The discovery of insulin in 1921 revolutionised T1D treatment. The starvation diet was quickly abandoned in favour of insulin therapy, with a cautious reintroduction of carbohydrates. For the next 50 years, dietary guidance for T1D emphasised controlled carbohydrate consumption and avoiding refined sugars. Strict meal plans and "exchange systems" (i.e., one exchange is the equivalent of 15 grams of carbs) were developed to monitor food portions, especially carbohydrates, to maintain blood glucose levels within a safe range.

In the 1960s, concerns emerged about the potential risks of a low carbohydrate, high saturated fat diet for individuals at elevated risk of cardiovascular complications [6]. This led to a shift in dietary recommendations for the general population, advocating for reduced fat intake (to less than 35% of total energy) and increased carbohydrate consumption to approximately 50-55% of total energy intake, predominantly from "complex" foods like peas and whole grains. Gradually, sucrose (plain white table sugar) found its way into recommendations, albeit with a suggested limit of no more than 10% of total energy intake.

In the following years, advancements in insulin delivery technologies, such as insulin pens and pumps, ushered in a new era of precision dosing tailored to an individual’s food preferences through carbohydrate counting and insulin dose calculators. Matching insulin dosage to carbohydrate intake (“functional insulin therapy”) became the gold standard in T1D management, providing patients with increased dietary flexibility.

For years, low-fat diets with limited restrictions on carbohydrates have been endorsed as a cornerstone of healthy eating, not only for individuals with T1D but also for the general population [7]. However, analyses of dietary patterns during the ongoing obesity epidemic reveal a concerning trend: the surge in calorie consumption appears to be predominantly driven by increased carbohydrate intake [8, 9]. Rates of obesity, heart disease, and type 2 diabetes (T2D) have soared [10, 11], with obesity in individuals with T1D reaching alarming levels [3, 12, 13], possibly influenced by the relaxation of dietary restrictions and sedentary lifestyles.

Today, LCDs are a popular option for many people living with T1D [14, 15]. Advocates argue that restricting carbohydrates can lead to more stable blood glucose levels, reduced insulin requirements, and improved overall health outcomes. However, there is still debate among healthcare providers and researchers about the optimal carbohydrate intake for people with T1D and some concerns about the long-term safety and sustainability of LCDs [16].

 

What is a Low Carbohydrate Diet (LCD)?

An LCD is a nutritional approach that limits carbohydrate intake, such as those found in sugary foods, bread, pasta, and some fruits and vegetables. Instead, the diet focuses on consuming foods high in protein and fat.

But what exactly constitutes a "low carbohydrate diet"? There is no universal agreement on the definition, but some suggest that it should be less than 130 grams of carbohydrates per day or less than 26% of total energy intake [17]. However, these levels can vary based on factors like age, sex, and activity levels (see Table 1 for definitions). The specific carbohydrate intake depends on the type of LCD followed. For instance, some diets may restrict carbohydrate intake to as low as 20 grams per day (ketogenic diet or very LCD), while others may allow up to 100-130 grams per day (LCD). This broad range and lack of a clear definition make it challenging to establish recommendations from research studies or derive statements about health effects.

Meat, poultry, fish, eggs, cheese, nuts, seeds, and low-carb vegetables such as spinach, kale, and broccoli are commonly consumed on an LCD. Foods high in carbohydrates, such as sugar-sweetened beverages, candy, cakes, cookies, and certain grains like rice and wheat, are typically avoided or strictly limited. When reducing carbohydrates in the diet, fat and protein intake are increased to meet energy requirements. A fat intake of approximately 70% of total energy intake is a common target for some people on an LCD or ketogenic diet.

 

Table 1. Suggested definitions for different carbohydrate diet types, with recommended carbohydrate intake.

In addition to classifying the diet by the amount of carbohydrates consumed, there are additional types of LCD (note, this may not be a complete list):

1.       Ketogenic Diet: This diet is very low in carbs, high in fat (often around 70% of total energy intake), and moderate in protein. The aim is to get your body into a state of ketosis, where it burns fat for fuel instead of carbohydrates. A typical ketogenic diet might include meals like eggs and avocado for breakfast, a chicken salad for lunch, and grilled fish with vegetables for dinner.

2.       Atkins Diet: This diet starts with a very low-carb phase and allows you to gradually increase your carbohydrate intake as you lose weight. The initial phase of the Atkins diet is very similar to the ketogenic diet.

3.       Paleo Diet: While not strictly an LCD, the Paleo diet eliminates grains, legumes, and processed foods, resulting in a lower carbohydrate intake. The diet focuses on lean meats, fish, fruits, vegetables, nuts, and seeds.

4.       Low-Carb, High-Fat Diet: This diet is similar to the ketogenic diet but may include slightly more protein and carbs. It encourages whole foods and restricts processed foods.

5.       Dukan Diet: This high-protein, low-carb diet is divided into four phases: two weight-loss phases and two maintenance phases. The first phase is very low in carbohydrates, while the later phases allow for more carbohydrates. Some health professionals have criticised the Dukan Diet as too restrictive, potentially leading to nutrient deficiencies.

6.       Zero-Carb Diet: Also known as the carnivore diet, this diet eliminates all carbohydrates and focuses on meat, fish, and eggs. It's a controversial diet and not recommended without medical supervision.

Potential Benefits of LCDs for People with T1D

In recent years, LCDs have gained in popularity, with social media platforms and news outlets touting their potential health benefits, particularly concerning glucose management. But what's the real story behind these claims? In this section, we'll take a closer look at some potential benefits of LCDs and aim to separate fact from fiction. From weight loss to improved blood glucose management, we'll explore the science behind this popular dietary approach and what it could mean for your health.

Weight Loss

First, let's discuss one of LCDs' most well-known potential benefits: weight loss. As obesity becomes increasingly common among people living with T1D [3, 13, 18-20], finding effective ways to manage weight is more important than ever. Studies have shown that obesity in T1D is linked to an increased risk of hospitalisation and complications like retinopathy and macrovascular disease [21, 22]. Compounding these issues are the data mentioned at the beginning of this chapter, indicating worsening HbA1c levels in people with T1D despite increased access to new technologies, such as insulin pumps and CGMs [1].

But how can LCDs help with weight loss?

There are several physiological explanations for why carbohydrate restriction may be beneficial for managing body weight in people with T1D.

1.       Increased Satiety: As the carbohydrate content of the diet is reduced, the relative proportion of energy derived from protein and/or fat increases, leading to increased satiety and often reduced total calorie intake [23]. Popular forms of LCDs like the Atkins diet (20 grams of carbohydrate per day with a gradual increase to 50 grams per day) or Protein Power (≤100 grams of carbohydrate per day) put no formal limit on caloric consumption due to the assumption that higher fat and protein intake will help control caloric intake due to greater satiety [24].

2.       Reduced Insulin Requirements: LCDs may reduce daily insulin requirements (most likely due to the reduced carbohydrate intake) [25, 26], which might facilitate weight loss. The rise in obesity among individuals with T1D may be linked to intensive insulin therapy [13, 18] coupled with a positive energy balance. Insulin is an anti-catabolic hormone that promotes fat accumulation and slows basal metabolism [18, 27, 28]. These effects are enhanced in people with T1D because insulin administration circulates systemically first (i.e., systemic hyperinsulinaemia), disproportionately affecting muscle and adipose tissue compared to hepatic (liver) tissue (i.e., hepatic hypo-insulinaemia) [28, 29]. This contrasts with individuals without diabetes, where endogenous insulin (i.e., insulin produced by the pancreas) must first pass through the portal vein to the liver. The fact that insulin first passes through the liver in people without diabetes has two consequences. First, it suppresses the process of gluconeogenesis, which is where the liver makes glucose from non-carbohydrate sources (fats and/or proteins). This would mean that the liver produces less glucose. Second, only 40–50% of the insulin that has passed through the liver continues into the systemic circulation to act on peripheral muscle and fat tissue, increasing peripheral glucose uptake and suppressing lipolysis [27].

3.       Reduced Hypoglycaemia: If insulin administration is reduced appropriately, LCDs may decrease the incidence of hypoglycaemia and help mitigate weight gain [3]. Acute hypoglycaemia is associated with food cravings, especially for carbohydrates, possibly leading to overeating behaviours [30]. Individuals with T1D often consume more carbohydrates than recommended to treat hypoglycaemia and unnecessarily treat with foods containing protein and fat [31]. Carbohydrates are also consumed in anticipation of hypoglycaemia (so-called ‘hypoglycaemia-defensive snacking’) to prevent going low [13].

 

Greater Insulin Sensitivity or Lower Risk of Insulin Resistance

If you're living with T1D, you may be familiar with the term "insulin resistance”. This means that the cells in your body become less responsive to insulin. This increasingly observed condition [12, 32-34] poses significant health risks, as it's an independent risk factor for microvascular and macrovascular complications, including nerve damage (neuropathy), kidney disease (nephropathy), eye damage (retinopathy), and heart and blood vessel diseases [32, 35-41]. In people with T2D, studies suggest that insulin resistance is linked to increased mortality [42], although a cause-and-effect relationship hasn’t been established.

But what causes insulin resistance in people with T1D? The mechanisms contributing to insulin resistance in T1D and T2D likely overlap. These include increased fat storage within the muscles (intramuscular triglyceride content) [43] and how cells generate energy (mitochondrial dysfunction) [44, 45]. Interestingly, chronic insulin use [29] and exposure to long-acting basal insulins such as insulin detemir or glargine have been linked to insulin resistance [46].

This is where LCDs may come into play. As said, LCDs have been associated with reduced insulin requirements in people with T1D [25, 26]. This reduction in insulin exposure might theoretically lead to increased lipolysis (breakdown of fat), thereby improving insulin sensitivity and further reducing insulin requirements, resulting in a “virtuous cycle” regarding weight management. However, this is based on limited animal and observational studies [47]. Further research is needed to fully understand the effects of LCDs on insulin sensitivity in people with T1D.

 

Greater Time in Range and Improved Glucose Management

The second proposed benefit of following an LCD is the potential to increase time in range and lower HbA1c levels. This is why LCDs are often recommended for individuals with T2D, as they can significantly improve blood glucose management. When consumed, carbohydrates are broken down into glucose in the bloodstream, which can cause significant spikes in blood glucose levels in people with diabetes. Reducing carbohydrate intake can minimise these spikes, leading to more stable glucose profiles.

Research shows that a lower daily carbohydrate intake is associated with improved time in the target glycaemic range in adults with T1D using hybrid closed-loop systems [48]. This suggests that current hybrid closed-loop algorithms may be particularly challenged by higher carbohydrate amounts. Therefore, LCDs may have a particular advantage in improving time in range for people using these systems.

Reports indicate that very low carbohydrate or ketogenic diets (20–50 grams of carbohydrate per day [49]) can result in HbA1c reductions as low as 5.3% in people with T1D. There is also anecdotal evidence suggesting positive effects of very LCDs on glycaemia [15, 50, 51]. However, it is important to note that while lowering HbA1c has well-established, recognised benefits [52], there is little evidence that reducing HbA1c below 7.0% in T1D further decreases overall premature mortality or morbidity risk [53]. Therefore, these very LCDs may be unnecessarily extreme, especially if they increase the risk of hypoglycaemia or diabetic ketoacidosis (DKA) [16].

Improved Mental Clarity

If you've ever struggled with brain fog and fatigue after a carb-heavy meal, you're not alone. When you consume a high-carbohydrate meal, your blood glucose levels can spike and then crash, leaving you feeling foggy and tired. This is due to reactive hypoglycaemia, or postprandial hypoglycaemia, where blood glucose levels drop relatively low within an hour or so after eating.

Many LCD advocates report improved mental clarity and focus. This could be because the rise in blood glucose is lower when consuming an LCD meal, leading to a reduced insulin response. Therefore, decreasing carbohydrate intake may help avoid these fluctuations and maintain stable blood glucose levels during and after meals.

 

Potential Risks of LCDs in People with T1D

While LCDs have gained popularity in recent years, it's important to consider the potential risks associated with this dietary approach, especially for individuals with T1D. One notable downside is the challenge of adhering to this diet, as following an LCD requires a solid understanding of nutrition to prevent nutrient deficiencies. LCDs can also be restrictive, making it difficult to stick to the diet long-term.

Very LCDs are generally discouraged in people with T1D due to concerns over potential complications, including DKA, increased oxidative stress, hypoglycaemia, dyslipidaemia (abnormal levels of lipids in the blood), nutrient deficiencies, and the difficulty of maintaining such diets over extended periods [54, 55]. Here, we will provide a bit more information about the potential risks.

 

Elevated Risk of Diabetic Ketoacidosis (DKA)

One of the most serious risks of an LCD or very LCD for people with T1D is DKA, which occurs when the body produces high levels of blood acids called ketones (see Chapter 1 for more information). DKA can be life-threatening if left untreated, and individuals with T1D who follow an LCD may be at an increased risk due to the reduced amount of insulin, as there are fewer carbohydrates in their diet.

Very LCDs of 30-40 grams of carbohydrates per day have been reported to reduce HbA1c levels to as low as ~5.5% in adults with T1D [14 15, 50]. It's important to note that the studies showing these results have limitations. These limitations include being case reports of individuals or having small sample sizes that are not representative of the wider population [50]. Therefore, caution should be taken when interpreting these findings. Additionally, there is little evidence that achieving an HbA1c lower than 7.0% actually provides additional health or mortality benefits, especially if it increases the risk of hypoglycaemia [56, 57].

Furthermore, very LCDs with ketogenic effects may increase the risk of DKA in individuals taking sodium-glucose co-transporter 2 (SGLT2) inhibitors (predominantly individuals with T2D), as total daily insulin intake drops significantly while using these medications [58]. A particularly concerning danger of combining a very LCD with SGLT2 inhibitors is the increased risk of DKA when glucose concentration is within the normal range (“euglycaemic DKA”). This situation is more challenging to detect, potentially leading to delayed recognition and treatment.

While very LCD may increase the risk of more frequent and severe hypoglycaemia, CGM with alerts for impending hypoglycaemia may help facilitate an LCD or very LCD [59, 60]. Although there is very limited scientific research on this topic, there is anecdotal evidence of people on AID systems following a very LCD experiencing moderate DKA after exercise. Those using an AID may be at an elevated risk as the system stops administering insulin due to the reduced carbohydrates in the diet. This topic needs further investigation, however.

 

Potentially Impaired Response to Emergency Glucagon Treatment

One significant concern with LCDs is the potential risk of an impaired response to glucagon therapy during severe hypoglycaemia. This is concerning because glucagon treatment is a lifesaving tool for managing severe hypoglycaemia in people with T1D (see Chapter 1 for more information). Glucagon works by signalling the liver to release stored glucose into the bloodstream, quickly raising blood glucose levels. Glucagon is typically administered as an injection or nasal spray. It is usually given by a family member, friend, or caregiver trained to administer the medication in an emergency.

Research indicates that people with T1D following a one-week very LCD (≤50 grams of carbohydrate per day) may have an impaired response to glucagon rescue during mild hypoglycaemia [61]. This is presumably due to reduced liver glycogen content caused by long periods of low carbohydrate intake. This effect could be even more pronounced in an individual who has exercised recently, as exercise will further reduce liver glycogen stores. A study by Ranjan et al. highlighted concerns about very LCDs in people with T1D. The study demonstrated that just one week of an isocaloric very LCD (≤50 grams of carbohydrate per day) reduced the treatment effect of glucagon for mild hypoglycaemia [61]. This raises important questions about the reliability of glucagon as a rescue treatment for severe hypoglycaemia in individuals following a very LCD. Further research is needed to determine if glucagon's efficacy as a rescue treatment in severe hypoglycaemia is impaired with very LCD.

 

Elevated Blood Lipids and Cholesterol

Another concern that has been raised with LCDs or very LCDs is that reducing dietary carbohydrates may lead to an increase in saturated fat intake to maintain calorie intake. Studies have shown that very LCDs may thereby lead to elevated total cholesterol and LDL cholesterol levels and dyslipidaemia in individuals with T1D [15, 51]. While the impact of an LCD on blood lipids in people without diabetes is inconclusive and somewhat controversial, more studies show positive than adverse effects on blood lipid profiles [62-71]. In cases with (very) poor glucose control, the benefits of an LCD (i.e., a reduced risk of complications due to improved glycaemic management) probably outweigh the possible adverse effects. Again, we would like to emphasise the lack of high-quality research that has been conducted in this area due to the challenges of undertaking this type of research. Therefore, following such diets needs to be considered on a case-by-case basis.

 

Risk of Nutrient Deficiencies

If you're considering a very LCD to manage your T1D, you should be aware of the potential risks of nutrient deficiencies. By omitting carbohydrate-rich foods like fruits, vegetables, whole grains, and dairy products, you may be missing out on valuable nutrients and fibre essential for optimal health. Evaluation and monitoring of vitamin and mineral status in individuals with T1D following an LCD are encouraged, and multivitamin supplements may be recommended to reduce the risk of deficiencies [72, 73].

 

Risk to Bone Mineral Density and Bone Fractures/Osteoporosis

Studies have shown that a very LCD may negatively affect bone health due to inadequate calcium and vitamin D intake [72]. This is especially concerning for individuals with T1D, as suboptimal glycaemic management has been linked to reduced bone mineral density [74, 75], and there are reports that nearly 20% of people with T1D between the ages of 20-56 years meet the criteria for being osteoporotic [76].

While there is limited data on the long-term effects of LCD in people with T1D, following a ketogenic diet has been shown to reduce bone mineral content in children with epilepsy [77]. It's essential to ensure that a nutrient-rich diet meets energy, vitamin, and mineral requirements for normal growth and development, particularly in children [78]. Failing to meet these needs in growing children may be particularly detrimental and lead to anthropometric and maturation issues, as well as increase the risk profile for cardiovascular disease due to increased fat intake [54].

For people with T1D, it is essential to work closely with a specialist dietitian to assess their overall diet and ensure that all requirements are met [16]. Very LCDs are generally not advised for youth with T1D, and the timing and amount of carbohydrate intake around exercise and physical activity present additional challenges. Current strategies recommend additional carbohydrate-based snacking for prolonged activities to help minimise the risk of hypoglycaemia [79].

 

Eating Disorders and Food Fixation

Managing T1D often places a lot of emphasis on food selection and portion size. Adding dietary restrictions, such as an LCD or very LCD, can increase the risk of becoming overly focused on food. Research has shown that individuals with T1D have a greater risk of developing eating disorders or other psychological disorders compared to people without diabetes [80-82].

Long-term adherence to LCDs has shown varying results [15, 83, 84], and sustainability is a critical component of any dietary recommendation. Given cultural norms, food's role in celebrations, restaurant meals, and travel, dietary restrictions may be difficult to manage in the short and long term. An LCD diet may also be socially isolating and increase the risk of developing disordered eating behaviours.

 

Conclusions

Rising obesity rates, the low number of individuals meeting HbA1c targets, and impaired metabolic health in people with T1D, despite advancements in insulins and other diabetes technology [1, 2], highlight the need for innovative and practical lifestyle changes. Sensible LCDs, with less than 130 grams of carbohydrates per day, may represent a strategy to improve glucose profiles and metabolic health.

However, managing blood glucose in individuals with T1D is complex. That’s why any strategy must be achievable, enhance the quality of life, and ensure the enjoyment of food while considering social and psychological factors. After all, food is more than just fuel – it's a source of pleasure and connection.

Nutritional guidance for people with T1D should be based on appropriate scientific evidence using sound methodologies. While LCDs may offer benefits for some individuals with T1D, it's essential to consider the potential risks and limitations and collaborate closely with healthcare professionals to ensure a balanced and sustainable approach to dietary management (see Table 2).

So, is an LCD right for you? The answer is not black and white, and it's crucial to work with your healthcare team to make informed decisions about your health. But one thing is clear: the use of LCDs is an intriguing area that warrants more high-quality research and attention in people with T1D!

Table 2. Potential pros and cons of low carbohydrate diets (LCD) for individuals with T1D

Pros of Low Carbohydrate Diets:

Cons of Low Carbohydrate Diets: