Efnaformúla fyrir laktósa; galaktósi til vinstri, glúkósi til hægri.
Í allri mjólk spendýra (og þar á meðal manna) er sykur, sem nefnist laktósi. Nafnið er dregið af latneska orðinu lac, mjólk, og því er hann einnig nefndur mjólkursykur. Sykurinn kemur hvergi annars staðar fyrir í náttúrunni, en hann er í misjafnlega miklu magni (2-8% af þyngd) eftir því um hvaða tegund lífvera er að ræða. Meðal manna er hlutfallið 6,5-7,5% en hjá kúm og geitum um 5%.
Laktósi er svo nefndur tvísykur og er hann byggður upp af tveimur einsykrum, glúkósa og galaktósa. Af öðrum tvísykrum má nefna reyrsykur (venjulegan sykur, sakkarósa) og maltsykur (maltósa). Reyrsykurinn er gerður úr glúkósa og frúktósa (ávaxtasykri) og maltsykur úr tveimur glúkósaeiningum.
Þá eru til fjölsykrur, sem leysast ekki upp í vatni, og eru úr þúsundum einsykurs-einingum, sem tengjast saman í langar keðjur. Af þeim eru til margar gerðir, en algengastar eru mjölvi (sterkja) og beðmi (sellulósi), sem eru hvorartveggju byggðar upp af glúkósa.
Við meltingu á tví- eða fjölsykrum brotna keðjurnar niður í einsykrur. Síðan frásogast einsykrurnar í smáþörmum, fara inn í blóðrás og berast til frumna, þar sem þær eru nýttar (meðal annars til brennslu) eða þeim breytt í önnur efni (til dæmis fitu eða fjölsykruna glýkógen).
Í meltingarvegi eru kirtlar, sem gefa frá sér enzým, sem losa um tengin á milli einsykranna, svo að út koma glúkósi og galaktósi. Enzýmið, sem vinnur á sykrunni laktósa, nefnist laktasi (endingin –asi táknar enzým) og myndast í kirtlum í smáþörmum.
Nú er það svo, að meirihluti fólks í veröldinni hefur ekki í sér þetta tiltekna enzým, laktasa, sem klýfur laktósa í glúkósa og galaktósa. Þessar einsykrur frásogast því ekki í smáþörmum þess. Þess í stað fer laktósi áfram niður í grófþarma, þar sem bakteríur taka að nýta sér hann sem fæðu og sundra honum.
Við þessa starfsemi baktería myndast niðurbrotsefni sykrunnar, sem er CO2. Við það fær fólk miklar kvalir í kviðarholið, þjáist af uppþembu og niðurgangi. Í Afríku og Asíu eru um 70% manna þar án laktasa en aðeins um 30% Evrópubúa. Þetta fólk þolir því ekki að drekka mjólk.
Nýverið hafa rannsóknir leitt í ljós, að fólk, sem þolir ekki sykurinn laktósa og drekkur því ekki mjólk, er miklu síður hætt við krabbameini í lungum, brjósti og eggjastokkum en öðru fólki, samkvæmt grein, sem birtist í tímaritinu Nature. Sjá: http://www.nature.com/bjc/journal/vaop/ncurrent/abs/bjc2014544a.html og http://www.healthcanal.com/cancers/57001-lactose-intolerants-at-lower-risk-of-certain-cancers-study.html.
Höfundar greinar taka þó skýrt fram, að það sé alls ekki víst, að mjólk valdi krabbameini. Á hinn bóginn kom í ljós, að fjölskyldumeðlimir þessa fólks, sem þolir ekki mjólk, eru í jafnmikilli hættu og aðrir við að fá krabbamein. Á þeirri forsendu var og sýnt fram á, að erfðaþættir væru ekki áhrifavaldar hér. Það þykir því margt benda til þess, að orsaka krabbmeina sé að leita í mataræði manna, að því Jianguang Ji, dósent í háskólanum í Lundi og einn af höfundum greinarinnar, heldur fram. Ekki er þó hægt að líta framhjá því, að tíðni krabbameins í brjóstum og eggjastokkum er hæst í Vestur-Evrópu og Norður-Ameríku, þar sem neyzla mjólkur og mjólkurafurða er mest.
Í ljósi þessarar vitneskju og niðurstaðna úr ítarlegri rannsókn, sem sagt var frá hér á þessum síðum fyrir skömmu (sjá hér: http://ahb.is/er-mjolk-oholl-rannsokn-bendir-til-thess/), hljóta heilbrigðis-yfirvöld (landlæknir) hér á landi að þurfa að endurskoða leiðbeiningar sínar til almennings um hollt og gott mataræði. Það er ekki einleikið, hvað landlæknisembættið tekur undir með MS um, að mjólk sé holl og góð, D-vítamíngjafi og rík af kalki (kalsíum) til að byggja upp sterk bein. Nú leyfir maður sér að efast um sannleiksgildi þessa eða öllu heldur, að nauðsynlegt er að gleyma því ekki, að kalk, D-vítamín og önnur bætiefni má fá úr grænmeti, ávöxtum og fiski alls ekki síður en úr mjólk.
ÁHB / 6. nóvember 2014
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