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expert reaction to study looking at carbohydrate intake and health

A study published in the Lancet Public Health looked at levels of carbohydrate intake, health and longevity.


Catherine Collins RD FBDA, NHS dietitian, said:

“No aspect of nutrition is so hotly contended on social media than the carb versus fat debate, despite the long term evidence on health benefits firmly supporting the higher carb argument.  Yet supporters of the cult of Low Carb High Fat (LCHF) eating, itself based on a lifestyle choice and the flimsiest of evidence supporting benefit, will no doubt disagree with this newest research on the subject.

“Such a stance is at odds with advice from WHO and government health bodies globally – including the UK’s Public Health England – that recommend a carb intake to provide around half our daily calorie needs.  This approach reduces our calories from fat to around a third of total calories, with protein making up around 20% of the total, and alcohol calories sneaking in, too, where consumed.

“So it comes as no surprise that this ARICs study observing the dietary habits and health issues of 15,000 middle aged people found exactly the same beneficial reduction in mortality on an ‘average carb diet’ as previously published large scale population studies have shown.  Not surprising, given that nutrition research from the 1950s onwards has helped define and refine these current global nutrition guidelines and their recommendations.

“In this study, low carb diets (<40% calories from carb) and very high carb diets (where more than 70% of calories come from carbohydrate) were both associated with an increased population mortality.

“On an ‘average’ 2000 kcal-a-day intake, a diet of 30% calories from carbs equates to only 150g a day, with sugars (natural or ‘added’) contributing around 50g of that total.  With a mere 100g of complex carb a day to play with, a lower intake of cereals, grains, and starchy vegetables is inevitable.  Such diets compromise the essentials of a healthy diet – dietary fibre to prevent constipation, support control of blood sugar and lower blood cholesterol levels, and the maintenance of healthy gut flora which is now considered to be a major player in health and disease.

“The sheer variety of nutrients achieved with a plant based, carb-rich diet cannot be replicate on a restricted carb one.  Reducing carbs means dietary fat intake must rise – and in doing so, this increases not only the post-meal cardiac risk associated with higher circulating fat levels, but also boosts saturated fat intake, too.

“Following a very low carb diet prevents the inclusion of plant based proteins like beans and other pulses, so the diet distorts towards a higher saturated fat intake derived from higher consumption of animal proteins like meat and cheese.  In this study, the lower the daily carb intake, the higher the intake of animal fats, with those on the lowest carb diets consuming twice as much animal fat than those with the highest carb intakes.  It’s worth bearing in mind that the greater the animal protein intake, the higher the saturated fat content of the diet.  This association is often overlooked by those promoting low carb/high protein diets for health.

“But even ‘healthier’ fats of the omega-6 polyunsaturates group have pro-inflammatory effects in the body, increasing cell inflammation and damage.  The sheer amount of saturates and omega-6 polyunsaturates consumed on a very low carb diet distorts the usually well tolerated effect of different fats on health with a more balanced approach.

“In summary, this paper will disappoint those who, from professional experience, will continue to defend their low carb cult, but contributes to the overwhelming body of evidence that supports a balanced approach to calorie intake recommended globally by public health bodies.

“Two points are worth noting from this research – first, that weight gain associated with increasing age is independent of the type of diet followed.  Following a high carb diet does not lead to a propensity for weight gain, another factoid promoted in the LCHF community.

“Secondly, and more importantly, it raises some timely and important issues about whether people with diabetes should be using a low carb or very low carb approach as long term treatment.  Diabetes of any type is characterised by metabolic abnormalities that increase the risk of cardiovascular disease and other diseases of ageing beyond those of the general population.  The feting and promotion of GPs promoting often bizarre low carb diets to manage diabetes has gained much media traction from the uncritical support of the LCHF movement that promote over-hyped personal beliefs as ‘fact’, and run counter to the dietary recommendations of Diabetes UK.  If nothing else, this study provides some redress to this one-sided debate, and adds caution to such practice for long term management.”


Prof Nita Forouhi, MRC Epidemiology Unit, University of Cambridge, said:

“This study takes on the contentious nutrition issue of low-carb diets for health head-on, and peels away layers of complexity with robust methods and thorough analyses to unravel a clear message: moderate carbohydrate intake of 50-55% of total energy, and not a low-carb diet, is optimal for longevity.  This finding is spot-on in line with the Public Health England dietary guidelines in the UK.

“Current guidelines have been criticised by those who favour low-carb diets, largely based on short term studies for weight loss or metabolic control in diabetes, but it is vital to consider long-term effects and to examine mortality, as this study did.  For populations where carbohydrate intakes are much higher, such as in Asia, there is also a message – there is a U-shaped relationship of carbohydrate intake with mortality, with very high intakes also adversely related.

“Some critics will argue that this is not clinical trial evidence but observational, yet doing clinical trials of diets for long-term outcomes is not realistic, and this observational study was rigorous.  It was large in size, included men and women of different ethnic groups, and applied modern advanced statistical approaches including accounting for confounding factors and checking for potential reverse causality.  The researchers also replicated their findings in a meta-analysis of all available published studies from around the world that examined this question.

“A really important message from this study is that it is not enough to focus on the nutrients, but whether they are derived from animal or plant sources.  When carbohydrate intake is reduced in the diet, there are benefits when this is replaced with plant-origin fat and protein food sources but not when replaced with animal-origin sources such as meats. Many low-carb diet regimes do not make this distinction, but it is important.”


Dr Ian Johnson, Nutrition researcher and Emeritus Fellow, Quadram Institute Bioscience, said:

“This is a large, well-conducted study, from an experienced and reliable group, in which the diets of over 15,000 adults were assessed, and then deaths from all causes were recorded over the next 25 years.  Of course this research strategy has some disadvantages in that diets can change a lot over a lifetime, and simply reporting mortality tells us little about overall health and disease.

“Nevertheless the risk of death was convincingly shown to be lowest in those participants who obtained about half their calories from carbohydrates.  The risk was highest in those who ate a low carbohydrate diet in which the calories were replaced mostly by fats and proteins from animal foods.  The researchers also showed that their results were consistent with a large number of previously published studies.

“The national dietary guidelines for the UK, which are based on the findings of the Scientific Advisory Committee on Nutrition, recommend that carbohydrates should account for 50% of total dietary energy intake.  In fact, this figure is close to the average carbohydrate consumption by the UK population observed in dietary surveys.  It is gratifying to see from the new study that this level of carbohydrate intake seems to be optimal for longevity.

“These findings emphasise that there is nothing to be gained from long-term adherence to low-carbohydrate diets rich in fats and proteins from foods of animal origin.  However, it is not just the quantity of dietary carbohydrates that matters, but also their sources and composition.  There is good evidence from other research to show that most should come from plant foods rich in dietary fibre and intact grains, rather than from sugary beverages or manufactured foods containing high levels of added sugars.”


Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:

“Carbohydrates are made by plants and are the main source of food energy in most human diets.  This observational study in the USA reports modest differences in risk of dying over a 25 year follow up among those who reported consuming diets with low (less than 40% energy) or high (greater than 70% energy) carbohydrate intakes.  An intake of carbohydrate providing 50-55% of the energy was associated with the lowest risk.

“Carbohydrates normally provide between 45-50% of energy in most Western countries including the UK.  The study also includes previously published studies in a meta-analysis which found the lowest risk was associated with a carbohydrate intake close to 46% of the energy.  Most dietary guidelines suggest carbohydrates supply at least 50% of the energy intake.

“The authors found higher risk of dying to be linked to diets high in meat and animal fat and low in carbohydrates (similar to some popular low carbohydrate high protein diets).  When some of the carbohydrate was replaced by proteins and fat from plant sources the risk of dying was found to be lower.  However, the study was unable to make a distinction between the type of carbohydrates consumed i.e. sugar and refined starchy foods versus unrefined cereals.  This is important because unrefined carbohydrates are the main source of dietary fibre and refined carbohydrates have a lower content several of the B vitamins, unless fortified.

“A major limitation of this study is the use of a food frequency questionnaire to assess dietary intake.  This method underestimates both energy and fat intake.  Energy intakes were estimate to be around 1600 kcal/day whereas adult men and women need 2400 and 2000 kcal/day to remain in energy balance.  Furthermore, it is well known that the overweight and obese mis-report food intake, leading to the observation that low reported energy intakes are associated with obesity!  One explanation for the finding in this and the other USA studies is that it may reflect the higher risk of death in the overweight/obese, who may fall into two popular diet camps – those favouring a high meat / low carbohydrate diet and those favouring a low fat / high carbohydrate diet.

“The accompanying editorial takes a swipe at USA dietary guidelines, seeming to blame them for the rise in obesity, which seems baffling, as most Americans don’t follow them.  However, both the main article and the editorial agree extreme intakes are undesirable and that the bottom line is that a balanced diet providing about half of the food energy from carbohydrates is best for health.”


* ‘Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis’ by Sara B Seidelmann et al. was published in The Lancet Public Health on Thursday 16 August 2018. 


Declared interests

Catherine Collins: “No conflict of interest.  Have used very low carb diets in patients with intractable epilepsy.”

Prof Nita Forouhi: “I am a member of the Joint SACN/NHS-England/Diabetes-UK Working Group on ‘lower carbohydrate diets compared to current government advice for adults with type 2 diabetes’. Views expressed are my own, not the Group’s.”

Dr Ian Johnson: “ITJ was a member of the SACN Carbohydrates Working Group 2009 – 2015.”

Prof Tom Sanders: “Honorary Nutritional Director of HEART UK.  Scientific Governor of the British Nutrition Foundation.  He is now emeritus but when he was doing research at King’s College London, the following applied: Tom does not hold any grants or have any consultancies with companies involved in the production or marketing of sugar-sweetened drinks.  In reference to previous funding to Tom’s institution: £4.5 million was donated to King’s College London by Tate & Lyle in 2006; this funding finished in 2011.  This money was given to the College and was in recognition of the discovery of the artificial sweetener sucralose by Prof Hough at the Queen Elizabeth College (QEC), which merged with King’s College London.  The Tate & Lyle grant paid for the Clinical Research Centre at St Thomas’ that is run by the Guy’s & St Thomas’ Trust, it was not used to fund research on sugar.  Tate & Lyle sold their sugar interests to American Sugar so the brand Tate & Lyle still exists but it is no longer linked to the company Tate & Lyle PLC, which gave the money to King’s College London in 2006.  Tom also used to work for Ajinomoto on aspartame about 8 years ago.  Tom was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain.  Tom has previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and Tom is a member of the Programme Advisory Committee of the Malaysian Palm Oil Board.  In the past Tom has acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC.  Tom’s research on fats was funded by Public Health England/Food Standards Agency.”

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