Definition

The term diabetes mellitus (DM) encompasses a group of heterogeneous diseases that vary in both pathophysiology and phenotypic presentation. The common factor among them is a disruption in glucose metabolism, either through an absolute insulin deficiency caused by beta-cell destruction or a relative deficiency due to beta-cell dysfunction and insulin resistance, leading to hyperglycemia. There are four main types of diabetes:

• Type 1 Diabetes Mellitus (autoimmune) 
• Type 2 Diabetes Mellitus 
• Gestational Diabetes 
• Other Types (e.g., monogenic diabetes of the young [MODY], among others) 

Type 2 diabetes is by far the most prevalent form, accounting for approximately 90% of all diabetes cases. ^1,2

Impact on Nutritional Status

Regardless of etiology, diabetes mellitus is marked by hyperglycemia. In type 1 diabetes, absolute insulin deficiency and consequent renal glucose loss lead to an initial catabolic state, which is often associated with weight loss and an increased risk of ketoacidosis. With well-regulated insulin therapy, both body weight and metabolism can normalize. 
In contrast, type 2 diabetes frequently arises as a complication of metabolic syndrome, resulting from dysfunction of insulin-producing cells coupled with increased insulin resistance in peripheral tissues and the liver. Patients with type 2 diabetes commonly have other metabolic comorbidities, such as dyslipidemia and obesity, or, with advancing age, sarcopenic obesity. 

Nutrient requirements of patients with diabetes. Adjustments are necessary for malnourished patients, depending on their physical activity levels and age. The weight used is the adjusted body weight (ADJ) for BMI 28 and above, otherwise the body weight before admission to hospital ³. 3. DM = diabetes mellitus; BW = body weight; d = day

Type 1 Diabetes

Currently, there are no clear guidelines for nutrition in type 1 diabetes. Caloric requirements and diet composition should be discussed individually, with particular attention to carbohydrate amount and quality. vidence suggests that a very-low-carb diet, with daily carbohydrate intake between 47–75 g, may benefit type 1 diabetics, though large-scale studies are lacking. ⁵ There is little data is available on the recommended daily intake of carbohydrates. However, it is essential for people with type 1 diabetes to learn how to estimate carbohydrate quantities accurately to adjust bolus insulin accordingly. In addition, it is recommended that at least 50% of carbohydrate intake should come from whole grain products or other sources of dietary fiber. In general, ≥14 g /1000 kcal dietary fiber is recommended daily. ⁵ Regarding the glycemic index (GI), a measure of how much a predefined amount of carbohydrates (50 g) in a certain food increases blood sugar, there is some conflicting data. A low-GI diet is recommended for people with type 1 diabetes in some, but not all, guidelines as the evidence on HbA1c reduction is inconclusive. ^5,6

Typ 2 Diabetes

Between 60–90% of people with type 2 diabetes are obese. ⁷ Many patients also present with cardiovascular conditions, such as hypertension, at diagnosis. To improve glycemic control and manage cardiovascular risk factors—especially in younger patients—a targeted weight reduction of 5–10% of initial body weight is recommended. ^5,8,9 Substantial weight loss can even achieve remission of diabetes and significant improvement in hypertension. ^10,11 Regarding the composition of the diet, there are several recommended diets, but no consistent guidelines for the exact composition. Certain diets are recommended because of their positive effect on weight. These include the Mediterranean diet, low/very-low-carb diet, as well as vegan or vegetarian diets. Especially with a vegan diet, it is crucial to ensure a balanced food selection. Supplementation of various micronutrients, e.g. B12, should be regularly checked and substituted if necessary. This diet is not recommended for adolescents, pregnant women or breastfeeding women.  
 
The definitions of the terms "low-carb" and "very-low-carb" diet are not uniform. Low-carb was used in the corresponding studies with a carbohydrate content of 26-45%, very-low-carb with a content of <26% of total calorie requirements. Low-fat diets and the DASH (Dietary Approaches to Stop Hypertension) diet can help prevent diabetes and support weight loss. The Mediterranean diet also positively impacts cardiovascular health. For all diets, focus should be on high-quality, balanced foods. At least half of grain products should be whole grains, and a high dietary fiber intake (≥14 g per 1000 kcal) is encouraged. Total carbohydrate intake should be moderated, avoiding refined sugars and highly processed foods. ^5,9,12 In addition, care should be taken to choose vegetables with a low starch content over those with a high starch content. Preference should be given to low-starch vegetables, and low- or no-calorie sugar substitutes can be used moderately to reduce calorie intake, though unsweetened alternatives are generally preferred. ^4,5 When selecting dietary fats, trans fats and saturated fats should be significantly reduced. Instead, preference should be given to unsaturated fats and omega-3 fatty acids, which are commonly found in native vegetable oils and fatty fish. ^4,5  Individual dietary preferences and any comorbidities should always be considered when making dietary choices.   

Elderly patients

Regardless of the type of diabetes, particular attention should be paid to an adequate intake of nutrients in older diabetes patients. For type 2 diabetics, weight loss should only be pursued after carefully weighing potential metabolic benefits against risks like sarcopenia, osteoporosis, or malnutrition. Overly restrictive diets should be avoided. ^13,14 HbA1c targets for older type 2 diabetics should be adjusted based on overall health and life expectancy, especially to minimize the risk of hypoglycemia. However, the HbA1c target should always be <8.0%. ⁷ For older patients with type 1 diabetes, HbA1c targets should also be reviewed and tailored according to life expectancy and functional status. For some patients, simplifying the insulin regimen may be beneficial, even if optimal glycemic control is not achieved, to reduce the risk of hypoglycemia—a potentially life-threatening complication, particularly in type 1 diabetes. Older patients are especially vulnerable to hypoglycemia, and many type 1 diabetics also experience reduced hypoglycemia awareness. ¹⁵

Physical activity 

Physical activity is an essential component of diabetes management for all types of diabetes. It helps maintain a healthy weight, enhances insulin sensitivity, reduces mortality and cardiovascular risk, and improves overall fitness. 16  For individuals with type 2 diabetes, at least 150 minutes of moderate-intensity activity or 75 minutes of vigorous activity per week, spread over at least three sessions, is recommended. Additionally, muscle-strengthening, flexibility, and balance exercises are advised. ⁴

For individuals with type 1 diabetes, regular physical activity through aerobic and resistance training is also recommended. However, they should be particularly mindful of the risk of hypoglycemia during exercise and receive proper training to manage it. Furthermore, any comorbidities, such as diabetic retinopathy or diabetic foot disease, should be considered when selecting suitable activities. ¹⁵

Alcohol Consumption

Special caution is advised regarding alcohol consumption, particularly for type 1 diabetics and type 2 diabetics who use insulin, sulfonylureas, or glinides. Alcohol, like other sedatives, can delay or dull the perception of hypoglycemia symptoms—a potentially life-threatening complication, especially in type 1 diabetes. Alcohol also inhibits hepatic gluconeogenesis and increases ketone body production, thereby raising the risk of hypoglycemia or diabetic ketoacidosis. Therefore, alcohol should only be consumed in small quantities (1 standard drink, equivalent to 15 g of alcohol, for women; 2 standard drinks for men), not immediately before or after physical activity, and only alongside carbohydrate intake. 5 Alcohol consumption can contribute to weight gain and, when consumed in high quantities, may lead to hyperglycemia. ⁴

Vitamins, Minerals, and Trace Elements

• Daily Requirement Coverage:
• Potentially increased vitamin C requirement: 200-500 mg/day ¹⁷
• Potentially increased folate (vitamin B9) requirement: up to 1,500 µg/day ¹⁷
• Increased risk of vitamin B12 deficiency with metformin therapy ¹⁷
• Substitution: Supplementation should follow guidelines if a deficiency is diagnosed ¹⁷
• Recommended Daily Sodium Intake: ≤2.3 g/day ⁵

 Aims of Nutrition Therapy for Type 1 & Type 2 Diabetes:

• Maintain/improve bodily function, especially in older patients 
• Prevent malnutrition/nutrient deficiencies 
• Avoid hypoglycemia/hyperglycemia 
• Delay or prevent diabetic complications 
• Encourage healthy eating habits 
• Support weight loss: 
• Aiming for at least a 5-10% reduction in body weight can improve insulin sensitivity, enhance glycemic control, and may even induce remission in overweight (BMI >25 kg/m² and <30 kg/m²) or obese (BMI ≥30 kg/m²) patients. ^5,7-9 Different BMI thresholds may apply to individuals of non-European descent. ^18,19
• Provide carbohydrate education
• Understanding the effects of food, especially carbohydrates, on blood sugar is essential for all people with diabetes. Patients on insulin therapy, in particular, require guidance on carbohydrate counting to ensure effective and safe management of blood glucose levels. ^6,9,15

Monitoring

Type 1 Diabetes

• For type 1 diabetics, blood sugar measurement usually involves continuous glucose monitoring (CGM) using blood sugar sensors that measure the glucose content in the interstitium of the subcutis. If there is any suspicion that a sensor measurement is inaccurate, for example in the case of interstitial edema or inappropriate symptoms, the value should be checked using capillary measurement (CBGM). ¹⁵
• The frequency of follow-up examinations depends on the glycemic control. However, a clinical laboratory follow-up should be carried out at least once a year. Close monitoring should be provided if the therapy is adjusted or if glycemic control is insufficient. ¹⁵
• Malnutrition in people with diabetes should also be treated when they are hospitalized. ¹⁴ Diabetes-specific oral nutritional supplements can be used if necessary. ²⁰

Type 2 Diabetes

• For people with type 2 diabetes, the frequency of blood glucose monitoring depends on the treatment regimen (e.g., oral antidiabetic drugs and/or insulin) and level of glycemic control. In particular, patients who require insulin therapy may benefit from CBGM and, in some cases, CGM. ^6,9
• Regular HbA1c measurements are recommended every 3 months, or at least every 6 months if glycemic control is stable. ⁶
• The frequency of follow-up examinations depends on glycemic control. However, a clinical laboratory follow-up should be carried out at least once a year. If the therapy is adjusted or if glycemic control is insufficient, more intensive care should be provided. 
• For patients taking metformin, annual monitoring of vitamin B12 levels is recommended. ^4,5,9
• The frequency of recommended blood sugar self-measurements varies greatly depending on the monitoring system used. 
• As with all patients, malnutrition in diabetic patients should be treated before hospitalization. ¹⁴ Diabetes-specific oral nutritional supplements are available if needed. ²⁰

Special Considerations

• Diabetes patients often have multiple comorbidities and an increased risk of micro- and macroangiopathic complications such as nephropathies, retinopathies or neuropathies, as well as an overall increased cardiovascular risk profile. Nutritional recommendations for these comorbidities should also be followed. 
• Gastroparesis: In cases of diabetic gastroparesis, dietary modifications are recommended. Smaller portions and more liquid or puréed foods can help ease symptoms. Raw and fiber-rich foods may worsen symptoms and should be avoided. ⁵
• Diabetics should be aware of the influence of other illnesses on their diabetes. For example, infections can affect blood sugar levels. More frequent blood sugar and ketone monitoring may be necessary in such cases. ¹⁵ Additionally, some medications (e.g., SGLT2 inhibitors, metformin) may need to be temporarily paused and replaced with insulin if required. ⁷

Medications/Supplements

• Metformin: The exact mechanisms of action of metformin are not all known. However, it leads to a reduction in hepatic gluconeogenesis, a decrease in intestinal glucose absorption and a reduction in peripheral insulin resistance. ²¹ Vitamin B12 should be monitored annually. The most common side effects include gastrointestinal discomfort and nausea. Metformin should be paused during prolonged fasting/illness due to the risk of lactic acidosis. ⁷
• SGLT-2 inhibitors: These inhibit glucose reabsorption in the proximal tubule via sodium-glucose cotransporter-2 (SGLT2), leading to increased glucose excretion. ²¹ They can support weight loss and lead to increased diuresis and fluid loss. In case of prolonged fasting or illness, SGLT-2 inhibitors should be discontinued due to the risk of ketoacidosis. ⁷
• GLP-1-RA: Glucagon-like peptide 1 receptor agonists (GLP-1-RA) stimulate the glucose-dependent release of insulin in the pancreatic beta cells and lead to an increased feeling of satiety as well as delayed gastric emptying and thus to reduced food intake, a decrease in blood sugar and, as a rule, to weight loss. The most common side effects are gastrointestinal, such as nausea, bloating, diarrhea or constipation. During illness or extended periods of fasting, a pause in treatment can be evaluated to increase appetite again. ⁷
• Sulfonylureas/Glinides: Sulfonylureas, together with glinides and insulins, are among the diabetes medications with the highest risk of hypoglycemia, as they increase glucose-independent insulin release from the pancreas. ^9,21 In general, their use has become increasingly rare. ⁷
• DPP4 inhibitors: Dipeptidylpeptidase-4 inhibitors (DPP4 inhibitors) inhibit the enzyme that breaks down incretins, which include GLP-1. They are generally well tolerated and have little effect on the risk of hypoglycemia. ⁹
• Insulins: Insulin therapy carries an increased risk of hypoglycemia, particularly with fast-acting (bolus) insulins used for blood sugar correction or to cover meals. Basal insulins (long-acting insulins) should not be discontinued, even during periods of fasting or illness; at most, they may need adjustment. Discontinuing basal insulin, especially in type 1 diabetic patients, can lead to ketoacidosis. ¹⁵