Low carbohydrate diets and diabetes

What is a low carbohydrate diet?

There is no standard definition of ‘low carbohydrate’ used in research.  The term low carbohydrate has been used to describe diets as low as 20g of carbs per day or for those with a more modest reduction in carbs (less than 45% of total energy intake).  However, Feinman et al. have proposed definitions for different levels of carbohydrate restriction to be used in research, outlined in Table 1.

Table 1: Definition of a low carbohydrate diet

In practice, low carbohydrate and very low carbohydrate diets restrict or exclude a wide range of foods including wholegrains, starchy vegetables, legumes, most fruits and some dairy foods, replacing them with non-starchy vegetables, meat, poultry, seafood, eggs, olive oil, coconut oil, nuts, and dairy foods such as cheese, butter, cream and Greek yoghurt.  They often encourage choosing higher fat cuts of meat and full-fat dairy foods.

Are we really eating too many carbs?

Advocates of low carbohydrate diets say that eating too much carbohydrate is contributing to our increasing waistlines and development of chronic disease, including diabetes. However, evidence doesn’t support these claims.

Current government guidelines recommend a carbohydrate intake between 45 and 65% of energy, coming predominantly from low energy density and/or low glycaemic index foods¹.  This range is based on meeting requirements for micronutrients (vitamins and minerals) as well as fat and protein needs. In the 2011-2012 Australian Health Survey the average Australian adult was consuming only 43.5% of energy as carbohydrate, below the lower end of this range².

Furthermore, at the time of the survey, only 30% of Australians were eating the recommended serving of non-discretionary grains, 31% the recommended servings of fruit and less than 4% were meeting the recommended serves of vegetables and legumes³.  Only one-third of grain foods consumed were wholegrain or high fibre products, compared to the recommended two-thirds. Instead the average Australian consumed 35% of their energy from ‘discretionary’ foods (foods which are energy-dense but provide little nutritional value)⁴.  While we are still awaiting additional findings from the more recent 2017-2018 National Health Survey, data on fruit and vegetable consumption show similarly low intakes to the previous survey⁵.  So, it seems that our problem is not that we are eating too many carbs but that we are choosing less healthy options.

What does the research say about low carbohydrate diets and diabetes?

Most of the research on the use of low carbohydrate diets in diabetes management has been conducted in people with type 2 diabetes (T2D) and several systematic reviews and meta-analyses have been published over the past few years. The authors of these reviews have come to similar conclusions: that while lower carbohydrate diets may result in improvements in glycaemic management in the short-term (3-6 months), they don’t appear to be significantly better than higher carbohydrate diets in the longer-term (12 months or more)^6–8. Some studies show greater short-term improvements in glycaemic markers with lower carbohydrate intakes^6–8.  However, research also suggests that very low carbohydrate diets are difficult to sustain, an important consideration when making dietary recommendations⁷.

Furthermore, similar benefits (including weight loss, improvements in glycaemic management and cardiovascular risk factors, and reductions in medication use in T2D) have been achieved with other dietary patterns, including Mediterranean, vegetarian and vegan diets, the latter two typically high in carbohydrate and lower in fat^{9, 10}.  This suggests that while carbohydrate restriction may be one option for the dietary management of T2D, it is not the only option, nor is it necessary for optimising glycaemic management or achieving weight loss.

There is currently limited evidence to support the efficacy and safety of low carbohydrate diets in individuals with type 1 diabetes (T1D). While there is some evidence to show lower HbA1c levels and reduced insulin needs in those following a low carbohydrate diet, most studies are small and uncontrolled^{11, 12}. Only three small randomised controlled studies have been published comparing higher and lower carbohydrate diets, showing varying findings^13–15.  One study found significant reductions in HbA1c and insulin use in the low carbohydrate diet group but no differences in cardiovascular risk markers or glycaemic variability¹⁵.  A second study reported more time spent in euglycaemia, less time in hypoglycaemia and less glucose variability on a low carbohydrate compared to high carbohydrate diet but no differences in mean glucose levels or cardiovascular risk markers. The researchers also noted impairments in cognitive function following the low carbohydrate diet¹⁴.  The third study found reduced glycaemic variability and time spent in hypoglycaemia on a low carbohydrate compared to high carbohydrate diet but no differences in time spent within the target range (3.9-10.0mmol/L) or cardiovascular risk markers¹³.

There are also a number of concerns cited around the use of low carbohydrate diets in those with T1D including the potential for micronutrient deficiencies, growth restriction in children, and the development of ketoacidosis, hypoglycaemia and dyslipidaemia^{11, 16, 17}. One study found that a low carbohydrate diet may reduce the effectiveness of glucagon to correct hypoglycaemia¹⁸. Again, long-term adherence to a restricted diet and the impact on social normalcy also needs consideration¹¹.

Current dietary recommendations for diabetes management

In their 2020 nutrition recommendations, the American Diabetes Association cite a lack evidence to be able to make specific recommendations about the ideal amount of carbohydrate for people with diabetes, including those with T1D, but highlight the importance of monitoring carbohydrate intake and considering the blood glucose response to dietary carbohydrate for improving postprandial glucose management¹⁹.  They also say that while there is some evidence for potential short-term (up to 1 year) benefits for the use of low carbohydrate diets for individuals with T2D and pre-diabetes, that long-term sustainability of such diets is an issue, and that this type of eating plan isn’t appropriate for certain groups including women who are pregnant or lactating, children, those with renal disease or disordered eating behavior, and people taking sodium-glucose cotransporter 2 inhibitors (SGLT2i).  They do, however, recommend limiting refined starches and added sugars and focusing on carbohydrates from vegetables, legumes, fruits, dairy foods and wholegrains. Similar recommendations are made in the Diabetes UK Evidence-based nutrition guidelines for the prevention and management of diabetes²⁰.

Diabetes Australia released a position statement on low carbohydrate eating in 2018, which suggests that low carbohydrate diets may be useful in lowering blood glucose levels and weight in the short-term (up to 6 months) in people with T2D but that further research is needed to establish the effectiveness and safety of low carbohydrate diets for people with T1D²¹.  They also recommend that people with diabetes who wish to follow a low carb diet should do so in consultation with their diabetes healthcare team, including seeking advice from an Accredited Practising Dietitian experienced in diabetes management.

Diabetes UK published a position paper in 2017, with similar recommendations to Diabetes Australia ²². They conclude that low carbohydrate diets can be safe and effective in the short-term in managing weight, and improving glycaemic control and cardiovascular risk in people with T2D but that a low carbohydrate diet should not be regarded as a more superior or a better approach than other strategies.  They also highlight the lack of evidence for the use of low carbohydrate diets in people with T1D and caution against the use of these diets in children due to serious concerns about the impact on growth.

Dietary recommendations for diabetes – the bottom line

Based on research to date, there is insufficient evidence to support specific macronutrient recommendations for individuals with diabetes ²³. Instead, diets varying in macronutrient distribution may be suitable to achieve nutritional goals²³. It is recommended that macronutrient distribution is individualised, taking into account total energy needs and metabolic goals. Emphasis should be on healthy eating patterns containing nutrient-dense foods, with less focus on specific nutrients^{24, 25}.  The eating plan also needs to align with an individual’s overall treatment plan including their medication regimen and physical activity levels²⁵.  Nutrition recommendations should also take into consideration factors including cultural background, personal preferences, co-occurring conditions and socioeconomic factors^{19, 24}.

A major benefit of low carbohydrate diets is the elimination of reduction in intake of processed starches and added sugars – foods that most people, diabetes or not, would benefit from removing from their diet.  Despite all the differences of opinion, most, if not all, health professionals would agree that there are many carbohydrate-containing foods that shouldn’t feature regularly in a healthy diet. This includes refined starches (such as biscuits, cakes, pastries, highly processed breakfast cereals and white bread) and added sugars (such as confectionary and sugar-sweetened drinks).   There is good evidence to support the health benefits of reducing intake of refined starches and added sugars, including for the prevention and management of T2D, and it is likely that most of the benefits of going low carb come from eliminating these foods, rather than restricting overall carbohydrate intake per se.

However, low carbohydrate diets also typically restrict foods (such as wholegrains, legumes, fruits and some vegetables) which are important sources of vitamins, minerals and dietary fibre and are the basis of eating patterns associated with a reduced risk of chronic disease and all-cause mortality.  Furthermore, there is evidence that low carbohydrate diets, particularly those high in animal foods, can worsen insulin sensitivity, increase T2D risk  and increase overall mortality^26–28.   Diets high in saturated fat are similarly associated with increased insulin resistance and risk of T2D^29–32.   And restricting carbohydrate intake and consuming more fat and protein, particularly from animal sources, has been shown to negatively impact the gut microbiome, potentially increasing inflammation and risk of gastrointestinal disease including colon cancer^33–36. There is also increasing evidence for the role of the gut microbiome in the development of obesity and T2D³⁷.

Low carbohydrate diets – role of the diabetes educator

Credentialled Diabetes Educators (CDEs) are able to provide general nutrition advice as part of the Diabetes Self-Management Education (DSME) process.  This includes general/introductory nutrition information on the role of food (including the role of carbohydrate foods) in diabetes management³⁸.  However, if someone with diabetes is considering a low carbohydrate diet, it is important that they are referred to an Accredited Practising Dietitian (APD) for individualised medical nutrition therapy.

It is also important to advise people who are taking diabetes medications, particularly insulin, sulphonylureas and SGLT2i, of the need to seek medical advice prior to starting a low carbohydrate diet. Significantly reducing carbohydrate intake can increase the risk of hypoglycaemia for those taking insulin or sulphoylureas, and these medications will likely need adjustment.  Restricting carbohydrate while taking SGLT2i may lead to euglycaemic diabetic ketoacidosis (DKA) so these medications should be ceased prior to significantly reducing carbohydrate intake^{39, 40}.   Regular blood glucose monitoring should be encouraged, to help with medication adjustment, along with monitoring of ketones in those at risk of DKA.