GP Tips: The 5:2 Fast Diet
Dr Kevin Fernando
“Doctor, what do you think of the Fast diet?”
This diet dictates that you eat normally 5 days weekly, and “fast” the
remaining 2 days. On fasting days, women
eat 500kcal daily, and men 600kcal.
In addition to weight loss, Moseley claims increased lifespan and
cognitive function, protection against Alzheimer’s dementia and “general
protection from disease” – bold claims indeed!
So What is the Evidence Base for the Fast Diet?
The Fast diet is based on the work of Dr. Krista Varady, a pioneer of alternate day fasting (ADF) so immediately there is a departure from the original literature where there were 3 or 4 fasting days weekly. A further discrepancy was that Varady’s subjects took their food allowance in one sitting (to promote a fasting state) whereas Moseley spreads it over 2 meals.
As you can imagine, there are very few human studies and many animal
studies. Furthermore, the published studies
are mostly short-term.
Weight Loss
Varady published a reasonably robust study in the American Journal of Clinical Nutrition during 2009 however involving just 16 obese subjects. Varady demonstrated a significant reduction in weight, lipid profile and BP with ADF. Average weight loss after 8 weeks was 5kg.
Cognitive Function & Dementia
These claims are based on 2 poor quality studies. The first (Johnson et al (2006) Medical Hypotheses), was an extrapolation from an ethically dubious study undertaken in Spain during the 1950s, which involved fasting nursing home residents and following them up to see who died first, compared with non-fasted residents (I kid you not!). The second study involved intermittently fasting genetically engineered mice and observing how they performed in a water maze test (Halagappa et al (2006) Neurobiological Disease)
This claim is based on a study by Harvie et al published in the International Journal of Obesity during 2010. This is a higher quality randomised study involving 107 overweight women and did actually use a protocol involving 2 fasting days weekly. Harvie et al demonstrated a mean weight loss of 6.4kg over 6 months, and also significant improvements in insulin sensitivity, and reductions in BP and lipid profiles.
Probably most telling of all was a systematic review of ADF authored
by Varady herself in 2007, which stated that ADF may protect against heart
disease, type 2 diabetes and cancer, however much more research is required.
Should We Recommend the Fast Diet to Patients?
Despite a somewhat shaky (albeit promising) evidence base, that both
departs and extrapolates from original literature, there is plenty anecdotal
evidence for significant weight loss with the Fast diet in both patients and
medical colleagues!
And actually the Fast diet is sustainable in the short to medium
term unlike many other fad diets (the Paleo diet! anecdotally, patients, friends and colleagues certainly say they find the 5:2 diet easier to comply with than they would have expected to, and whilst we lack comparative evidence of sustainability and weight loss with other diets this alone may make it a reasonable choice to advise to patients)
Ultimately, it’s not rocket science – it’s simply about total
calorie reduction – if our (male) patients regularly consume 3800kcal less per
week, which can only be a good thing.
If supporting type 2 diabetes patients to undertake the Fast diet,
it is important to either down-titrate or cease any hypoglycaemic medication on
fast days.
Finally, the main theory of Moseley is that humans evolved during
times of severe dietary restriction, and that fasting was the norm and
conferred health benefits. Surely then,
we should be completely abstaining from food during the fast days? Some food for thought…Boom Boom…
References
Harvie MN et al (2011) “The effects of intermittent or continuous
energy restriction on weight loss and metabolic disease risk markers: a
randomized trial in young overweight women” Int J Obes (Lond) 35(5): 714-727
Varady KA & Hellerstein MK (2007) “Alternate-day fasting and
chronic disease prevention: a review of human and animal trials” Am J Clin Nutr
86: 7-13
Varady KA et al (2009) “Short-term modified alternate-day fasting: a
novel dietary strategy for weight loss and cardioprotection in obese adults” Am
J Clin Nutr 90: 1138-43
