FACTS BEHIND THE HEADLINES Is a high-carb diet 'poison' to diabetics

bs_bs_banner FA C T S B E H I N D T H E H E A D L I N E S DOI: 10.1111/j.1467-3010.2012.02003.x Is a high-carb diet ‘poison’ to diabetics? S. Stanner British Nutrition Foundation, London, UK A recent headline in The Times suggesting carbohydrate may be poisonous to diabetics (The Times 2012) no doubt caused concern among many of the 2.9 million people diagnosed with diabetes in the UK. Although alarmist, as this article described possible benefits of low-carbohydrate diets rather than any dangerous effects of carbohydrate per se, the headline has raised awareness of the continued debate about the optimal macronutrient content of weight loss diets for diabetes sufferers. Why the interest in low-carbohydrate diets? Nutritional therapy is fundamental for the effective management of type 1 and type 2 diabetes, playing a vital role in glycaemic control. This helps to reduce the risk of long-term complications of uncontrolled diabetes – namely diabetic retinopathy, neuropathy and cardiovascular disease (CVD). Weight management too remains a key aspect of the treatment of type 2 diabetes, as 90% of sufferers are overweight or obese. Some studies have shown low-carbohydrate diets to be effective as a means of losing weight, as well as optimising blood glucose control in people with type 2 diabetes, at least in the short-term. However, there has been much debate about whether they are safe and effective as a long-term strategy. This article will consider the potential benefits vs. risks of low-carbohydrate/high-protein diets and the possible implications for dietary recommendations for people with diabetes. What is a low-carbohydrate diet? Similarly, the UK Department of Health (DH 1991) recommends that carbohydrate makes up 45–60% of daily energy intake (DH 1991). This equates to around 225–300 g of carbohydrate in a 2000 kcal (8400 kJ) diet. A diet containing less than 130 g of carbohydrate a day is regarded by the American Diabetes Association (ADA) as ‘low carb’. This is the minimum daily intake of digestible carbohydrate recommended by the ADA, based on providing adequate glucose as the fuel for the central nervous system without reliance on glucose production from ingested protein or fat. Restrictions and food eliminations causing carbohydrate to fall below this intake raise concerns about insufficient intake of certain essential micronutrients and fibre that may require dietary supplementation. Although there is also no agreed definition among researchers regarding the amount of carbohydrate in low-carbohydrate diets, a critical appraisal by Accurso et al. (2008) suggested the following definitions, relating to a 2000 kcal diet: • low-carbohydrate diet: less than 26% energy (or <130 g); • moderate-carbohydrate diet: 26–45% energy (or 130–225 g); • very low-carbohydrate ketogenic diet: less than 6% energy (or <30 g). Large reductions in dietary carbohydrate are, by necessity, paralleled with increases in the proportion (and often actual level) of energy from fat and protein. Low-carbohydrate diets may therefore be described as high-protein diets and can differ in their fat content and fatty acid profile. Those diets leading to increases in absolute fat intake may be associated with an increased intake of saturated fatty acids. There is no formal definition of a low-carbohydrate diet. The Dietary Guidelines for Americans (2010) recommend a diet composed of 45% to 65% carbohydrate for the general population (USDA & USDH 2010). Are low-carbohydrate diets effective for weight loss in people with diabetes? Correspondence: Sara Stanner, Science Programme Manager, British Nutrition Foundation, High Holborn House, 52-54 High Holborn, London, WC1V 6RQ, UK. E-mail: [email protected] There has been considerable interest in the role of manipulating dietary macronutrient composition as a mechanism for achieving weight loss. Comparisons of the effectiveness of low-carbohydrate/high-protein diets vs. low-fat diets among the general population have shown the former to be as effective, if not more, in 350 © 2012 The Author Journal compilation © 2012 British Nutrition Foundation Nutrition Bulletin, 37, 350–354 High-carb diet reducing weight in the short-term (six months). In a systematic review of 13 randomised controlled trials in adults with a mean body mass index of 28 kg/m2, Hession et al. found significantly greater weight loss (and improved cardiovascular disease risk factors) in those following low-carbohydrate compared to low-fat diets at six months, and better, or similar, weight loss at one year (Hession et al. 2009). The authors highlighted higher attrition rates in the low-fat compared with the low-carbohydrate groups suggesting a patient preference for the low-carbohydrate/high-protein approach but emphasised the need for longer term studies demonstrating weight loss maintenance after one year. A recent systematic review and meta-analysis of 1141 obese patients from 23 studies confirmed this finding, with low-carbohydrate diets being associated with significant decreases in bodyweight [-7.04 kg (95% CI -7.20/-6.88)], body mass index [-2.09 kg/m2 (95% CI -2.15/-2.04)] and abdominal circumference [-5.74 cm (95% CI -6.07/-5.41)] (Santos et al. 2012) in the shortterm but the authors again concluded that the effects of such diets on long-term health remain unknown. Weight loss on the low-carbohydrate diet is probably caused by a combination of restriction of food choices and the enhanced satiety produced by the high-protein content. There may also be an additional effect of ketosisinduced appetite suppression. Thus, the main mode of action is reduction in energy intake caused by carbohydrate restriction (Bravata et al. 2003). The effectiveness of low-carbohydrate diets has also been investigated in people with diabetes, although findings are more conflicting. In a review of six studies investigating the effects of hypocaloric reducedcarbohydrate diets in people with type 2 diabetes, Dyson (2008) concluded that there is evidence that such diets promote reductions in bodyweight but emphasised that conclusions are hampered by the lack of control groups and short duration of most of the studies. In a meta-analysis of 13 studies looking at the effects of restricted carbohydrate diets in type 2 diabetes, Kirk and colleagues (2008) showed improvements in haemoglobin A(1c) (HbA1c), fasting glucose and triglycerides but an equivocal effect on weight. Similarly, a systematic review of randomised trials greater than 12 weeks comparing low-carbohydrate diets with low-fat diets, usualcare diets or low-glycaemic index diets on weight reduction in type 2 diabetes found five studies showing greater weight reduction with low-carbohydrate diets but four of these demonstrated no significant difference (Castañeda-González et al. 2011). The longest intervention studies did not show any difference in weight change between the different diets. The recently pub- © 2012 The Author Journal compilation © 2012 British Nutrition Foundation Nutrition Bulletin, 37, 350–354 351 lished findings from the Diabetes Excess Weight Loss (DEWL) trial also supported this view, i.e. that diets with different macronutrient contents can lead to similar weight loss within the context of a reducedcalorie diet. This was a randomised controlled trial of higher protein vs. high carbohydrate over 2 years in type 2 diabetes (Krebs et al. 2012). This trial, which involved 419 overweight or obese participants (70% of whom completed the trial), found that those following a lowfat, high-protein diet (30% energy as protein, 40% energy as carbohydrate and 30% energy as fat) or a low-fat, high-carbohydrate diet (15% energy as protein, 55% as carbohydrate, 30% energy as fat) lost weight (2–3 kg) and reduced their waist circumference (2–3 cm) over 12 months and largely maintained this weight loss for the following 12 months. However, they found no significant difference between the two diets in these diabetic subjects. Although some limitations of this trial are evident, for example, the high dropout rate and lack of robust biomarkers of dietary adherence, the authors suggested that in a free-living situation, with realistic and sustainable support in a community setting, prescription of an energy-reduced low-fat diet with either increased protein or carbohydrate results in similar modest losses in weight and waist circumference over a 2-year period. They therefore emphasised the need for flexibility with macronutrient compositions for weight management in those with diabetes. Effect of low-carbohydrate diets on glycaemic control and other CVD risk factors Significant weight reduction for overweight or obese people with type 2 diabetes will improve glycaemic control and cardiovascular risk factors. Lowcarbohydrate/high-protein diets have been shown to improve lipid and glycaemic profiles in obese nondiabetic individuals (Skov et al. 1999; Due et al. 2004; Layman et al. 2005; Claessens et al. 2009). Studies specifically in type 2 diabetes are more limited and generally short, but improvements have been demonstrated in blood sugar levels. For example, a recent trial of 61 adults with type 2 diabetes randomised to either a lowcarbohydrate diet or a traditional low-fat diet, found an improvement in the blood sugar levels of those in the low-carbohydrate group within six months of starting the diet. The average insulin dose was significantly different in the two groups at six months (P = 0.046), with around a third less insulin required for those on the low-carbohydrate diet (Guldbrand et al. 2012). Some studies also suggest that low-carbohydrate diets can lead 352 S. Stanner to improvements in HbA1c levels. Gannon and Nuttall (2011) conducted direct comparisons of highprotein, low-carbohydrate diets compared with highcarbohydrate, low-protein diets in subjects with type 2 diabetes. They found that high-protein, lowcarbohydrate diets reduced fasting plasma glucose, 24-hour glucose area under the curve and HbA1c. However, evidence to date in those with diabetes is mainly from short-term trials lasting one year or less (Diabetes UK 2011a) and improvements have not been consistently demonstrated. Larsen et al. (2011) compared low-fat diets that were high in protein (30% energy) or high in carbohydrate (55% energy) in overweight and obese subjects with type 2 diabetes over 12 months. They found both types of diet to reduce HbA1c levels at 12 months but found no significant difference between the two groups. Both groups also experienced reductions in weight, serum triglycerides and total cholesterol and increases in high density lipoprotein (HDL) cholesterol but no change in blood pressure. Overall, they found no superior long-term metabolic benefit of a high-protein diet over a high-carbohydrate diet in the management of type 2 diabetes (Larsen et al. 2011). Studies to date, including the trial by Larsen and colleagues, have used intensive interventions and/or provided a significant proportion of participants’ food, making the translation of such effects to the general diabetic population difficult. The recent DEWL trial comparing low-fat, high-protein or high-carbohydrate diets over two years, using deliverable group-based interventions in a community setting, also found no significant differences in HbA1c levels or in lipids and blood pressure (Krebs et al. 2012). Are low-carbohydrate diets safe for diabetics? One of the main side effects associated with a lowcarbohydrate diet is the risk of hypoglycaemia (low blood sugar), which can be even greater during physical activity. It is therefore important to consider overall diabetes control and monitor blood glucose levels closely and adjust medication accordingly. Other potential side effects of a low-carbohydrate diet include headaches, poor concentration and constipation (Diabetes UK 2011a). Concern has been raised about the high-protein and sometimes high saturated fat content of lowcarbohydrate diets. The increased protein intake is of particular concern for those with microalbuminuria or established diabetic nephropathy as it could cause a deterioration in renal function. However, studies have not demonstrated changes in renal function at least over the short-term. For example, the DEWL trial did not find any change in microalbuminuria or any significant rise in creatinine among participants with modest increases in protein intake over two years. If individuals following a low-carbohydrate diet compensate for the lack of carbohydrates by consuming foods that are high in fat, particular saturated fatty acids, there may be adverse effects on cholesterol levels and subsequent CVD risk. A high-fat intake may also provoke inflammation in the arteries of diabetics, which plays a role in coronary artery disease and other manifestations of atherosclerosis. A recent study by Harte et al. (2012) showed that high-fat meals led to large numbers of gut-derived bacteria in the blood, with higher numbers in type 2 diabetics caused by intestinal permeability (or a ‘leaky gut’). This increased permeability of the gut lining seems to allow a greater transfer of bacterial fragments from the intestine into the blood. As a result, inflammatory reactions are triggered, which can lead to conditions such as heart disease and may help explain why diabetics are at a greater risk of developing heart problems, weight gain and cardiovascular conditions. However, most studies have not shown adverse effects of low-carbohydrate diets on blood cholesterol or other CVD risk factors over the short-term and low-carbohydrate/high-protein diets can be implemented in the context of a relatively low-fat/low saturated fatty acid intake. Current evidence, therefore, indicates that low-carbohydrate diets in the short-term (up to one year) are safe for people with diabetes to follow. However, studies of longer duration are needed to determine the safety of such diets in the longer term. Implications for dietary recommendations for those with diabetes Dietary carbohydrate is an important component of a healthy, varied diet. The majority of the carbohydrate derived from a normal diet reaches the body’s peripheral tissues as glucose. Glucose is utilised as a metabolic fuel and is the obligate energy source for the brain, renal medulla and erythrocytes. Carbohydrate-containing foods provide many important nutrients such as the B-vitamins and minerals, as well as fibre (IOM 2002). In the UK, dietary advice for people with diabetes is similar to that for the general population – a varied diet, which includes carbohydrate from fruits, vegetables, wholegrains and pulses (and thus of high fibre and low glycaemic index), reduced salt intake, low-fat dairy products, lean meats, fish, including oil-rich fish and © 2012 The Author Journal compilation © 2012 British Nutrition Foundation Nutrition Bulletin, 37, 350–354 High-carb diet other sources of protein (i.e. lower fat choices to control intake of saturated fatty acids) (NCCCC 2008). In the short-term, low-carbohydrate diets may be beneficial for people with diabetes (e.g. helping to achieve a healthy weight, improving glycaemic control and increasing HDL cholesterol levels) but there is no evidence that such diets are more successful over the longterm than traditional approaches. Recently, both the ADA and Diabetes UK have recognised that a range of approaches to weight loss should be considered, with the overall aim that energy intake should be less than energy expenditure (ADA 2008; Diabetes UK 2011b). The ADA nutrition recommendations published in 2008 state that either low-carbohydrate or low-fat calorierestricted diets may be effective in the short-term (up to one year) (ADA 2008). Similarly, Diabetes UK published a position statement in 2011 (Diabetes UK 2011a), which concluded that low-carbohydrate diets may be effective in facilitating weight loss in people with type 2 diabetes in the short-term, but there is no evidence that this approach is more successful in the longterm than any other approach (Davis et al. 2009). Diabetes UK also changed their advice in 2011 to state that low-carbohydrate diets, in people with type 2 diabetes, can lead to improvements in HbA1c in the shortterm (i.e. up to one year) (Diabetes UK 2011a). Therefore, the most suitable means of achieving weight loss and good glycaemic control should be negotiated between the patient and their dietitian. Although there is now good evidence to indicate it is safe for diabetics to consume a low-carbohydrate diet in the short-term (i.e. up to one year), there is a lack of evidence on the effectiveness and safety of such a dietary regimen in the longer term. There is also still a need for more research to determine the effectiveness of different degrees of dietary carbohydrate restriction on weight, glycaemic control, hypertension and lipid profile in people with type 2 diabetes (Worth & Soran 2007), as well as to investigate the long-term health impact of these diets (Davis et al. 2009). With this in mind, a cautionary approach to recommending low-carbohydrate diets for those with diabetes seems prudent until adequate evidence is available on which to base further guidance. 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