In­ter­mit­tent fast­ing has fast be­come one of the world’s most pop­u­lar meth­ods of los­ing weight, but how ex­actly does it work, and is it safe?


Ev­ery­one in diet and fit­ness cir­cles is talk­ing about in­ter­mit­tent fast­ing. While the rea­son for its sud­den pop­u­lar­ity can be at­trib­uted to its strong as­so­ci­a­tion with in­ter­na­tional celebri­ties such as Jen­nifer Lopez, Bey­oncé, Hugh Jack­man and many oth­ers, Dr Nandipha Ndhlovu, a gen­eral prac­ti­tioner with a spe­cial in­ter­est in aes­thetic medicine, has an­other sug­ges­tion.

“The no­tion that some form of fast­ing might have health ben­e­fits is noth­ing new, but [in­ter­mit­tent fast­ing] has gained cur­rency in re­cent decades be­cause of re­search show­ing that calo­rie re­stric­tion can ex­tend the life­spans of many life forms such as bac­te­ria, mice, rats and mon­keys,” Dr Ndhlovu ex­plains. “This has not been proven in hu­mans for ob­vi­ous rea­sons – ethics!” she ex­claims.

As the name sug­gests, in­ter­mit­tent fast­ing is an eat­ing pat­tern that cy­cles be­tween fast­ing and eat­ing, with­out nec­es­sar­ily spec­i­fy­ing what types of food you should eat on non-fast­ing days.

While there are dif­fer­ent meth­ods of in­ter­mit­tent fast­ing, the gen­eral idea is that you eat as nor­mal on some days of the week, and then on other days you re­duce your calo­rie in­take quite con­sid­er­ably, aided by fast­ing.

“On a fast­ing day, a per­son’s in­take is of­ten lim­ited to non-caloric flu­ids such as wa­ter, tea, cof­fee (with low-fat milk) or it may al­low a very re­stricted amount of daily calo­ries,” Mbali Mapholi, di­eti­cian and spokesper­son for the As­so­ci­a­tion for Di­etet­ics in South Africa, ex­plains. It’s rec­om­mended that all liq­uids must have a caloric in­take of less than 50 kJ, which means that no al­co­hol is al­lowed.


There are a va­ri­ety of meth­ods that you can choose from. Dr Ndhlovu takes us through the three most com­mon ones. AL­TER­NATE-DAY FAST­ING

This diet is also known as the 24-hour diet. The di­eter eats un­re­stricted for one day and doesn’t eat at all the next day and so on.


This pop­u­lar method has been en­dorsed by celebri­ties such as Jimmy Kim­mel. “The rou­tine is also known as the 5:2 diet, in which the di­eter eats nor­mally for five days a week and greatly re­strict food in­take two days a week. The caloric in­take dur­ing these two days must not ex­ceed 500-600 kJ for fe­males, and 1 000 kJ for males,” Dr Ndhlovu ex­plains. TIME-RE­STRICTED FAST­ING

Also known as the 16:8 diet, this type dic­tates that the di­eter must eat all meals within an 8-hour win­dow and fast for 16-hours. For ex­am­ple, you’d only eat be­tween 12-8pm and fast for the re­main­ing 16-hours.


Di­etary re­stric­tions such as those seen in in­ter­mit­tent fast­ing can ma­nip­u­late mi­to­chon­drial net­works in cells, Mapholi ex­plains, adding that re­cent re­search has sug­gested that this ma­nip­u­la­tion of cells does have the po­ten­tial to in­crease your life­span, but the re­search is not ab­so­lute.

“In terms of weight loss, cur­rent re­search in­di­cates that in­ter­mit­tent fast­ing and con­tin­u­ous calo­rie re­stric­tion seem to pro­duce weight loss re­sults, so there may be an ar­gu­ment for con­sid­er­ing it as a weight loss method,” Mapholi says with a bit of he­si­ta­tion.

“How­ever, if it was to be con­sid­ered an op­tion, it’d be im­por­tant to high­light the po­ten­tial risks re­lated to fast­ing and low-calo­rie in­takes, as well as the con­sid­er­a­tion of the over­all lim­ited ev­i­dence base and un­cer­tainty about the long-term out­comes of in­ter­mit­tent fast­ing,” she adds.


While there have been nu­mer­ous stud­ies that have sug­gested that, over and above the weight loss ben­e­fit, in­ter­mit­tent fast­ing can im­prove meta­bolic health and re­duce a per­son’s chances of de­vel­op­ing type 2 di­a­betes and heart dis­ease – again, the re­search isn’t with­out lim­i­ta­tions. Mapholi cau­tions that we shouldn’t nec­es­sar­ily ac­cept all of these as fact.

“Many of these stud­ies were per­formed on ro­dents and most of them were not done over a sig­nif­i­cantly long enough pe­riod of time,” Mapholi ex­plains.

“In­ter­mit­tent fast­ing, if un­su­per­vised by a med­i­cal pro­fes­sional de­pend­ing on a per­son’s age, med­i­cal his­tory and life­style, can be detri­men­tal to health by in­creas­ing in­sulin re­sis­tance while de­creas­ing im­por­tant detox­i­fy­ing enzyme func­tions, mus­cle mass and body nu­tri­ents,” she stresses.

Dr Ndhlovu adds that more se­vere calo­rie re­stric­tion may cause other health prob­lems such as re­duced lean body mass and bone min­eral den­sity, and an in­creased risk for anaemia and gall­stones. “Me­tab­o­lism will slow down in peo­ple who re­strict their calo­rie in­take for a long pe­riod, which may take years to re­cover. If they then re­vert to their pre­vi­ous di­ets, calo­rie re­stric­tors could see them put on more weight than they lost,” Dr Ndhlovu says.

She warns that there are cer­tain in­di­vid­u­als who should not con­sider go­ing on the fast at all, they in­clude: Di­a­bet­ics (con­sult with your doc­tor) Peo­ple with other meta­bolic dis­or­ders High risk in­di­vid­u­als: the young, el­derly and/or un­der­weight Preg­nant woman


As is with any di­etary change, there are im­por­tant con­sid­er­a­tions you must con­tem­plate be­fore de­cid­ing on an eat­ing pat­tern/diet. Mapholi says the most im­por­tant thing to do be­fore be­com­ing an in­ter­mit­tent faster is to con­sult with a di­eti­cian or a doc­tor.

“You can­not re­ally make this health de­ci­sion on your own with­out con­sult­ing a health­care pro­fes­sional, as there’s no con­clu­sive re­search on the safety of the diet, es­pe­cially in the long term. The health­care pro­fes­sional will con­duct a med­i­cal as­sess­ment to iden­tify whether the fast­ing method is safe for the pa­tient or not,” Mapholi says, em­pha­sis­ing that fad di­ets come and go. “In­ter­mit­tent fast­ing is not ex­cluded from this, it will soon blow over and an­other fad diet will make waves.”

Both Mapholi and Dr Ndhlovu agree that noth­ing can re­place a bal­anced reg­i­men of a mod­er­ate caloric in­take and phys­i­cal ac­tiv­ity. In fact, Dr Ndhlovu even goes as far as to say that she does not per­son­ally ad­vo­cate for the diet, mainly be­cause it’s not easy to sus­tain.

In­ter­mit­tent fast­ing is clearly a diet that can be great for some but no so much for oth­ers. What’s im­por­tant is find­ing an eat­ing plan that works for you, fits into your life­style and one that you can main­tain over a long pe­riod of time.

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