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 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”. 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 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.