The Nobel Laureate (immunology) Sir Peter Medawar once said
that “If politics is the art of the possible, then science is the art of the
soluble” and there is no better way of solving a problem than breaking it down
to ever smaller units and then building it up again. In cell biology, this is
easy. Isolate an enzyme and study its characteristics in the test tube. Then
see what happens when an intact cell is put through its paces. Lucky cell
biologists! Studying free-living humans poses an entirely different challenge
with the boundaries of investigation set by factors ranging from ethics to
practicalities of modern day life. Notwithstanding these challenges, the study
of how the human diet influences our health must proceed with the highest
possible rigour. In certain areas we can claim tremendous success such as the
role of nutrients in neural tube defects, in age-related blindness, in blood
lipids, in blood pressure, in bone disease and the like. In obesity, we have
let ourselves down badly and nothing highlights this more than a recent
systematic review of the data on diet and obesity concluded by The Swedish
Council on Health Technology Assessment. Founded in 1987, this Council[1] is
an independent national authority, tasked by the government with assessing health
care interventions based on ‘systematic literature reviews’ of published
research.
Last week (November 27th, 2013) they launched a
report: “Diet among obese individuals”[2]. In
this instance, the data refer to those who are clinically obese with a BMI
greater than 30kg/m2. The systematic review covered all dietary
intervention studies and those observational studies that lasted at least 6
months. The review covered all known publications up to the end on May this
year. The authors used the internationally accepted GRADE[3]
system to rank the scientific quality of the data. Studies with inconsistent
results or imprecise findings/objectives or confounded by non-controlled
factors were excluded. The accepted studies were used to collectively yield a
conclusion as to the strength of the evidence linking diet to the treatment of
clinical obesity. The following ranking was used: ++++ for high quality
evidence, +++0 for moderate quality, ++00 for low quality and +000 for very low
quality evidence. The results are presented for a variety of nutritional
comparisons and then for foods.
If the document is searched for all conclusions ranked at the
highest level (++++), only three appear.
They are:
·
“There is
strong scientific data to indicate a link with increasing coffee intake and a
reduced risk of diabetes among obese individuals”
·
“There is
strong scientific data available to indicate that initiating dietary
intervention with a VLED (very low energy diet) regimen of 8–12 weeks can
achieve greatly increased weight loss over up to 12 months for obese
individuals, but after two years the effect of the regimen is marginal”
·
“There is strong scientific data available to indicate that physical
activity as a supplement to dietary intervention with energy restriction has no
significant supplementary value for weight reduction after 6 months for obese
individuals”
It is remarkable that only three conclusions
reach what would be regarded as strong evidence. The report is however large
enough for all “activists”, scientists and non-scientists, to find their own
gems in the findings. For example, the Internet is awash with claims that this
report slams low fat diets and applauds low carbohydrate diets. However, the report is quite specific about
comparisons between moderate low carbohydrate diets and low-fat diets in the
clinically obese: “There is moderately
strong scientific data to indicate that advice on moderate low carbohydrate
diets compared with advice on low fat diets for obese individuals has a more
beneficial effect on weight at 6 months. At 12 months, the effect on weight is
the same (+++0). There is inadequate data available to assess whether there is
any difference between advice on the two diets with regard to weight at 24
months (+000)”. Twist it how you
like but the facts are we have no long-term evidence on which to base such
important food and nutrition policies. The same conclusions ring through most
of the fat-carbohydrate comparisons in the report.
It is also worth looking at some of the
conclusions on foods. On “Sweet drinks” the following is one key conclusion:
“There is limited scientific data available to indicate that reduction of sweet
drinks is linked to weight loss and lower blood pressure among obese
individuals (++00)”. For “chips” the
report finds: “There is no data available to assess any effect of potatoes or
chips on body weight (no studies are available)”. For “Fruit and Vegetables”, the comments are:
·
“There is limited scientific data
to indicate that advice on increased intake of fruit and vegetables, compared
with advice on reduced fat intake, leads to slightly less pronounced weight
loss at 6 months among obese individuals (++00). There is inadequate data
available to determine whether there is any difference in effect on waist size
(+000). For a longer period (12 months or more), there is inadequate data
available to determine whether advice on increased intake of fruit and
vegetables has a beneficial effect on body weight or waist size (+000).
• “There is inadequate scientific
data available to determine whether intake of fruit and vegetables demonstrates
a link with future weight change among obese individuals”.
One must bear in mind that this study was
focused on weight management in the clinically obese and that what is relevant
to that sector may not be relevant to the prevention of obesity or the
long-term treatment of moderate overweight. Nonetheless, this influential
report will serve if nothing else to show that of the 43 conclusions as regards
to the role of nutrients in the management of clinical obesity, only one single
conclusion (Very Low Energy Diets) met the top ++++ GRADE rating. Of the 51
conclusions on foods and lifestyle, two met this standard (coffee and physical
activity). How poor is that.
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