—and therefore to tire morequickly—than those with adequatelevels.

The study’s first phase provided 10postmenopausal women with a con-trolled diet adequate in magnesium for35 days. In the next phase, a low-mag-nesium diet provided less than half therecommended daily intake for 93 days.The last phase provided a diet adequatein magnesium for 49 days. The volun-teers were subjected to exercise tests atthe end of each dietary phase, along withbiochemical and physiological tests.

After consuming the low-magnesiumdiet, volunteers showed a significantoverall loss of magnesium. They hadlowered muscle levels of magnesium,and their red blood cells were at the lowend of the normal range.

Too littlemagnesium inyour diet willmake your bodywork harder.Good sources ofmagnesiuminclude branmuffins,pumpkin seeds,barley,buckwheatflour, low-fatvanilla yogurt,trail mix,halibut steaks,garbanzo beans,lima beans,soybeans, andspinach.

PEGGY GREB (K11083-1)

LackEnergy?Maybe It’sYourMagnesiumLevel

8 Agricultural Research/May 2004

9Agricultural Research/May 2004

The data shows that during the low-magnesium-status phase, the volunteersused more oxygen during physical ac-tivity, and their heart rates increased byabout 10 beats per minute. “When thevolunteers were low in magnesium, theyneeded more energy and more oxygento do low-level activities than when theywere in adequate-magnesium status,”says Lukaski.

The study was published in the May2002 issue of the Journal of Nutrition.

The body stores about half its mag-nesium inside the cells of tissues andorgans. The other half is combined withcalcium and phosphorus inside bones. Atiny amount—just 1 percent—of thebody’s magnesium circulates within theblood at a constant level.

These findings are consistent withother studies showing that too little mag-nesium makes the body work harder.“The effects are likely to occur in indi-viduals with low magnesium, regardlessof whether the person is athletic or sed-entary,” says Lukaski. “That means thatathletes wouldn’t be able to work or trainas long as they would if they had bettermagnesium levels. People need to eat ad-equate magnesium to make sure theirhearts and muscles are healthy enoughto meet the demands of daily living.”

The Recommended Dietary Allow-ance (RDA) for magnesium, based on thecurrent Dietary Reference Intakes(DRIs), for women over age 31 is 320milligrams (mg) daily and for men overage 31 is 420 mg daily. The DRIs are aset of categories of nutrient requirementsby age and/or gender groupings that arereleased as an authoritative, ongoingseries by the National Academy ofSciences. Starting in 1941, RDAs wereupdated periodically as a separate groupto reflect current scientific knowledge.Now, RDAs appear as one of severalcategories that together make up themore detailed DRIs. ARS provides keydata for use by experts who help establishthe DRIs.

Another study at the Grand Forkscenter, headed by physician Leslie M.Klevay, with biochemist David B. Milne,

found an association between ordinarydiets low in magnesium and irregularheartbeats. Klevay heads the center’sTrace Elements and CardiovascularHealth Laboratory. The study provided22 postmenopausal women, aged 47 to78, with a diet of conventional foodscontaining either less than half or morethan the RDA for magnesium of 320 mgdaily. All volunteers ate both controlleddiets—each for 81 days—throughout thestudy, though they were not told whichdiet was given first.

After each 81-day diet phase, magne-sium concentrations were measuredthrough blood tests. The scien-tists also wanted to evaluate thecharacteristics of electricalimpulses that provide pumpinginstructions to the heart. Electro-cardiograms were taken at theend of each phase to measurevolunteers’ heart rhythms.

When the women ate a dietcontaining just 40 percent of theRDA for magnesium, theyshowed signs of magnesium de-pletion in their blood serum, redblood cells, and urine. At thesame time, they experiencedsmall increases in heartbeats thatarise in abnormal areas withinthe heart, which are called ectopic heart-beats. They indicate increased myocar-dial irritability.

“These changes indicate that 130 mgof magnesium is too little and that theRDA may be correct,” says Klevay. “Peo-ple who live in soft-water areas, who usediuretics, or who are predisposed tomagnesium loss or ectopic beats mayrequire more dietary magnesium thanwould others.”

Perhaps not surprisingly, people wholive in areas where the water is hard—and therefore contains more minerals, in-cluding magnesium—have been found tohave a reduced risk for heart disease.Magnesium is sometimes provided intra-venously to people shortly after they’vehad a heart attack, though at least onestudy showed no evidence of a “survivalbenefit” from the practice.

Only about

32 percent32 percentof the U.S.

population

met the RDA-

DRI for

magnesium.

Klevay’s study was published in theMarch 2002 issue of the American Jour-nal of Clinical Nutrition.

Data from the nationwide food con-sumption survey, the USDA-ARS Con-tinuing Survey of Food Intakes byIndividuals (CSFII 1994-96), indicatethat only about 24 percent of womenbetween ages 40 and 69 were meetingthe 1989 RDA for magnesium. Thecurrent RDA-DRI for magnesium is 40mg higher than the 1989 RDA for thatage group.

In addition, the ARS Community Nu-trition Research Group (CNRG) reports

data from its online Com-munity Nutrition Map(CNMap) indicating thatonly about 32 percent ofthe U.S. population metthe RDA-DRI for magne-sium. The CNRG is oneof six laboratories in theBeltsville (Maryland)Human Nutrition Re-search Center. The map-ping project combines acollection of food-and-nutrition-consumptionindicators (obtained fromseveral sources, includ-ing CSFII 1994-96) with

Geographic Information System soft-ware. This data provides a snapshot ofthe percentage of people meeting therequirements for different nutrients.CNMap can be accessed at www.barc.usda.gov/bhnrc/cnrg/cnmapfr.htm.—ByRosalie Marion Bliss, ARS.

This research is part of Human Nutri-tion, an ARS National Program (#107)described on the World Wide Web atwww.nps.ars.usda.gov.

Henry C. Lukaski, Forrest H. Nielsen,and Leslie M. Klevay are with the USDA-ARS Grand Forks Human NutritionResearch Center, 2420 2nd Ave., GrandForks, ND 58202; phone (701) 795-8429[Lukaski], (701) 795-8455 [Nielsen], fax(701) 795-8230, e-mail hlukaski@gfhnrc.ars.usda.gov, fnielsen@gfhnrc.ars.usda.gov, lklevay@gfhnrc.ars.usda.gov. ★

9Agricultural Research/May 2004