A low-calorie diet (LCD) is usually greater than 800 kcal per day, and typically ranges from 1,200 to 1,600 kcal per day [7]. The LCD is designed to induce an energy deficit of 500 to 1,000 kcal per day, producing a weight loss of 1–2 lb. per week [7]. An LCD is most commonly prescribed using a traditional food regime [7, 8]. Fat restriction may be combined with an LCD to assist with reducing energy intake [8]. In 1998, the National Heart, Lung, and Blood Institute (NHLBI) published evidence-based clinical guidelines for the treatment of adult obesity [9]. These guidelines recommend an LCD, with energy prescriptions of 1,000–1,200 kcal per day for women and 1,200–1,500 kcal per day for men. The NHLBI guidelines also recommend restricting fat intake to no more than 30 % of daily energy intake, with the restriction designed to assist with reducing energy intake, thereby aiding with meeting the reduced-energy goal.

The Diabetes Prevention Program (DPP) provides an example of weight, cardiometabolic, and dietary outcomes commonly found when an LCD with fat restriction is prescribed [10]. Within the DPP, overweight and obese individuals with elevated fasting glucose levels who were randomly assigned to the lifestyle intervention condition received an LCD (ranging from 1,200 to 1,800 kcal per day depending on initial body weight), with a fat restriction of 25 % energy from fat. This intervention also included a moderate- to vigorous-intensity physical activity goal of 150 min per week, and provided a behavioral intervention to assist with changing dietary and physical activity behaviors to aid with achieving a weight loss goal of 7 % of initial body weight. The other two conditions that participants were randomized to were a medication (metformin) or placebo intervention. In DPP, participants were followed for an average of 2.8 years, and in the lifestyle intervention, the mean weight loss was −5.6 kilograms (kg), which was significantly greater than the two other conditions, medication, or placebo, that had not been prescribed the LCD. The pattern of weight loss and maintenance of weight loss seen in the lifestyle intervention showed the greatest degree of weight loss occurring at 6 months (approximately −7.0 kg), which was maintained for another 6 months, with weight regain occurring over longer follow-up. However, at 4-year follow-up, the lifestyle intervention still maintained a weight loss of almost −4.0 kg.

For cardiometabolic outcomes, fasting glucose and glycosylated hemoglobin (HbA₁) significantly decreased more so in the lifestyle intervention as compared to the medication and placebo intervention [10]. Over the mean 2.8-year follow-up, the lifestyle intervention showed a significantly reduced prevalence of hypertension and dyslipidemia as compared to the medication and placebo interventions. Additionally, significantly fewer lifestyle participants required medication for treatment of elevated triglycerides or high levels of low-density lipoprotein (LDL)-cholesterol than the other conditions. However, as weight loss alone enhances cardiovascular outcomes and the lifestyle intervention had the greatest weight loss, it is not clear if the enhanced cardiovascular outcomes found in the lifestyle intervention are a consequence of the LCD or other factors, such as greater weight loss, found within the lifestyle intervention.

Analyses of 1-year changes in dietary intake showed that the lifestyle intervention had significantly greater reductions of energy (−452 kcal per day) and percent energy from fat (−6.6 %) intake than the medication and placebo interventions [11]. Additionally, the lifestyle intervention showed significantly greater increases in intake of dietary fiber and weekly intake of servings from fruits and significantly greater reductions in weekly intake of servings from red meat and sweets than the other two conditions [11].

A very-low-calorie diet (VLCD) provides ≤800 kcal per day [8, 22, 23]. VLCDs were developed to enhance weight loss by creating a greater energy deficit than what occurs with the LCD. The VLCD is designed to preserve lean body mass, thus large amounts of dietary protein, usually 70–100 grams (g) per day or 0.8–1.5 g protein per kg of ideal body weight, are prescribed [8, 22]. One recommended source of protein for VLCDs is from a milk-, soy-, or egg-based powder, which is mixed with water and consumed as a beverage [22]. These powders also include 100 % of the recommended daily allowance for essential vitamins and minerals [8, 22]. Another recommended source of protein is from lean meat, fish, and fowl, and this form of VLCD is called a protein-sparing modified fast, which must be supplemented with a multivitamin and 2–3 g per day of potassium [22]. VLCDs require consumption of 2 liters (L) per day of noncaloric fluids [22]. VLCDs are considered to be appropriate only for those with a BMI ≥ 30 kg/m², and are increasingly used with individuals prior to having bariatric surgery to reduce overall surgical risks in those with severe obesity [8, 22].

One meta-analysis compared VLCDs to LCDs and examined short-term (varied from 8 to 50 weeks) and long-term (varied from 18 to 66 months) weight loss outcomes [22]. Results indicated that in the short-term, weight loss favored the VLCD (−16.1 ± 1.6 % vs. −9.7 ± 2.4 % of initial weight; p < 0.001), but long-term outcomes were similar between the two diets (−6.3 ± 3.2 % vs. −5.0 ± 4.0 % of initial weight; p > 0.2). The lack of difference between the two diets during the longer-term follow-up was due to greater weight regain occurring in the VLCD conditions. Thus, this meta-analysis suggests that in the long term, the VLCD does not enhance weight loss outcomes as compared to diets of higher energy prescriptions (LCD).

Improvements in glycemic control and blood lipids are also found with VLCDs; however, the degree of improvement appears to be a function of the amount of weight loss rather than any particular factor of the diet [22, 23]. Research on the effect of VLCDs on appetite regulation is very limited, with most research focused on the effect of VLCDs on binge eating, particularly in those with Binge Eating Disorder, and reported outcomes are mixed [23].

Energy-focused dietary prescriptions are successful at producing weight loss and improving cardiometabolic outcomes. Within these types of weight management dietary prescriptions, the improvements in cardiometabolic outcomes appear to mostly be a function of the degree of weight loss achieved, rather than any specific component of the dietary prescriptions. Those prescriptions that are better able to lower energy intake, such as the PMR plan and VLCDs, produce greater weight loss; however, the maintenance of the weight loss becomes more challenging if the degree of energy restriction is such that it cannot be maintained or is not designed to be maintained (VLCDs). Intervention research in the area of the pattern of consumption of energy is beginning to suggest that the frequency of eating bouts and when the greatest amount of energy is consumed during the day may be important in improving weight loss outcomes, but more research is needed in the area.

Very little research has examined changes in overall diet quality in energy-focused dietary prescriptions, but research that has been conducted shows that besides decreases in energy intake, reductions in fat (especially if this was part of the dietary prescription), red meat, and sweets intake; and increases in fruit and micronutrient (particularly with PMR plans) intake are found with these energy-focused dietary prescriptions. Research is lacking regarding the various energy-focused dietary prescriptions differing abilities to enhance appetite regulation.

A low-carbohydrate diet does not have a standard definition; however, most interventions define a low-carbohydrate diet as consuming no more than 20 g of carbohydrate per day [26–28]. In low-carbohydrate diets, energy is not restricted, yet research has found that energy intake does decrease when a low-carbohydrate diet is prescribed [29]. This reduction in energy intake, rather than ketosis as initially theorized [24], is the hypothesized mechanism by which a low-carbohydrate diet produces weight loss [30, 31]. A low-carbohydrate diet recommends consumption of conventional foods high in protein and fat, with a focus on consuming mono- and polyunsaturated fats [24]. Carbohydrates are to be consumed from non-starchy vegetables [26]. Once a desired weight is achieved, carbohydrate intake may gradually increase (5 g per day of carbohydrate per week), primarily in the form of vegetables, limited fruits, and eventually small amounts of whole grains and dairy products, to 50 g of carbohydrate per day [24].

A systematic review of RCTs examining the effect of low-carbohydrate diets on weight loss found that low-carbohydrate diets reduced body weight over a >3-month time period when compared to corresponding baseline values [32]. For more long-term outcomes, a meta-analysis examining the effect of low-carbohydrate and energy-restricted, low-fat diets on weight loss found the low-carbohydrate diet produced a significantly greater weight loss at 6 months (−4.3 kg; −5.6 to −3.0 kg, 95 % confidence interval), but not at 12 months (−1.0 kg; −3.5 to 1.5 kg, 95 % confidence interval) [33]. Finally, a 2-year trial also found that weight loss was not significantly different between a low-carbohydrate diet and an energy-restricted, low-fat diet (low-carbohydrate = −6.3 kg [−8.1 to −4.6 kg, 95 % confidence interval]; energy-restricted, low-fat = −7.4 kg [−9.1 to −5.6 kg, 95 % confidence interval]) [34]. Thus, weight loss appears to be greater in low-carbohydrate diets in the short term (<6 months), however over the long term (>12 months), weight loss appears to be comparable between a low-carbohydrate and an energy-restricted, low-fat diet.

For cardiometabolic outcomes, the results of one meta-analysis found that low-carbohydrate diets positively impact HDL-cholesterol and triglycerides, but negatively impact total cholesterol and LDL-cholesterol as compared to energy-restricted, low-fat diets at 6 months [33]. Little research has evaluated overall diet quality of a low-carbohydrate diet prescription. However, the carbohydrate goal in these diets does mean that intake of grains, fruit, starchy vegetables, and dairy is low. Research on the impact of a low-carbohydrate diet on appetite regulation is limited. A secondary data analysis of a 2-year RCT assessing appetite found that individuals who consumed a low-carbohydrate diet reported being less bothered or distracted by hunger than those on an energy-restricted, low-fat diet [27].

A high-protein diet is defined as consuming 20–30 % energy from protein [47]. For weight loss, high-protein diets also include an energy restriction. Consumption of a high-protein diet is believed to enhance weight loss due to two mechanisms: increasing dietary-induced thermogenesis [48, 49] and enhancing satiation [47, 50, 51]. Greater dietary-induced thermogenesis boosts overall energy expenditure, which could increase the degree of energy deficit incurred, assisting with weight loss. The enhanced satiation experienced with a diet high in protein may assist with appetite regulation, increasing adherence to a diet that is reduced in energy content [25]. A high-protein diet can be achieved through consumption of conventional foods, particularly meats, dairy, eggs, beans, and nuts; however, high-protein, portion-controlled liquid, and solid meal replacement products are available that may enhance dietary adherence to an energy-restricted, high-protein dietary prescription.

For weight loss outcomes, a meta-analysis compared energy-restricted, high-protein diets (mean percent energy from protein: 30.5 ± 2.4 %) to energy-restricted, standard protein diets (mean percent energy from protein: 17.5 ± 1.5 %) [52]. A subgroup analysis that only included studies with a duration ≥12 weeks found weight loss was not significantly greater in energy-restricted, high-protein diets compared to energy-restricted, standard protein diets (weighted mean difference = −0.49; −1.34 to 0.37, 95 % confidence interval).

A completers analysis from a 52-week RCT found that an energy-restricted, high-protein, low-fat diet had similar increases in HDL-cholesterol, and reductions in total cholesterol, LDL-cholesterol, triglycerides, glucose, insulin, blood pressure, and C-reactive protein as compared to an energy-restricted, high-carbohydrate, low-fat diet [53]. As weight loss was not significantly different between conditions and both groups lost significant weight over time, weight loss may be the contributor to improvements in these cardiometabolic measures.

While associations have been found between high-protein diets and satiation, RCTs investigating the impact of energy-restricted, high-protein diets on satiation are limited. One 12-week trial investigated the impact of an energy-restricted, high-protein diet (30 % energy from protein) vs. an energy-restricted, normal protein diet (18 % energy from protein) on weight loss and appetite [54]. Appetite sensations, specifically hunger, fullness, and desire to eat, were not significantly different between groups.

To better understand the impact of differing macronutrient alternations on weight loss, Sacks and colleagues [55

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