What Is a Low-Calorie Diet?

A low-calorie or a hypocaloric diet is a diet designed to lower daily caloric intake.¹ A conventional low-calorie diet usually aims for daily caloric intake of 1,500-1,800 kcal/day for males and 1,200-1,500 kcal/day for females, lower than the average daily caloric requirements of 2,600-2,800 kcal/day for males and 2,000-2,200 kcal/day for females.^1,2 A very low-calorie diet aims for daily caloric intake of less than 800 kcal/day, typically ranging between 400 kcal and 800 kcal daily. The very low-calorie diet is recommended only for a short period of time (<12 weeks) for rapid weight loss.

Can Low-Calorie Diets Improve Metabolic Health? 

For individuals who are obese (body mass index >30 kg/m²), guidelines recommend achieving significant weight loss through lifestyle changes and pharmacological treatment for improved health and well-being.³ Obesity is known as the primary culprit of a host of cardiometabolic diseases,⁴ including cardiovascular disease and type 2 diabetes mellitus (T2DM).⁵ Adhering to a healthy eating pattern with reduced caloric intake is currently recommended for weight loss and weight loss maintenance in obese individuals as well as for improved glycemic control and blood pressure management.^3,6–11 Making dietary changes and engaging in physical activity to create a 500-700 kcal/day energy (ie, calorie) deficit are recommended.

Weight Loss – Studies on low-calorie diets differ vastly in their design, where some focus on restricting certain macronutrients (eg, low-carbohydrate or low-fat diets), others on creating fixed energy deficits, while others focus on method of delivery (eg, meal replacements).¹³ Despite the varied designs, research indicates that low-calorie diets, especially very low-calorie diets, can be effective in achieving weight loss and potentially improving cardiometabolic risk factors. For instance, the Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy (CALERIE) Study, a phase 2 multicenter randomized controlled trial, studied the effects of 25% caloric restriction in healthy non-obese individuals for 2 years.¹⁴ The study found that the intervention was effective in improving both weight and various cardiometabolic risk factors. Those assigned to the caloric restriction group saw a mean weight reduction of 7.5 kg (vs. 0.1 kg weight increase in control group). Additionally, significant improvements in lipid profiles, systolic and diastolic blood pressure, and insulin sensitivity were seen associated with caloric restriction. Similar effectiveness in weight loss is seen with caloric restriction in obese individuals. For instance, two systematic reviews and meta-analyses report that very low-calorie diets are effective in achieving weight loss.^15,16 One of the reviews analyzed 12 randomized controlled trials on very-low-energy diets (VLEDs) and found that behavioral programs combined with VLEDs led to a significantly greater weight loss over a one-year period than when only behavioral programs were implemented.¹⁵ Another analyzed five studies comparing very low-calorie diets (<800 kcal/day) to low-energy liquid-formula diets (>800 kcal/day) in obese individuals with and without T2DM and found that both forms of diet that restrict calories were effective in achieving weight loss in both those with T2DM (8.9-15 kg total weight loss) and those without T2DM (7.9-21 kg total weight loss) over the course of 4-52 weeks of intervention.¹⁶

Diabetes – Studies indicate that low-calorie diets can be an effective method for improved weight and glycemic control in individuals with T2DM.^2,17,18 A systematic review and meta-analysis of 16 randomized controlled trials found that very low-calorie diets resulted in significant improvements in hemoglobin (Hb) A1C and fasting glucose levels in individuals with T2DM when compared to non-dietary therapies.¹⁷ Other reviews have similarly observed that very low-calorie diets can effectively promote weight loss, improve insulin response, normalize HbA1C levels, and potentially reverse T2DM.² One landmark randomized controlled trial called the Diabetes Remission Clinical Trial found that a dietary intervention in T2DM with very low-calorie diet and nutritional support resulted in significant weight loss and remission of diabetes when compared to the control group who received routine care.¹⁸ At 1-year and 2-year follow-up, 24% and 11% of those in intervention achieved 15 kg weight loss and 46% and 36% were in remission, respectively, showing that a low-calorie diet can result in significant and long-term improvement in diabetes when weight loss is sustained.

Heart Health – Most research has focused on the overall effect of weight loss on cardiovascular health and has consistently shown that healthy weight loss can significantly improve cardiovascular health. For instance, the Look AHEAD (Action for Health in Diabetes) Study compared the effects of intensive lifestyle intervention (ILI) involving diet modification (1,200-1,800 kcal/day) and physical activity to an intervention involving only diabetes support and education (DSE) in individuals with T2DM.¹⁹ The study concluded that ILI resulted in a significantly greater weight loss than DSE and that it also resulted in significant improvements in systolic and diastolic blood pressure, high-density lipoprotein cholesterol levels, and triglyceride levels, along with the finding that HbA1C was also improved with ILI – all pertinent parameters in determining cardiovascular health. Additionally, few studies that have studied the direct impact of low-calorie diets on cardiovascular health show promising results. For instance, a small cohort study investigated the effects of weight loss from a very low-calorie liquid diet (624 kcal/day) on cardiovascular parameters in individuals with T2DM.²⁰ The study found that weight loss induced by the intervention was associated with a significantly decreased 10-year cardiovascular risk as well as estimated heart age in both individuals who did and did not experience remission of T2DM. Overall, research indicates that dietary interventions, such as a low-calorie diet, that lead to healthy weight loss can improve heart health. Those seeking to improve cardiovascular health should keep in mind that beyond achieving weight loss, the American Heart Association places a great emphasis on choosing the right kinds of foods for heart health.²¹ While implementing a healthier lifestyle and eating pattern, focus on eating a diet characterized by a high intake of fruits, vegetables, whole grains, fat-free or low-fat dairy products, poultry, fish, nuts, beans, and seeds, and a low intake of sodium, sweets and sugars, saturated fat, trans fat, cholesterol, and red meats.

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