Introduction

A healthy diet is important for healthy living, as the old saying goes, "You are what you eat." In today's world, a healthy lifestyle is even more important due to the increasing incidence of diabetes and obesity. According to the International Diabetes Federation (IDF), approximately 425 million people worldwide suffer from diabetes, and if current trends continue, 629 million people aged 20–79 years will have diabetes by 2045. Nutrition is key in the prevention of type 2 diabetes and obesity, but there are no evidence-based data defining the best dietary approach to prevent and treat these chronic diseases (1).

Low-carbohydrate diets (low-carb diet, LCD) are popular among physicians and patients, but the appropriateness of reducing carbohydrate intake in obese patients and patients with diabetes is still being discussed. Despite evidence that reducing carbohydrate intake lowers body weight and improves blood glucose level in patients with type 2 diabetes, little data are available on long-term sustainability, safety, and effectiveness (2).

In recent decades, LCD and ketogenic diets have become popular in weight loss, not only in the scientific community but also in the lay community. Books on this topic have become best-selling specialized books. These dietary approaches are effective for weight loss, but increasingly there is evidence that caution is needed, especially when these diets are followed by individuals at a very young age or with certain diseases (3, 4).

In the past, when insulin was not yet available for diabetes treatment, LCD was recommended as primary therapy, but nutritional recommendations from that time were different from the values used today (5). Many low-carbohydrate diets appeared, for example, the Atkins diet, Zone diet, and South Beach diet (6). The term low-carbohydrate diet covers a very heterogeneous set of nutrients, has no unambiguous definition, and clinical studies often do not provide information on carbohydrate content and quality. For this reason, it is difficult to compare results from different scientific studies. A typical diet usually consists of 45%–50% of daily energy intake from carbohydrates. LCD provide less than 45% of daily energy intake from carbohydrates (6, 7). According to some studies, LCD usually contain less than 100 g carbohydrate intake per day, with total nutrient intake consisting of 50%–60% fats, less than 30% carbohydrates, and 20%–30% proteins (8).

Diets with very low carbohydrate content are ketogenic diets, in which daily carbohydrate intake is less than 50 g (6). After a few days of drastically reduced carbohydrate intake, energy production begins to depend on fat burning with increased production of ketone bodies, i.e., ketosis develops. Ketone bodies (intermediates produced in the liver, which include acetone, acetoacetic acid, beta-hydroxybutyric acid) are an alternative energy source to glucose for the central nervous system (9). Increased production leads to higher ketone-body levels (0,5–3,0 mmol/l), therefore the ketogenic diet can be indicated for the treatment of refractory epilepsy (10,11), including children with GLUT1 deficiency (glucose transporter) (12). People following ketogenic diets lose weight due to lower insulin levels, a diuretic effect, and reduced hunger. The most common negative effect is the “keto flu,” a temporary condition with symptoms such as dizziness, fatigue, sleepiness, and constipation (7, 13). In most very low-carbohydrate diets, food selection is strictly restricted, which leads to inadequate nutrition. Long-term adherence also causes limited food choices (14).

Low-carbohydrate diet and diabetes mellitus type 1 and type 2

Diabetes mellitus type 1 is an autoimmune disease characterized by destruction of pancreatic beta cells and absolute insulin deficiency. In type 2 diabetes, the organism either resists the effects of insulin, which regulates blood glucose levels, or does not produce enough insulin to maintain normal blood glucose level (15, 16).

Affected patients have disturbed glucose metabolism and are prone to chronic complications caused by hyperglycemia and acute complications from hypoglycemia and ketoacidosis. Standard treatment consists of daily insulin administration and dietary adjustment. The best way to prevent diabetic complications is to control glycemia and achieve normal glycated hemoglobin (HbA1c 7.0% or 53 mmol/mol), which is considered the primary goal in diabetes treatment. According to the American Diabetes Association (ADA), LCD are those in which total carbohydrate intake is less than 130 g/day or 26%. Before insulin was known, strict low-carbohydrate diets for diabetes treatment were accepted (10 g/day) (15). A recent large observational study (1020 patients with type 1 diabetes) reported that lower total carbohydrate intake was associated with lower HbA1c levels (17). In type 1 diabetes, blood glucose fluctuations are caused by carbohydrate intake from food and insulin mostly from exogenous sources (18). Reducing dietary carbohydrates lowers dosing errors when determining the required amount of exogenous insulin and reduces blood glucose variability (16). As a result, hyperglycemic and hypoglycemic episodes become less frequent and less severe, as well as overall insulin requirements are lower (20). In accordance with recommendations of the National Health and Medical Research Council (NHMRC) for type 1 diabetes treatment, patients are advised to consume carbohydrates at 45%–65% of total energy intake. These recommendations are also supported by healthcare experts (21, 22).

Low-carbohydrate diet and gestational diabetes

Gestational diabetes is diabetes diagnosed for the first time during pregnancy. Gestational diabetes causes increased blood glucose levels, which can affect pregnancy and the child's health. Future mothers can manage gestational diabetes by eating healthy foods, exercising, and using medication when necessary. Blood glucose control helps keep the pregnant woman and baby healthy and prevents difficult labor. In women with gestational diabetes, blood glucose usually returns to the optimal value (4.0–5.9 mmol/l) shortly after delivery. If a pregnant woman had gestational diabetes, the risk of developing type 2 diabetes is higher (23).

Based on diagnostic criteria, healthy nutrition is the universal first-line treatment of gestational diabetes (24, 25). Based on decades of clinical experience, in which diet was followed as the basis of treatment for gestational diabetes, LCD was the first choice (26, 27, 28).

Low-carbohydrate diet and obesity

The goal of LCD is usually weight loss and planned weight reduction, which are caused by restricted glucose intake. Glucose is the main source of energy in the body. If blood insulin level drops due to restricted glucose intake, the organism is forced to oxidize fats (29). In addition, diets include higher intakes of proteins and fats, which provide more energy to the body and create a longer feeling of satiety (30). Exact percentages of these macronutrients differ depending on the diet, but usually the diet is oriented either toward a high-fat or a high-protein low-carbohydrate diet. This means 40% of daily energy intake from fats in high-fat, low-carbohydrate diets, or 25% of daily energy intake from proteins in low-carbohydrate diets with high protein (30, 31). Regardless of the specific LCD pattern, the diet consists of greater consumption of eggs, meat, and low-starch vegetables. Conversely, fruit, grains and starchy vegetables are often completely excluded from the diet (29).

Popular low-carbohydrate diets

Atkins diet: In his book (Diet Revolution), Dr. Atkins makes weight-loss recommendations based on an appraisal of scientific literature. The program starts with substantial carbohydrate restriction (<20 g/day) in the induction phase. Then, the degree of restriction is adjusted individually, depending on the person’s response to the diet. The weight-loss program includes detailed instructions for gradually adding small amounts of carbohydrates. The program aims to help people find their individual carbohydrate tolerance levels. The Atkins diet has four phases: induction, continuation, pre-adjustment, and maintenance. During induction, people on the diet are instructed to restrict carbohydrates to 20 g/day. If weight loss reaches the target level, phase 1 lasts about 2 weeks. In the second phase, people begin adding a variety of foods and usually increase carbohydrates by 5 g/day each week. During the pre-adjustment phase, foods containing carbohydrates from the list of acceptable foods are added until weight is considered balanced. The allowed carbohydrate amount is individualized. In the final phase, the tolerated carbohydrate amount is linked to weight balance in phase 3. Restarting phase 1 is recommended to treat relapses (32).

Zone diet (Zone diet): The book The Zone: A Dietary Roadmap gives advice on losing weight based on ratios of carbohydrates, proteins and fats, as well as “favorable” and “unfavorable” types of each (33). Recommendations focus on proteins and carbohydrates that have a low glycemic index and fats that are primarily monounsaturated and rich in omega-3 fatty acids. In contrast, the diet has low fat content, and excludes other sources of saturated fat, such as butter, most vegetable oils (except olive oil and similar oils), grains, added sugars, starchy vegetables, and egg yolks. The diet consists of five meals and two snacks per day, each with “good” carbohydrate, protein, and a specific amount of fat. People who choose the diet have food options to choose from. They eat within an hour after waking and do not have long intervals between meal occasions, and do not take more than 500 calories at one time. The goal is to adjust the diet to protein needs with a target daily energy intake consisting of 40% carbohydrates, 30% protein and 30% fat (32).

South Beach Diet: The original book South Beach Diet: The Delicious, DoctorDesigned, Foolproof Plan for Fast and Healthy Weight Loss gives weight-loss advice based on recommendations the physician obtained from his practice (34). The South Beach diet uses a qualitative approach by excluding, at the beginning of the diet, some carbohydrate sources with a higher glycemic index. Portion size is not specified and it is recommended to eat until hunger is no longer felt. The diet has three phases. In phase 1, a normal diet should be consumed, consisting of chicken, beef, turkey, fish, lots of vegetables, eggs, cheese, nuts and salads made with olive oil. Fruit, starchy vegetables and grains are not allowed during this two-week period. In phase 2, the list of foods is broadened to include a wider range of fruits and vegetables, with greater emphasis on choosing low glycemic index foods (e.g., apples, berries, grapefruits and whole-grain bread). In phase 3, consuming more foods labeled as medium glycemic index foods (usually starches) is allowed, and occasional snacking (e.g., chocolate cake) is permitted once target weight is reached (32).

Positive and negative effects of low-carbohydrate diet on health

LCD can help prevent severe health conditions such as metabolic syndrome, diabetes mellitus, high blood pressure and cardiovascular disease, or improve them. In fact, almost any diet that helps us lose excess weight can reduce or even reverse cardiovascular risk factors for cardiovascular disease and diabetes and improve blood cholesterol and glucose values (29).

As with any diet, LCD also has a positive and a negative side because of the deficiency of one of the macronutrients. The advantage of LCD is that it ensures weight loss. Patients who maintain LCD show an average reduction in body weight of 6.2 kilograms over 6 months to one year. Similar studies comparing LCD with other diets found that LCD has a greater effect on short-term weight loss. In addition, participants observed a decrease in blood pressure (35). Another study from 2009 illustrated the efficacy of LCD in reducing triglyceride levels in patients’ blood (36). Low-carbohydrate eating may also help in treatment of some chronic diseases including epilepsy, chronic seizures and type 2 diabetes. In children with diagnosed epilepsy or chronic seizures, one study found a 38% reduction in seizure frequency after following LCD (37). LCD helps patients with type 2 diabetes maintain a healthy insulin level and shows potential in normalizing blood glucose levels (35).

If carbohydrates are suddenly and drastically removed from the diet, various temporary adverse health effects may occur, such as headache, bad breath, weakness, muscle cramps, fatigue, skin rash, constipation, or diarrhea. Some diets further restrict carbohydrate intake so much that over the long term they can lead to vitamin or mineral deficiency, loss of bone mass and gastrointestinal disorders and may increase the risk of various chronic diseases. Because LCD may not provide required nutrients, these diets are not recommended for children and adolescents. Their developing bodies need nutrients that are found in grains, fruits and vegetables. It is not clear what possible long-term health risks a low-carbohydrate diet may pose, since most research studies lasted less than one year. Some health professionals believe that if large amounts of fats and proteins from animal sources are consumed, the risk of heart disease or certain cancers may increase. If a low-carbohydrate diet is followed, in which a higher fat intake and probably higher protein intake are consumed, it is necessary to include foods with healthy unsaturated fats and healthy proteins. Foods containing saturated and trans-fatty acids, such as meat, high-fat dairy products, biscuits and pastries, should be limited (29).

Conclusion

Low-carbohydrate diet could be an interesting alternative in the treatment of certain chronic diseases, such as obesity, diabetes, and epilepsy, but also Parkinson's disease, Alzheimer's disease, and multiple sclerosis. The FDA (Food and Drug Administration) recommends an average of 300 g carbohydrates per day (38). With a low-carbohydrate diet, about 30–50 g carbohydrates are consumed (35). However, it is difficult to follow such nutrition long term at such an intake of red meat and other fatty and salty foods, which are unhealthy for our health when consumed long term. It is also important to realize that dietary habits leading to rapid weight fluctuation are associated with increased mortality. Instead of maintaining LCD, which would last only a few weeks to months, it is important to adopt a change that is sustainable in the long run. A balanced diet rich in fruits and vegetables, lean meat, fish, whole grains, nuts, seeds, olive oil and adequate hydration supports a healthier life. Priority should also be regular monitoring of cardiometabolic risk markers. People with type 2 diabetes require careful monitoring of hypoglycemia.

Authors: Mgr. Nikola Pelechová, doc. MUDr. Kvetoslava Rimárová, CSc., mim.prof. Ústav verejného zdravotníctva a hygieny, Lekárska fakulta UPJŠ, Košice Práca podporovaná grantovým projektom KEGA 007/UPJŠ-4/2018, KEGA 008UPJŠ-4/2020 a vnútornými projektami IPEL VVGS-2019-1221 a IPEL VVGS-2019-1383.

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