Fast­ing has tran­si­tioned from sci­ence-jour­nal fod­der to main­stream slim­ming tool. As it es­ca­lates among the health-con­scious, we ask if the re­search stacks up, or is in­ter­mit­tent fast­ing sim­ply star­va­tion re­branded?

Women's Health (UK) - - CONTENTS - words CAROLINE WIL­LIAMS

In­ter­mit­tent fast­ing: recipe for health or disas­ter?

We sus­pect you first heard about fast­ing from that one friend who dis­cov­ers and tri­als all new health trends just be­fore they reach peak take-up, no doubt over a late din­ner at a restau­rant with counter seat­ing where she es­chewed food in favour of re­gal­ing you with the sci­ence be­hind eat­ing in­ter­mit­tently. She’s done the 5:2, tried the 16:8 (‘it’s the same sci­ence, just eas­ier to fol­low!’) and is cur­rently evan­ge­lis­ing about the five-days-ev­ery-three-months fast­ing-mim­ick­ing diet.

Since 5:2 came into our lives in 2012, in­ter­mit­tent fast­ing – the con­cept of shun­ning calo­ries for hours or days at a time – has been touted as a way to re­duce your risk of heart dis­ease, diabetes and cancer, while melt­ing belly fat and of­fer­ing your di­ges­tive sys­tem a re­boot. The premises of the 5:2 (cut­ting back to 500 calo­ries two days a week) and the 16:8 (eat­ing all your day’s food within an eighthour win­dow) are ba­si­cally the same. When you give your body a break from eat­ing, it be­gins feed­ing off your fat re­serves. An­other it­er­a­tion, the fast­ing-mim­ick­ing diet (FMD) – which we first learnt about in 2015, but is now gar­ner­ing main­stream in­ter­est as fast­ing rises in pop­u­lar­ity, is a five­day pro­gramme based on the re­search of Dr Val­ter Longo, a bi­ol­o­gist at the Univer­sity of South­ern Cal­i­for­nia. It’s de­signed to make your body think all it’s be­ing fed is wa­ter, while keep­ing ac­tual star­va­tion at bay via a calo­rie-con­trolled, sci­en­tif­i­cally de­vel­oped diet. But it doesn’t come cheap – it’s £225 for the five-day kit, de­vised by Dr Longo and sold na­tion­wide by a com­pany called Prolon.

Hang on a minute. De­priv­ing your­self of food goes against all your in­stincts, not to men­tion the di­etary ad­vice we’re al­ways hear­ing. So is this lat­est nutri­tion trend the prod­uct of sci­en­tific de­vel­op­ments – or is the diet in­dus­try hav­ing the last laugh?


The sci­ence is fairly sim­ple. When you put your­self on the 5:2 or the 16:8, you starve your body of the en­ergy it usu­ally gets from food, which forces it to seek its fuel in­ter­nally. First, you’ll utilise the sugar in your blood­stream; when that’s gone, it’s time to tap into the liver’s glyco­gen stores. Af­ter a day or so of eat­ing fewer calo­ries than your body needs, th­ese too will run out, so your body has no op­tion but to start burn­ing fat.

But pro­po­nents of the FMD in­sist that when you fast for longer than a day or two, the ben­e­fits go be­yond weight loss. Yes, the five-day plan pushes your body into ke­to­sis – the meta­bolic state in which the body burns fat for en­ergy, made fa­mous by the Atkins diet (and keto breath) – but some­thing else is go­ing on, too.

A short pe­riod of se­vere calo­rie-de­pri­va­tion forces the body to save as much en­ergy as pos­si­ble by be­gin­ning to break down and di­gest its own cells, a process called au­tophagy. Sounds scary, but Dr Longo in­sists you bear with. The body is in­tel­li­gent enough to start with dead or dam­aged cells that aren’t worth the en­ergy to sus­tain – the ones that con­trib­ute to age­ing and are more likely to turn can­cer­ous. In other words, the body be­comes health­ier, seem­ingly younger and at less risk of dis­ease. Un­til the FMD came along, the only way to trig­ger au­tophagy was to go on a wa­ter-only diet – some­thing no di­eti­tian would rec­om­mend with­out close mon­i­tor­ing. In this way, Dr Longo’s fast of­fers a sys­tem clean-up that prom­ises to torch fat while also boost­ing gen­eral cel­lu­lar health and get­ting nu­tri­ents via the food.


Most of the stud­ies that sell fast­ing as a nu­tri­tional win point to re­sults found in mice. Pe­ri­odic fast­ing of ro­dents in a lab set­ting re­duced fat, in­creased life­span and cut in­ci­dences of cancer and in­flam­ma­tion – linked to ev­ery­thing from heart dis­ease to de­pres­sion. And where some of the mice’s cells were lost to au­tophagy in the or­gans, mus­cles and bones, they were quickly re­placed with new ones.

But you’re not a mouse. In hu­mans, stud­ies com­par­ing short-term fasts – the likes of 5:2 and 16:8 – with calo­rie-con­trolled di­ets con­sis­tently sug­gest they are no more ef­fec­tive at shift­ing weight and im­prov­ing meta­bolic mark­ers than any other kind of diet. And when it comes to the pro­tec­tive im­pact of pe­ri­ods of pro­longed fast­ing, the ev­i­dence is lim­ited. A re­cent pi­lot trial of the FMD on hu­mans in Dr Longo’s lab showed find­ings of a 3% av­er­age body fat re­duc­tion, low­ered blood glu­cose and lower lev­els of in­flam­ma­tory mark­ers. But the ev­i­dence for au­tophagy was cir­cum­stan­tial at best.

Dr Longo in­sists that even healthy peo­ple could ben­e­fit from a pe­ri­odic fast. The sci­en­tist whose re­search led to the de­vel­op­ment of the 5:2 diet is un­con­vinced. ‘We have no ev­i­dence of ben­e­fits for peo­ple who are a healthy weight,’ says Dr Michelle Harvie, a re­searcher at the Univer­sity of Manch­ester who first looked into fast­ing as a way to help women at risk of breast cancer to lose weight. ‘All the ev­i­dence is for weight loss among the over­weight.’ As for the prom­ise of longevity, we’ll just have to wait and see.

Oth­ers point out that fast­ing isn’t the only way to lure your body into a re­plen­ish­ing state. The al­ter­na­tive is called ex­er­cise, and the the­ory goes that both fast­ing and get­ting your sweat on put the body into a state of mild stress, stim­u­lat­ing it to re­pair it­self. It also gives the brain a boost by trig­ger­ing the re­lease of a chemical called brain-de­rived neu­rotrophic fac­tor (BDNF), which in­creases the brain’s ten­dency to make new neu­rons and cre­ate and main­tain con­nec­tions. This might ac­count for the clear head that many peo­ple boast of dur­ing a fast and, in­deed, af­ter ex­er­cise.


Should you have con­fessed to friends a few years ago that you were starv­ing your­self, you’d have been met with con­cern. To­day, re­place the word ‘starve’ with ‘fast’ and you’re more likely to be met with a know­ing smile. But do the two mean the same thing?

Yes, ac­cord­ing to Dr Longo – and that’s the point. In or­der for fast­ing to trig­ger a sys­tem re­boot, the body needs to think that it’s starv­ing. ‘Your body tips over into star­va­tion quite quickly – but that’s a good thing,’ he ex­plains. ‘Af­ter a cou­ple of days, it starts killing off cells.’

All very well if you’re do­ing it in a con­trolled way, like the FMD, but take it too far and it can prove fa­tal – peo­ple have died from do­ing wa­ter-only fasts.

And even con­trolled fasts can in­duce an emo­tional fall­out.


De­pri­va­tion stud­ies have con­sis­tently found that dras­ti­cally culling calo­ries can lead to an un­healthy re­la­tion­ship with food. As Duane Mel­lor, se­nior lec­turer in hu­man nutri­tion at Coventry Univer­sity, points out, you only have to look at the glut­tonous at­ti­tude of some 5:2 di­eters on non-fast days to un­der­stand how fast­ing can af­fect your eat­ing. ‘If you haven’t got your calo­rie in­take un­der con­trol when you’re not fast­ing, you could be vul­ner­a­ble to a binge pat­tern,’ he says.


So should we be fall­ing for the hype? Epi­demi­ol­o­gist Dr Ben­jamin Horne, who con­ducted a re­cent re­view of the ev­i­dence, found that fast­ing is gen­er­ally safe when done ev­ery other day for a year or five days in suc­ces­sion once a month. But it’s im­por­tant to know the risks – faint­ing, de­hy­dra­tion, and mal­nu­tri­tion. ‘No study that I know of has given us in­for­ma­tion re­gard­ing the thresh­old for mal­nu­tri­tion, which likely de­pends on the in­di­vid­ual,’ he says. And with no re­search com­par­ing dif­fer­ent fast­ing meth­ods in hu­mans, it’s hard to know which works best.

While it’s ac­cepted that there are peo­ple who shouldn’t fast – chil­dren, di­a­bet­ics, preg­nant and breast­feed­ing women and any­one with a BMI un­der 18.5

– if your goal is to burn fat, then stud­ies sug­gest fol­low­ing a short-term fast can be ef­fec­tive. As for the longevity-boost­ing po­ten­tial of the FMD, more re­search is needed. The only thing that’s re­ally clear about fast­ing is that its ben­e­fits – and re­sults – are sub­jec­tive. Yes, it can be safe, but that doesn’t mean it’s the health­i­est op­tion. Try it, by all means, but also ask your­self if your ef­forts wouldn’t be bet­ter spent on an­other Bdnf-booster – a gym pass.

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