Popular diets – what is the evidence?
Wednesday 8th January 2014
A conference for health professionals, staged by the British Nutrition Foundation (BNF) in London today, addressed the evidence behind popular weight loss diets, at a time of year when slimming products and regimens are in peak demand.
5:2 has fast become a familiar phrase in our slimming vocabulary and delegates heard that the latest research shows intermittent energy restriction diets actually can work, if a slimmer follows the right one. Dr Michelle Harvie, Research Dietitian, Genesis Prevention Centre, University Hospital of South Manchester, presented the findings of her team’s research, conducted over 8 years, which resulted in an original version of the popularly known 5:2: ‘The 2-Day Diet’.
Dr Harvie’s research shows that following a diet which requires two days of low carbohydrate or low calorie intake of around 500-600 calories, and five days of healthy eating can be effective for weight loss. The key is that, contrary to current popular thinking, followers will not succeed if they ‘eat whatever they want’ on the five non-fasting days.
Dr Harvie said: “Energy restriction is difficult to maintain over the long term and people tend to find it easier to follow a diet with intermittent energy restriction. So, while a regular diet might require a 25 percent calorie restriction every day for successful weight loss, our research looked at the success rates for including 2 days of 75 percent calorie restriction, interspersed with 5 days of healthy eating. The 2 low carb diet leads to a lower intake on non-diet days and an overall lower calorie intake and that’s how the diet works.
“As well as weight loss, the results showed dieters achieved an improvement in insulin resistance when compared with regular diets. In addition, our research shows that a diet based on just one day of low calorie or low carbohydrate fasting, combined with six days of healthy eating will help with long-term weight maintenance."
Prof Gary Frost, Chair of Nutrition and Dietetics, Imperial College London, went on to say that humans have a default to eat and that if it was easy to lose weight our species would have died out years ago. Prof Frost’s research examines the potential impact of glycaemic index (GI), the level of glucose in the blood in response to intakes of different carbohydrate-containing foods, something that’s widely recognised as very important in the understanding and treatment of diabetes. His research shows that GI has some impact on appetite regulation and that better weight maintenance can be achieved through a low GI diet, especially when low GI is accompanied by a high intake of fermentable dietary fibre. However, substantial change to the GI of the diet is required to achieve weight loss.
Prof Mark Berry, Head of Plant Biology & Biochemistry at Unilever R&D Colworth, presented fascinating data from a human study into how the features of a ‘Palaeolithic diet’ might be applied to food and drinks today with benefits for health.
According to Prof Berry, the Palaeolithic diet included a high quantity and broad variety of different plants, around 100 different varieties per year, which is many more than we tend to consume in the average diet even if achieving ‘5-a-day’. There were, therefore, many more phytochemicals and more fibre in the Palaeolithic diet.
Prof Berry said: “While our research showed little impact on glucose response in the body in those consuming a Palaeolithic diet, there is a huge difference in glucose-dependent insulino-tropic polypeptide (GIP) in the modern diet compared with the Palaeolithic diet. In preliminary studies a Palaeolithic diet also had a significant impact on satiety hormones and feelings of fullness after eating. These factors, combined with the low energy density, high fibre, low starch and high phytochemical attributes of the Palaeolithic diet have the potential to have a positive impact on Type 2 Diabetes and obesity.”
High protein and low carbohydrate diets
The potential merits of high protein, low carbohydrate diets have been hotly debated in recent years and Dr Alexandra Johnstone of the Rowett Institute of Nutrition and Health, presented findings from recent scientific studies into the efficacy of these diets. Dr Johnstone stressed that different weight loss strategies are effective in different people but that research evidence shows that high protein diets are undoubtedly useful for weight loss in some people, through appetite control.
Dr Johnstone said: “There’s no magic bullet. However, the high satiety effects of increased protein in the diet seems to be a contributing factor to the success of high protein, low carbohydrate diets, when there is typically an increase of around 20 percent in the actual amount of protein consumed. Data shows that the amount of weight lost on high protein diets is around double that lost on a comparable low fat diet at the six month mark. However there is little difference in weight loss at one year as dieters lose momentum.
“However, long-term, high protein, very low carbohydrate diets have some risks too, including a negative long term impact on colonic health and possible compromise to gut health, although there appears to be no adverse impact on renal function in those with healthy kidney function. So, they should be used as a weight loss tool, and should not be considered as a diet for life.”
Low and very low calorie diets
Finally, the conference addressed the efficacy of low and very low calorie diets in weight loss and examined when they are most effectively applied in the treatment of medical conditions.
Prof Anthony Leeds of the Faculty of Life Sciences, University of Copenhagen and Central Middlesex and Whittington Hospitals in London, presented scientific data into weight loss using formula diets, food and a mixture of both, in studies with calorie intakes ranging from 400 to 1,500 kcals per day, and the use of these diets in the medical management of obstructive sleep apnoea, diabetes, pre-operative ’liver shrinkage’, psoriasis and osteoarthritis.
Prof Leeds said: “It is possible to define the amount of body weight loss needed to achieve specific clinical effects and this is extremely useful in the management and treatment of these diseases.
“It is clear that low and very low calorie diets are effective in achieving weight loss. Research also shows that meal replacement diets, anti-obesity drugs and high protein diets can all also help with weight maintenance after weight loss following this approach.”
In summing up the proceedings, Prof Judy Buttriss, Director General of the British Nutrition Foundation said: “It was a really fascinating afternoon that provided the evidence base to support several popular dietary approaches that can be used as tools for effective weight loss and maintenance.
“Speakers emphasised that these tools are not mutually exclusive and there’s currently no evidence that one is any better than another in the long term. People looking to tackle their weight problem would perhaps be advised to choose one of these, along with regular physical activity, rather than other less researched approaches.”
Notes to Editors
- Prof Anthony Leeds declared his commercial interest as salaried medical director but holds on shares or share options.
- The British Nutrition Foundation symposium ‘Popular Diets – what is the evidence was held in London on Wednesday 8th January 2014
- BNF was established 45 years ago and exists to deliver authoritative, evidence-based information on food and nutrition in the context of health and lifestyle. The Foundation’s work is conducted and communicated through a unique blend of nutrition science, education and media activities. BNF’s strong governance is broad-based but weighted towards the academic community. BNF is a registered charity that attracts funding from a variety of sources, including contracts with the European Commission, national government departments and agencies; food producers and manufacturers, retailers and food service companies; grant providing bodies, trusts and other charities. Further details about our work, governance and funding can be found on our website (www.nutrition.org.uk) and in our Annual Reports.
- © British Nutrition Foundation 2014