Eighty percent of people with type 2 diabetes mellitus are obese or overweight.1 Excess adipose tissue can lead to endocrine dysregulation,2 contributing to the pathogenesis of type 2 diabetes, and obesity is one of the strongest predictors of this disease.3
For obese people with type 2 diabetes, diet and exercise can lead to weight loss and many other benefits, such as better glycemic control, less insulin resistance, lower risk of diabetes-related comorbidities and complications, fewer diabetic medications needed, and lower health care costs.4–7 Intensive lifestyle interventions have also been shown to induce partial remission of diabetes and to prevent the onset of type 2 diabetes in people at high risk of it.5–7
A very-low-calorie diet is one of many dietary options available to patients with type 2 diabetes who are overweight or obese. The protein-sparing modified fast (PSMF) is a type of very-low-calorie diet with a high protein content and simultaneous restriction of carbohydrate and fat.8,9 It was developed in the 1970s, and since then various permutations have been used in weight loss and health care clinics worldwide.
MOSTLY PROTEIN, VERY LITTLE CARBOHYDRATE AND FAT
The PSMF is a medically supervised diet that provides less than 800 kcal/day during an initial intensive phase of about 6 months, followed by the gradual reintroduction of calories during a refeeding phase of about 6 to 8 weeks.10
During the intensive phase, patients obtain most of their calories from protein, approximately 1.2 to 1.5 g/kg of ideal body weight per day. At the same time, carbohydrate intake is restricted to less than 20 to 50 g/day; additional fats outside of protein sources are not allowed.9 Thus, the PSMF shares features of both very-low-calorie diets and very-low-carbohydrate ketogenic diets (eg, the Atkins diet), though some differences exist among the three (Figure 1).
Patients rapidly lose weight during the intensive phase, typically between 1 and 3 kg per week, with even greater losses during the first 2 weeks.8,9 Weight loss typically plateaus within 6 months, at which point patients begin the refeeding period. During refeeding, complex carbohydrates and low-glycemic, high-fiber cereals, fruits, vegetables, and fats are gradually reintroduced. Meanwhile, protein intake is reduced to individually tailored amounts as part of a weight-maintenance diet.
LIPOLYSIS, KETOSIS, DIURESIS
The specific macronutrient composition of the PSMF during the intensive phase is designed so that patients enter ketosis and lose as much fat as they can while preserving lean body mass.9,11Figure 2 illustrates the mechanisms of ketosis and the metabolic impact of the PSMF.
With dietary carbohydrate restriction, serum glucose and insulin levels decline and glycogen stores are depleted. The drop in serum insulin allows lipolysis to occur, resulting in loss of adipose tissue and production of ketone bodies in the liver. Ketone bodies become the primary source of energy for the brain and other tissues during fasting and have metabolic and neuroprotective benefits.12,13
Some studies suggest that ketosis also suppresses appetite, helping curb total caloric intake throughout the diet.14 Protein itself may increase satiety.15
Glycogen in the liver is bound to water, so the depletion of glycogen also results in loss of attached water. As a result, diuresis contributes significantly to the initial weight loss within the first 2 weeks on the PSMF.9
WHO IS A CANDIDATE FOR THE PSMF?
The PSMF is indicated only for adults with a body mass index (BMI) of at least 30 kg/m2 or a BMI of at least 27 kg/m2 and at least one comorbidity such as type 2 diabetes, hypertension, dyslipidemia, obstructive sleep apnea, osteoarthritis, or fatty liver.12 Patients must also be sufficiently committed and motivated to make the intensive dietary and behavioral changes the program calls for.
The PSMF should be considered when more conventional low-calorie approaches to weight loss fail or when patients become discouraged by the slower results seen with traditional diets.8 Patients undergoing a PSMF are usually encouraged by the initial period of rapid weight loss, and such diets have lower dropout rates.16
This diet may also be recommended for obese patients who have poorly controlled type 2 diabetes and growing resistance to medications, to bring down the blood glucose level. Another use is before bariatric surgery to reduce the risk of obesity-related complications.8 Patients who regain weight after bariatric surgery may also benefit.
MEAL REPLACEMENTS OR A DIET PLAN?
The PSMF program at Cleveland Clinic is based on modified preparation and selection of conventional foods. Details of the program are described in Table 1. Protein sources must be of high biologic value, containing the right mix of essential amino acids (eg, lean meat, fish, poultry, egg whites).9
Some commercially available very-low-calorie diets (eg, OPTIFAST, Medifast) that are advertised as PSMFs consist mainly of meal replacements. In the program at Cleveland Clinic, meal replacements in the form of commercial high-protein shakes or bars can be used occasionally for convenience and to maintain adherence to the diet.
However, preparation of PSMF meals from natural, conventional foods is thought to play an important role in long-term behavior modification and so is strongly encouraged. Patients learn low-fat cooking methods, portion control, and how to make appropriate choices in shopping, eating, and dining out. These lessons are valuable for those who struggle with long-term weight loss. Learning these behaviors through the program may help ease the transition to the weight-maintenance phase and beyond. For some patients, cooking is also a source of enjoyment, as is the sight, smell, and taste of nonliquid foods.10
In addition, patients appreciate being able to eat the same foods as others in their household, except for omitting high-carbohydrate foods. It has also been reported that patients on a food-based PSMF were significantly less hungry and preoccupied with eating than those on a liquid formula diet.17