1. BMI: Badly Misunderstood Idea
BMI standsforBody Mass Index,andisusedto indicate apatient’smass(oftenreferredtoastheir
weight) inrelationtotheirheight(1).Thisallowsthe weightsof differentpeople tobe compared,
despite variationsin height.
BMI can be calculatedusingthe followingformula:
BMI = weight(kg)/height²(m)
It isalso possible tocalculate yourownBMIon:
http://www.nhs.uk/Tools/Pages/Healthyweightcalculator.aspx.Thislinkallowsyoutoenterimperial
data (i.e.heightinfeetandinches,weightinstonesandpounds),andwill convertittometricdata
(i.e.heightinmetres,weightinkilograms)forthe calculation.
BMI allowsustoidentifyif apatientisunder- oroverweight,orif theyare a healthyweight.To
demonstrate this,Ihave calculatedmyBMI usingcalculationbyfollowingthe NHSHealthyWeight
Calculatorabove:
My BMI (2)
At the top of the image ismy BMI, for a weightof 64 kg, anda heightof 165.1 cm. My BMI of 23.4
indicatesthatI am a healthyweightformyheight.Italsogivesme ahealthyweightrange of 50.4 kg
to 68.4 kg; if I gain or lose anyweight,Ican now identifyif Ihave developedaweightissue.
My BMI alsotakesintoaccount my age and gender.Metabolismslowswithage,meaningthatfewer
nutrientsabsorbedfromthe intestineswill be usedstraightaway.Excessnutrientswill be converted
to, andstoredas fat inadipose tissue (a.k.afatcells),whichisfoundsurroundinginternal organs
(visceral fat),andbeneaththe skin(subcutaneousfat).Thismayalterthe healthyweightrange
given.Gendermayalsochange the healthyweightrange;since womenare oftensmallerthanmen,
and therefore have fewercellular content,theyneedlessenergyandnutrientstomaintaingood
health.
BMI waspatentedbya Belgianstatistician,Adolphe Quetelet,inthe early1800’s. Although
Quetelets’workwasrecognisedbyotherscientistsandmathematicians,BMIwasnot a commonly
usedtermuntil manyyearslater(3). Followingthe endof WorldWar 2, life insurance companiesin
the USA observedarise inthe numberof deathsrelatedtoweightissues.Thisobservation
stimulatedinterestinBMI,andwhetheritcouldbe usedas a reliable indicatorof weight-related

2. health(4).However,BMIwas still notregardedasan effectiveindicatorof weight-relatedhealth
issues.
It wasn’tuntil muchlater,inthe mid1980’s, that BMI startedto getthe recognitionithastoday.
Two scientists,GarrowandWebster,developedcategoriesdepictingthe riskof developinghealth
issuesatvaryingBMIs (5),as a response tothe post-warobservations.These categorieswere
developedusingstatisticssuchasstandarddeviations,todetermineatwhat weightpeople beganto
developdifferenthealthissuessuchasdiabetes(type 2).Those people whohave aBMI lowerthan
18.5 are saidto be underweight;thismayindicate malabsorptionsyndromes(whichpreventssome
of the nutrientsconsumedinthe dietfrombeingabsorbed),malnutrition,andevenanorexia
nervosa.A BMI of 25 or above indicatesthatsomeoneisoverweight,andatriskof developing
diabetestype 2,cardiovasculardisease,hypertension,andstrokes.ThisriskincreasesfromaBMI of
30 upwards,withindividualsfallingintothiscategorybeingconsideredobese.Thisriskfurther
increasesinthose withaBMI of 40 or above,withindividualsfallingintothiscategorybeing
consideredasmorbidlyobese.Betweenthe underweightandoverweight categoriesliesthe healthy
weightrange.Althoughthere isstill ariskof developinganyof the healthissueslistedabove,the risk
issmallestinthisrange (1).
Eventually,in1995, the WorldHealthOrganisationbeganusingBMIas a measure of weight
alongside health(4).AlthoughBMIhasbecome recognisedasauseful indicationof health,thereis
still some debate astowhetheritisa truly reliablemeasurementthatcanbe usedto distinguishill-
health.BMI isnot an indicatorof bodycomposition; itmayoverestimate how muchfata muscular,
or shorterpersoncarries,andmay underestimatehow muchadipose tissuethinpeople carry(1).It
alsocannot identifythe distributionof fataroundabody; twopeople withthe same BMImay have
completelydifferentfatdistributions.Some peoplemaycarry fatsubcutaneously,andothersina
visceral fashion.Visceral fatisassociatedwitharaisedriskof healthissuessuchasstroke and
cardiovasculardisease.Therefore,despite havingthe same BMI,anindividual carryingmore visceral
fat ismore likelytodevelopthese conditions,thansomeonewhocarriestheirfatsubcutaneously
(4).Because of this,BMI shouldnotbe usedalone,butinconjunctionwithothermeasurements
such as fat tomuscle ratio(recordedusingcallipers.Callipersare gentlyclampedonthe skin,usually
aroundthe waist,hips,thighs,upperarms,andback, the same way thatfat may be pinchedbyhand.
The size of the fat caughtin the callipersismeasuredinmillimetres,andthisgivesanindicationof
howmuch subcutaneousfata personcarries),dietaryrecordsanddiaries,exercisetests(where
metabolicrate maybe calculatedbymeasuringhow muchoxygenthe patientuses,andhow much
carbon dioxide theyexhale),andthe medical historyof the patient.Itshouldalsobe notedthat
otherfactors suchas smoking,alcohol intake,andeventhe workplace will affectsomeone’shealth
i.e.some workplacesrequire peopletobe regularlyexposedtoradiation,whichisaknown
carcinogen(1).
To summarise,BMIis simplyaguideline thattellsuswhetherwe are at a desirable weightforour
height,andwhatcan be done tomaintainor correct this.If youare worriedaboutyourweight,and
fall outside of yourhealthyweightrange,bookanappointmenttosee yourGPand discusswhatthe
bestcourse of actionisfor your health.
By Emma Steer, 1st
Year BSc Nutrition
References:

3. (1). AspdenW.,Caple F.,ReedR.,JonesA.,WeyersJ.,Chapter43 "AnthropometricandBody
CompositionMeasurements", Practical SkillsinFoodScience,NutritionandDietetics,2011,Pearson
EducationLimited,London
(2).BMI calculator,31/10/2013, NHS Choices,URL
[http://www.nhs.uk/Tools/Pages/Healthyweightcalculator.aspx],date accessed[31/03/2015]
(3). WilsonS.,BodyMass Index,07/11/2007, HowStuffWorks.com, URL
[http://health.howstuffworks.com/wellness/diet-fitness/weight-loss/bmi4.htm],date accessed
[31/03/2015]
(4). The OriginsandLimitationsof BMI,2009, Cutthe Waist,URL
[http://www.cutthewaist.com/bmi.html],date accessed[31/03/2015]
(5). StephensonW.,BMI:Does the BodyMass Index needfixing?,29/01/2013, BBC News,URL
[http://www.bbc.co.uk/news/magazine-21229387], date accessed[31/03/2015]