A low-calorie diet is a nutrition plan where you consume fewer calories than your body uses throughout the day, creating an energy deficit that forces the body to burn fat for energy. While these diets are generally used for weight loss, it's important to understand both their benefits and potential risks.
What is a Low-Calorie Diet?
A low-calorie diet, sometimes referred to as a hypocaloric diet, involves reducing your daily calorie intake. Typically, a daily deficit of 500 to 1,000 calories is recommended, which can result in a safe weight loss of approximately 1-2 pounds per week.
Popular hypocaloric diet plans aim to reduce daily calorie intake to promote weight loss and improve metabolic health. Common approaches include calorie counting, during which individuals track every calorie consumed, as well as portion control, which involves moderating serving sizes without necessarily counting each calorie. In contrast, fad diets such as the Atkins, ketogenic, or detox diets often promise quick results but may lack scientific support or nutritional balance. Although some of these diets are technically hypocaloric due to their restrictive nature, not all emphasize reduced calorie intake as their core principle. For example, the ketogenic diet emphasizes high-fat and very-low-carbohydrate intake, leading to ketosis, which may contribute to weight loss. A related and increasingly popular strategy is intermittent calorie restriction, which includes plans like alternate-day fasting (ADF) or the 5:2 diet. These methods, which involve cycling between days of very low-calorie intake and normal eating, emphasize timing and frequency of intake rather than continuous restriction. In clinical settings, structured hypocaloric plans such as the HBD (hypocaloric balanced diet) used in trials, or VLCKDs, may be tailored based on patient goals, be it migraine control, insulin resistance, or cardiovascular health. Although various hypocaloric diets can be effective, sustainability, nutritional adequacy, and individual health status should guide the choice of diet.
Very low-calorie diets (VLCD) are a more extreme form of calorie restriction. If you have a BMI over 30 (which your doctor will call “obese”), then a very low-calorie diet may result in a loss of 3 to 5 pounds per week, for an average total weight loss of 44 pounds over 12 weeks.
Benefits of Low-Calorie Diets
Hypocaloric diets are highly effective for weight loss and obesity management, as they often lead to reductions in both fat and fat-free mass. Even losing as little as 5% of your body weight may improve medical conditions, including diabetes, high blood pressure, and high cholesterol. As a result, hypocaloric diets typically improve metabolic markers, including fasting glucose, insulin levels, hemoglobin A1c, and lipid profiles, such as total cholesterol and triglycerides.
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Cardiovascular Health
Calorie restriction has been shown to preserve cardiovascular health by reducing blood pressure levels and improving blood lipid ratios.
Longevity
Both animal and human data suggest that calorie restriction may extend lifespan and delay the aging process.
Mental Health
In a 2024 systematic review and meta-analysis published in Advances in Nutrition, adults with metabolic conditions such as type 2 diabetes mellitus, hypertension, or obesity who followed hypocaloric diets experienced a significant improvement in their depression scores but had no notable impact on anxiety. It remains unclear exactly which dietary components are most effective and whether these observable improvements are due to weight loss, nutritional changes, or psychosocial factors.
Risks and Considerations
If not well-balanced, following a hypocaloric diet may lead to nutritional deficiencies, particularly in essential vitamins and minerals. This is particularly important when food variety is limited or extreme calorie restriction is followed for extended periods.
Psychological Effects
Psychologically, individuals may experience increased hunger, irritability, fatigue, or mood fluctuations due to energy deficits. These effects can reduce adherence and increase the risk of binge eating or discontinuation of the diet.
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Specific Populations
Pregnant or breastfeeding women, children, and elderly individuals with frailty should avoid hypocaloric diets. For others, particularly those with obesity-related complications, structured calorie reduction combined with nutritional guidance can be beneficial. Very low-calorie-diets are not recommended for pregnant or breastfeeding women, and are not appropriate for children or teens except in specialized treatment programs. They also may not be OK for people over age 50, either, depending on the potential need for medications for pre-existing conditions, as well as the possibility of side effects.
Side Effects of Very Low-Calorie Diets
People on a very low-calorie diet for 4 to 16 weeks report minor side effects such as fatigue, constipation, nausea, and diarrhea. These conditions usually improve within a few weeks and rarely prevent people from completing the program.
Gallstones
Gallstones are the most common serious side effect of very low-calorie diets. Gallstones are more common during rapid weight loss. When the body experiences a calorie deficit, it starts to break down fat for energy. The liver then secretes more cholesterol and when combined with bile, can form gallstones.
Nutritional Imbalance
To be healthy, you need a balance of foods from different food groups. It's difficult to get good nutrition and feel satisfied on a very low-calorie diet. Most people on very low calorie diets need vitamin and mineral supplements to ensure they are getting proper nutrition.
Low-Carbohydrate Diets: An Alternative Approach
A low-carb diet limits the amount of carbohydrates you eat. A low-carb diet limits carbohydrates, often called carbs - such as those found in grains, starchy vegetables and fruit. A low-carb diet focuses on foods high in protein and fat. Many types of low-carb diets exist. A low-carb diet is generally used for weight loss.
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In general, you digest complex carbs more slowly. Complex carbs also have less effect on blood sugar than refined carbs do. Refined carbs such as sugar or white flour are often added to processed foods. The body uses carbs as its main energy source. During digestion, complex carbs are broken down into simple sugars, also called glucose, and released into your blood. Insulin is released to help glucose enter the body's cells, where it can be used for energy. Extra glucose is stored in the liver and in muscles.
What to Eat and Avoid
In broad terms, a low-carb diet focuses on proteins and some nonstarchy vegetables. A low-carb diet generally limits grains, legumes, fruits, breads, sweets, pastas and starchy vegetables, and sometimes nuts and seeds. A daily limit of 0.7 to 2 ounces (20 to 57 grams) of carbohydrates is typical with a low-carb diet. These amounts of carbohydrates provide 80 to 240 calories. Some low-carb diets greatly limit carbs during the early phase of the diet. In contrast, the Dietary Guidelines for Americans recommend that carbohydrates make up 45% to 65% of your total daily calorie intake.
Weight Loss and Other Benefits
Most people can lose weight if they limit calories and boost their physical activity. Low-carb diets, especially very low-carb diets, may lead to greater short-term weight loss than do low-fat diets. Cutting calories and carbs may not be the only reason for the weight loss with low-carb diets. Some studies show that you may shed some weight because the extra protein and fat helps you feel full longer. Low-carb diets that focus on healthy sources of carbs, fat and protein may help lower the risk of type 2 diabetes and heart disease.
Ketosis and Potential Risks
Severe carb limits can cause your body to break down fat into ketones for energy. This is called ketosis. It's not clear what kind of possible long-term health risks a low-carb diet may pose. If you opt to follow a low-carb diet, think about the fats and proteins you choose. Limit foods with saturated and trans fats, such as meat and high-fat dairy products.
Very-Low-Calorie Ketogenic Diets (VLCKD)
Comprehensive very-low-calorie diet (VLCD) programs are the preferred treatment for selected obese individuals. Typical VLCD patients have median body mass indexes of 36 kg/m2 and have median ages of 40 years. About 70% are female. Commonly associated medical problems include hypertension in 50%, hyperlipidemia in 41%, and diabetes mellitus or glucose intolerance in 14%. Typical weight loss with VLCD is around 21 kg in 16 wk. Reductions of 8-13% in blood pressure, 5-15% in serum total cholesterol, 5-20% in low-density lipoprotein-cholesterol, 15-50% in triglycerides, and decreases in blood glucose and glycohemoglobin in diabetic individuals accompany weight loss. VLCD-associated side effects can be managed medically without discontinuing treatment. Lifestyle education promotes long-term weight maintenance of approximately 56% 2 yr after VLCD treatment.
Medical Nutritional Therapy and T2DM
Medical nutritional therapy aiming at weight loss is a mainstay of treatment for obese subjects with type 2 diabetes mellitus (T2DM). An interplay between human obesity and T2DM was strongly confirmed in numerous epidemiological studies and both diseases are rapidly growing in parallel worldwide with major health consequences. In fact, weight loss has been associated with an improvement not only in glycemic control but also in other cardiovascular risk factors commonly altered in subjects with T2DM. Nonetheless, long-term non-pharmacologic weight loss interventions for adults with T2DM have shown limited efficacy. Thus, alternative weight loss strategies that are safe and effective in subjects with T2DM are in need.
Optimal Caloric Restriction and Macronutrient Distribution
The optimal degree of caloric restriction and macronutrient distribution of medical nutritional therapy in T2DM is not well defined. A systematic review of weight loss interventions in subjects with T2DM revealed that interventions including very low-calorie diets (VLCD) along with moderate physical activity and behavioral intervention produced the largest effect. Although the number of randomized clinical trials assessing the efficacy of VLCD in subjects with T2DM is limited, data suggest considerable weight loss, improved beta-cell function, and improved quality of life associated with short-term VLCD. However, in 2008 the American Diabetes Association stated as part of its nutrition recommendations for diabetes that VLCD appeared to have limited utility in the treatment of T2DM and should only be considered in conjunction with a structured weight loss program. On the other hand, evidence suggests that there are not an ideal percentage of calories from carbohydrate, protein and fat for all people with diabetes; therefore, macronutrient distribution should be based on individualized assessment of current eating patterns, preferences and metabolic goals. Although numerous studies have attempted to identify the optimal mix of macronutrients for the meal plans of people with diabetes, recent systematic review found that there is no ideal mix that applies broadly for successful weight loss in subjects with T2DM and that macronutrient proportions should be individualized.
High-Protein Diets
It has been claimed that high-protein diets may help promote weight loss, maintain lean body mass, and improve lipid and plasma glucose profiles in obese subjects with our without T2DM and prevent hepatic steatosis in obese animal models. However, concern has been raised that increased protein intake, could cause deterioration of renal function particularly in those with microalbuminuria or established diabetic nephropathy, and that high-protein interventions are not feasible in a 'real-world setting. In addition, short-term studies have shown that reducing total carbohydrate intake is associated with improved insulin sensitivity and glycemic control. Conversely, current standards of care of the American Diabetes Association for the subject with T2DM state that the recommended daily allowance for digestible carbohydrate is 130 g per day to provide adequate glucose as the required fuel for the central nervous system without reliance on glucose production from ingested protein or fat.
VLCK Diet Study
Against this background, the primary aim of our study was to evaluate the short-term safety and tolerability of a low-carbohydrate, ketogenic diet (<50 g of carbohydrate daily; VLCK diet) as part of an interventional weight loss program including lifestyle and behavioral modification support (Diaprokal Method) in subjects with T2DM. As secondary aims, we compared weight loss and changes in metabolic parameters between subjects following the interventional weight loss program or a low-fat hypocaloric diet together with a lifestyle and behavioral modification program made available by the health-care provider.
Methods
Eighty-nine men and women participated in our prospective, open-label, multi-centric randomized clinical trial with a duration of 4 months and parallel group design. Eligibility criteria for the study included age between 30 and 65 years, previous diagnosis of T2DM and body mass index between 30 and 35 kg m−2. Exclusion criteria included duration of T2DM longer than 10 years, insulin therapy, hemoglobin A1c (HbA1c) ⩾9% and fasting C-peptide <1 ng ml−1. In addition, subjects presenting with impaired renal function (defined as an estimated glomerular filtration rate <60 ml min-1 per 1.73 m2), impaired liver function (defined as liver enzymes greater than equal to twofold the upper normal limit), alcohol intake ⩾40 g per day for men and ⩾24 g per day for women, pregnancy, lactation, or severe eating or psychiatric disorder according to the investigator criterion were excluded from the study.
Study participants were recruited in the Endocrinology departments of seven participating Centers across Spain. Centralized approval was granted by the Ethics Committee of one of the participating Centers (Institut Municipal d’Assistència Sanitària, Hospital del Mar) and thereafter ratified by the local Ethics Committee at each participating site. Written informed consent was obtained from all study participants prior to randomization. Randomization to one of the two study groups was stratified by participating Center.
Study Design and Dietary Interventions
The 4-month dietary intervention in subjects randomly assigned to the interventional weight loss following a VLCK diet (VLCK diet group) as part of a commercial weight-loss program (DiaproKal Method) based on a high-biological-value protein preparations diet and natural foods or to a low-calorie diet (LC diet group) based on the ADA (American Diabetes Association) guidelines. The intervention for both groups included an evaluation by the specialist physician conducting the study, an assessment by an expert dietician, group meetings and exercise recommendations. Individual counseling to support lifestyle and behavioral modification throughout the study was performed according to a structured support program by an endocrinologist and a registered dietitian at each participating center in the LC diet group. The registered dietitian in the VLCK diet group was an employee of the company supporting the interventional program and used the same structured support plan as in the LC diet group. The program included nine individual sessions and a telephone contact every 15 days in both study arms.
VLCK Diet Methodology
The methodology in VLCK diet group was similar to that used in another recently published study evaluated the efficacy of a VLCK diet as part of a commercial weight loss program (Pronokal Method) in obesity. Each protein preparation contained 15 g protein, 4 g carbohydrates, 3 g fat and three specific active ingredients, (20 μg chromium, 0.8 g Ginseng and 0.4 mg Biotin); and provided 90-100 kcal. This method has three stages: active, metabolic stabilization and maintenance. The active stage consists of a very low-calorie diet (600-800 kcal per day), low in carbohydrates (<50 g daily from vegetables) and lipids (only 10 g of olive oil per day). The amount of high-biological-value proteins ranged between 0.8 and 1.2 g per each Kg of ideal body weight, to ensure meeting the minimal body requirements and to prevent the loss of lean mass. This method produces three ketogenic phases. In phase 1, the patients eat high-biological-value protein preparations five times a day, and vegetables with low glycemic index. In phase 2, one of the protein servings is substituted by a natural protein (for example, meat and fish) either at lunch or at dinner. In the phase 3, a second serve of the natural protein low in fat substituted the second serve of biological protein preparation. Throughout these ketogenic phases, supplements of vitamins and minerals, such as K, Na, Mg, Ca and omega-3 fatty acids, were provided in accordance to international recommendations. This active stage is maintained until the patient loses most of weight loss target, ideally 90%. Hence, the ketogenic phases were variable in time depending on the individual and the weight loss target, but they lasted between 30 and 45 days in total.
In the metabolic stabilization stage, the ketogenic phases were ended by the physician in charge of the patient based on the amount of weight lost, and started a low-calorie diet. At this point, the patients underwent a progressive incorporation of different food groups and participated in a program of alimentary re-education to guarantee the long-term maintenance of the weight lost. The maintenance stage consists of an eating plan balanced in carbohydrates, protein and fat. Based on each individual’s basal metabolic rate as determined by the Harris Benedic equation, the calories consumed ranged between 1500 and 2250 kcal per day and the target was to maintain the lost weight and promote healthy life styles.
LC Diet Methodology
The LC diet was aimed at a daily energy restriction of 500-1000 kcal according to each individual’s basal metabolic rate. Macronutrient dietary composition aimed at a daily intake of <30% of calories coming from fat, 10-20% from protein and 45-60% from carbohydrates.
Safety and Tolerability Assessment
Safety parameters included renal function (plasma creatinine, blood urea nitrogen, urinary albumin-to-creatinine ratio and estimated Glomerular Filtration Rate using the Modification of Diet in Renal Disease study equation MDRD-eGFR), liver function (alanine aminotransferase, aspartate aminotransferase and total bilirubin) and plasma uric acid, sodium and potassium. Beta-hydroxibutirate was measured from capillary blood (Optium Xceed Blood Glucose and Ketone Monitoring System; Abbott Laboratories, Chicago, IL, USA). The method performed to detect microalbuminuria was the albumin/creatinine ratio (μg mg−1) measured in spot urine samples. Diagnosis of microalbuminuria was defined when the spot collection was 30-300 μg mg−1 creatinine.
Safety parameters were assessed at baseline and at 2 weeks, 2 months (visit 5) and 4 months (visit 9, end of the study) following randomization. Capillary ketones were assessed at each study visit. Tolerability was assessed as the percentage of patients completing the 6-10 weeks pre-defined period of VLCK diet, and the incidence of pre-defined or unexpected adverse events (AE) throughout the study period.
Anthropometrical and Biochemical Assessment
Body weight, body mass index and waist circumference were performed according to previously describe standardized procedures. As glucose homeostasis parameters fasting plasma glucose, HbA1c and insulin were quantified. The HOMA-IR (Homeostasis Model Assessment for Insulin Resistance) was estimated as previously reported and a HOMA-IR>3.2 was considered as indicative of insulin resistance. Lipid profile analysis included fasting plasma triglycerides and total-, low-density lipoprotein and low-density lipoprotein cholesterol.
Dietary adherence and patient satisfaction were assessed by the Eating Self-Efficacy Scale and Liker Scale (1=very unsatisfied, 2=unsatisfied, 3=indifferent, 4=satisfied, 5=very satisfied), respectively. A dropout rate of 15% was anticipated in both study groups. Analysis of the safety and tolerability (safety population) variables was performed with an intention-to-treat analysis with baseline or last observation carried forward when the complete set of data for an individual was not available. Changes in body weight, BMI and waist circumference between groups were compared in the 'efficacy population', composed by those with at least one efficacy measurement available after randomization. Data on continuous variables are expressed as mean±s.d. unless stated otherwise. Categorical variables are described as percentage and number of valid observations. Other secondary measures were compared between groups at each study visit. No imputations for missing values were performed. Differences between groups were evaluated using parametric or non-parametric test as appropriate (χ2 or Fisher’s test for categorical variables, and analysis of variance or Mann-Whitney U-test). Statistical significance was set at a P-value <0.05.
Results of the VLCK Diet Study
The main clinical characteristics of the study participants are shown in Table 1. A total of 89 subjects were randomized to the low-calorie, ketogenic diet (VLCK diet) group (n=45) or the usual care low-calorie (LC diet) group (n=44). Attrition by completion of study visits was not different between groups (VLCK diet: 11.1% (5/45), LC diet: 18.2% (8/44); P=0.384). Anthropometric and metabolic parameters at baseline were comparable between the two study groups (Table 1).
Diet-Induced Changes in Safety Parameters
As expected by design, capillary blood β-hidroxibutirate concentration was larger in the VLCK diet group over the VLCK diet time period and for the remaining of follow-up (Figure 1a). Ketonemia positive (⩾0.3 mmol l−1) were detected in 91.1% of subjects of VLCK diet group during follow-up. The largest mean capillary ketonemia in the VLCK diet group during the study was recorded at 2 weeks follow-up (1.15±0.96 mmol l−1). The study participant with a ketonemia of 4.2 mmol l−1 did not present a random glucose >250 mg dl−1 or a pH<7.3. Despite this fact, no significant differences in the laboratory safety parameters were found between the two study groups (Figure 1). Changes from baseline in the urinary albumin-to-creatinine ratio (Figure 1b) and estimated Glomerular Filtration Rate using the Modification of Diet in Renal Disease study equation (Figure 1c) in the VLCK diet group were not statistically significant through the intervention period, and no differences were observed between the two weight loss strategies. Regarding to microalbuminuria diagnosis (UARC⩾30-300 μg mg−1), it was present in 6.3% in the VLCK diet group and in 17.6% of the LC diet group without reach statistically significant differences between groups (P=0.156) at the end of the study. Likewise, creatinine and blood urea nitrogen did not change significantly within study groups at the 2- or 4 months evaluations relative to baseline nor between groups (data not shown). Alanine aminotransferase and aspartate aminotransferase were slightly albeit significantly larger in the VLCK diet group as compared with the LC group at 2 weeks (alanine aminotransferase: 45.16 vs 26.85 IU ml−1, P<0.005; aspartate aminotransferase: 38.53 vs 22.15 IU ml−1, P<0.001) but not at the end of follow-up (4 months), (Figure 1d). Percentage of subjects in the VLCK diet group who presented with alanine aminotransferase or aspartate aminotransferase plasma concentration threefold higher than the upper limit of the normal range was not significantly different compared with controls (0%; P=0.157). Bilirubin plasma concentration remained invariable all over the study and did…