Assessment of body composition

Accurate assessment of body composition is essential for evaluating the impact of different nutritional approaches in PLwO [9]. Several techniques are available to measure FM, FFM, and other relevant parameters [14].

Dual-energy X-ray absorptiometry (DEXA) is widely used for its precision in distinguishing fat distribution and lean mass [15]. Bioelectrical impedance analysis (BIA) offers a more accessible and cost-effective tool, though its reliability may be influenced by hydration status [16, 17]. Imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) provide detailed assessments of adipose tissue distribution but are less feasible in routine clinical settings due to higher costs and logistical demands [18].

Air displacement plethysmography (ADP), commonly referred to as the BOD POD, is another reliable method for estimating body composition and is often used in both clinical and research environments [19]. The ViScan, a specialized abdominal BIA device, enables distinction between visceral and subcutaneous adiposity and has demonstrated clinical relevance in assessing obesity-related risk [20]. Although skinfold measurements are still employed, their accuracy is limited by operator variability and reliance on prediction equations [21].

In practice, the choice of assessment technique should consider resource availability, clinical context, and the specific objectives of the evaluation—particularly in monitoring changes in fat mass and lean mass throughout dietary treatment.

Caloric Restriction and Energy Deficit (LED, VLED)

Low-energy diets (LED) and very-low-energy diets (VLED) are structured nutritional interventions aimed at inducing significant weight loss through severe caloric restriction [23]. Table 1 shows the composition of LED and VLED. VLEDs are typically formulated as commercially prepared liquid products designed to replace all regular meals and are fortified with essential micronutrients to prevent deficiencies [23].

Table 1 Effects of nutritional approaches on body composition

The primary goal of VLEDs is to achieve rapid weight loss, approximately 1.0–2.5 kg per week, while preserving LM as much as possible [30]. However, even when protein intake is as low as 50 g/day, studies have reported that approximately 25% of total weight loss consists of LM, while 75% is fat loss [31].

As for body composition, studies showed that they were effective in inducing substantial short-term weight loss. For example, a randomized controlled trial (RCT) conducted by Harshal et al. compared VLEDs to moderate energy-deficit approaches in women living with obesity and polycystic ovary syndrome (PCOS), and body composition was measured at baseline and follow-up visits by DEXA [32]. After eight weeks, the VLED group exhibited significantly greater weight loss and reductions in WC and total and trunk fat compared to the conventional approach. However, both interventions resulted in losses of LM and FFM, while BMC and bone mineral density (BMD) remained unchanged [32].

Additionally, an observational prospective study by Serafim et al. evaluated body composition by BIA in 120 PLwO undergoing bariatric surgery (BMI: men 50.6 ± 1.12 kg/m2, women 49.1 ± 0.58 kg/m2) during a VLED [33]. The study reported significant reductions in weight and BMI, along with decreases in WC and hip circumference. Both FM and FFM showed significant reductions [33].

While aggressive caloric restriction can be an effective strategy for inducing rapid weight loss, its long-term effectiveness remains controversial. Meta-analyses have shown that VLEDs do not necessarily result in superior long-term weight loss compared to less restrictive approaches like LED [22]. Moreover, an RCT involving 49 women living with obesity compared long-term weight loss outcomes between a 1200 kcal/d diet of conventional foods and a VLED [34]. Women on the conventional diet, who lost an average of 11.9 kg over 6 months and attended 39 group behavioral maintenance sessions, maintained a loss of 12.2 kg after one year. Conversely, those who followed the VLED initially lost 21.5 kg over the same period but retained only 10.9 kg of that weight loss despite receiving the same maintenance sessions [34].

The effectiveness of VLEDs and LEDs in promoting weight loss and altering body composition is primarily driven by creating a substantial caloric deficit, resulting in negative energy balance [30]. This deficit forces the body to mobilize energy stores, particularly adipose tissue, to meet energy demands [30]. The significant reduction in carbohydrate intake associated with VLEDs induces glycogen depletion, which is accompanied by a loss of water weight, thereby contributing to rapid initial weight loss [22]. Additionally, the reduction in insulin levels and enhancement of lipolysis facilitate fat oxidation [22].

Despite their effectiveness in promoting rapid weight loss, VLEDs present several drawbacks that may compromise their long-term success and safety.

Excessive caloric restriction, especially when protein intake is inadequate, can lead to substantial loss of LM, negatively affecting metabolism and physical function [31]. Furthermore, the rapid weight loss associated with VLEDs can increase the risk of gallstone formation and exacerbate pre-existing medical conditions if not properly supervised [35]. Another significant limitation is the difficulty in maintaining weight loss over the long term.

Studies suggest that individuals following VLEDs may regain a substantial portion of lost weight unless accompanied by structured maintenance programs and behavioral interventions [34]. Additionally, adverse effects such as fatigue, dizziness, muscle cramps, constipation, and hair loss have been commonly reported during prolonged VLED use [22]. Continued research is necessary to identify strategies that minimize adverse effects while preserving LM and promoting sustainable weight loss.

Low-Fat Diets

The composition of the low-fat diet (LFD) and very low-fat diet (VLFD) is reported in Table 1.

As for body composition, a systematic review by Hooper et al. examined 32 RCT involving approximately 54,000 PLwO [36]. The analysis demonstrated that reducing dietary fat, without intentional caloric restriction, consistently resulted in modest reductions in body weight, FM, and WC. However, the reduction in fat intake likely caused a reduction in total energy intake, indirectly contributing to weight loss [36].

VLFD has been primarily studied in the context of vegetarian and vegan diets [24]. Indeed, a recent RCT involving 168 adults aged 25 to 75 years with a BMI between 28 and 40 kg/m2 compared the effects of a low-fat vegan diet to a control diet [37]. Body composition was assessed using DEXA, revealing that the vegan group experienced significant reductions in body weight, primarily due to a decrease in FM and VAT.

While these diets have shown positive effects on weight loss, studies that include body composition analysis are limited.

Overall, while LFDs can produce modest weight loss and fat reduction, their impact on LM preservation remains unclear [37].

An RCT involving 148 PLwO compared a low-fat diet to a low-carbohydrate diet (with carbohydrate intake below 40 g per day), and body composition was measured using BIA [38]. Results indicated that adherence to the low-carbohydrate diet was associated with greater reductions in body weight and FM and, notably, an increase in FFM percentage. Specifically, higher adherence to the low-carbohydrate diet corresponded to more weight loss, FM reduction, and improved FFM preservation [38].

The rationale behind LFDs is that reducing dietary fat, the most energy-dense macronutrient, helps to create a caloric deficit [39]. Experimental studies manipulating fat content in diets have shown that higher-fat diets lead to greater weight gain or less weight loss due to their higher energy density [39].

However, the long-term effectiveness of LFDs remains unclear, as participants often adapt to the reduced energy density over time, potentially diminishing their impact on weight loss.

Low-Carbohydrate Diets and Ketogenic Diets

The composition of the low carbohydrate diet (LCD) and ketogenic diet (KD) is reported in Table 1.

In recent years, KDs have attracted great interest in the treatment of PLwO and obesity-related metabolic disorders. However, although KDs are a dietary intervention designed to induce nutritional ketosis, different diets with different macronutirients compositions have been called this name, and this may result in bias and mistakes in the interpretation of current evidence. To clarify this variability, Table 2 also reports the main types of ketogenic dietary protocols currently used in clinical and research settings, highlighting their distinguishing macronutrient profiles. For PLwO, very low-calorie ketogenic diets (VLCKD), recently renamed very low energy ketogenic therapy (VLEKT) to avoid misconceptions with very low carbohydrate diets[40] are currently the most used in clinical practice. Of note, this definition emphasizes a more precise approach, characterized by both a minimal carbohydrate intake (< 50 g/day) and a significantly reduced caloric intake (typically under 800 kcal/day), which distinguishes it from other dietary approaches and better reflects its therapeutic application [40].

Table 2 Ketogenic protocols

The European guidelines for managing PLwO have recently shown the effects of VLEKT on body weight and body composition [26]. In the short, medium, and long terms, the authors found that VLEKT caused a notable reduction in body weight. Additionally, VLEKT has shown greater success in reducing body weight, namely WC and FM, when compared to other weight loss dietary treatments of the same length. Crucially, FFM preservation and the selective burning of fat in visceral rather than subcutaneous adipose tissue compartments optimize body composition and result in weight loss during VLEKT [26].

Furthermore, VLEKT, has shown greater reductions in FM compared to the Western diet [41]. Paoli et al., in an RCT prospective study of 16 athletes of normal weight, found a significantly higher decrease of FM (p = 0.0359), VAT (p = 0.0018), WC (p = 0.0185), and extracellular water (p = 0.0060) in the KD compared to the WD group. Body composition was assessed using DEXA. Lean soft tissue, quadriceps muscle area, maximal strength, and REE (resting energy expenditure) showed no changes in both groups [41]. The mechanisms underlying the potential benefits of KD and LCD on body composition remain debated.

KD is proposed to enhance fat oxidation through reduced insulin levels, promoting greater fat loss [42]. Additionally, KD appears to offer notable benefits in appetite regulation and spontaneous reduction in energy intake, particularly under ad libitum conditions [43,44,45,46,47]. This appetite-suppressing effect may contribute to fat loss without purposeful caloric restriction, potentially making KD a valuable tool for weight management [43,44,45,46,47]. However, rigorous studies comparing isocaloric, protein-matched KD to non-KD conditions have not demonstrated significant metabolic advantages in fat loss, suggesting that the greater weight loss observed with KD may be largely attributed to higher protein intake, which enhances satiety and reduces overall energy intake [48]. Furthermore, a comprehensive review by Hall and Guo, including 32 isocaloric, protein-matched studies, found no evidence supporting a metabolic advantage of carbohydrate restriction [49].

While LCDs and KDs are effective in reducing body weight and FM, their ability to preserve or enhance LM remains inconsistent. Future research should focus on distinguishing whether the benefits are primarily due to macronutrient composition or overall caloric reduction. Nonetheless, KD may hold specific advantages in terms of appetite control and adherence, making it a promising dietary approach for weight and FM management.

High-Protein Diets

High-protein diets (HPDs) lack a universally accepted definition, but they are generally described as providing at least 25% of total energy from protein [24]. Table 1 shows the composition of HPDs. As for body composition, research conducted by Layman et al. demonstrated that consuming protein at twice the RDA (1.6 g/kg) consistently outperformed the standard RDA (0.8 g/kg) in preserving LM and reducing FM [50, 51]. However, recent studies by Longland et al. have shown that, under conditions involving high-intensity interval training and resistance exercise, higher protein intake (2.4 g/kg) resulted in gains in LM and substantial FM. In contrast, a lower intake (1.2 g/kg) only managed to preserve LM and led to less FM [52].

Arciero et al. further supported these findings through their studies on “protein-pacing” strategy involving the consumption of 4–6 meals per day with over 30% of protein per meal (equating to > 1.4 g/kg/d). This approach has demonstrated superior outcomes in improving body composition compared to traditional lower-protein diets under hypocaloric conditions for FM, central adiposity, and increases in LM [53, 54].

Protein is known for its high thermic effect and considerable metabolic cost. Enhanced protein intake has been associated with the preservation of resting energy expenditure during caloric restriction [27].

Mediterranean Diet and Quality-Based Approaches

The composition of macronutrients in the Mediterranean diet (MD) is reported in Table 1.

The MD is recognized as a promising nutritional strategy for managing weight and body composition [9]. Its main characteristics include a high intake of vegetables, fruits, nuts, whole grains, and extra-virgin olive oil, with moderate consumption of fish and poultry, while limiting sweets, red meat, and dairy products [55]. This dietary pattern is rich in monounsaturated fats (MUFA) and fiber, with low levels of saturated fats and a balanced omega-6/omega-3 ratio [56]. Interventional studies suggest that the effects of the MD on body weight are more closely related to energy intake than macronutrient composition. Notably, even when calorie intake is not restricted, this dietary pattern does not lead to weight gain [57].

The largest RCT to date involved 7,447 participants (most of whom with overweight or obesity), divided into three groups: MD supplemented with olive oil, MD supplemented with nuts, and a low-fat diet. The median follow-up period was 4.8 years [58]. In the MD groups, fats accounted for approximately 42% of daily energy intake, with no calorie restriction or promotion of physical activity, despite a high prevalence of overweight and obesity.At the end of the study, all groups exhibited slight weight reductions but increased WC. Compared to the low-fat diet group, neither of the MD groups showed significant differences in body weight. However, the MD was associated with reduced central adiposity, as reflected by adjusted WC differences after 5 years [58].

A meta-analysis of 16 RCTs (n = 3,436) found that the MD was associated with greater weight loss compared to control diets, particularly when combined with calorie restriction and increased physical activity [59]. Another meta-analysis of five RCTs (n = 998) reported that the MD was more effective than low-fat diets for weight loss but showed similar results to low-carbohydrate and the American Diabetes Association diets. Effects on BMI and WC were comparable to those on weight reduction [60].

Cross-sectional studies have also reported an inverse association between adherence to the MD and central adiposity [61,62,63]. The positive effects on central adiposity and VAT are attributed to high MUFA and polyunsaturated fatty acid (PUFA) intake and reduced saturated fat intake [56].

Short-term studies demonstrated that the isocaloric MD rich in extra-virgin olive oil prevents central fat accumulation better than low-fat diets without significantly affecting body weight in PLwO [64]. Other interventions reported reduced VAT with the MD over 2 months [65, 66]. A protein-enriched, calorie-restricted MD for eight weeks resulted in substantial reductions in weight, VAT, and FM, while preserving FFM in PLwO awaiting bariatric surgery [66].

A systematic review of 18 trials involving 7,186 PLwO concluded that the MD reduces central adiposity as shown by decreased WC, waist-hip ratio, and VAT, although the most consistent results were seen for WC [67].

Overall, the MD appears to be effective in reducing body weight, particularly when energy-restricted and combined with exercise. Even when not energy-restricted, it does not promote weight gain. The MD also demonstrates potential for reducing central adiposity and metabolically harmful VAT, making it a beneficial choice for PLwO or those at risk of cardiovascular and metabolic diseases.

Fasting and Time-Restricted Eating

The composition of macronutrients during fasting is reported in Table 1. Fasting encompasses various dietary protocols (Table 3) aimed at cycling between periods of eating and fasting to achieve caloric restriction and metabolic benefits [68]. The primary approaches to fasting can be categorized into alternate-day fasting (ADF), whole-day fasting (WDF), intermittent fasting (IF), and time-restricted feeding (TRF) [68].

Table 3 Fasting protocols

ADF is one of the most extensively studied forms of fasting [69]. It involves a 24-h fasting period alternated with a 24-h ad libitum feeding period [69]. Research indicates that total energy intake tends to be reduced on feeding days, leading to weight loss and FM reduction even without deliberate caloric restriction [70]. ADF also shows potential for preserving LM, although some studies have reported LM loss due to severe energy deficits. Notably, Catenacci et al., in a RCT in 45 PLwO (BMI ≥ 30 kg/m2, aged 18–55 years) and evaluated body composition by DEXA and RMR [71] found that ADF, involving zero caloric intake on fasting days alternating with unrestricted feeding days, produced comparable results to daily caloric restriction in terms of body composition, in particular a significant reduction in total FM and trunk FM after 8 weeks of intervention, and even demonstrated superior outcomes after six months of unsupervised weight loss maintenance [71].

A variant of ADF, known as alternate-week energy restriction, involves one week of approximately 1300 kcal/day followed by a week of usual diet [72]. This approach has proven as effective as continuous energy restriction in reducing body weight and WC over both short-term (8 weeks) and long-term (1 year) periods [72].

TRF typically involves limiting food intake to specific periods each day, generally ranging from 16 to 20 h of fasting, followed by a feeding window of 4 to 8 h [73]. One of the most studied forms of TRF is Ramadan fasting, which involves refraining from eating and drinking from sunrise to sunset for approximately one month, resulting in weight loss through reductions in both LM and FM [74].

More structured studies on TRF include an 8-week study by Tinsley et al., which applied a protocol of 20 h of fasting followed by a 4-h feeding window, four days per week, in healthy, recreationally active men and assessed body composition by DEXA [75]. Despite a significant reduction in caloric intake on fasting days, muscle growth was similar in both the TRF and normal diet groups when combined with resistance training. However, the TRF group exhibited a slight tendency toward LM loss, although strength improvements were comparable between groups [75].

In contrast, Moro et al. investigated a 16-h fasting and 8-h feeding protocol in resistance-trained participants and evaluated the body composition by DEXA [76]. Their findings demonstrated significantly greater FM loss in the TRF group compared to those following a normal diet, with no significant impact on LM. Enhanced fat loss was suggested to be linked to elevated adiponectin levels, which may promote mitochondrial biogenesis and enhance energy expenditure. However, the TRF group also experienced unfavorable hormonal changes, such as reduced testosterone and triiodothyronine levels [76].

Furthermore, Deying et al. [29] conducted a 12-month study involving 139 PLwO (BMI 28–45 kg/m2), randomly allocated to either a TRF (8:00 a.m. to 4:00 p.m.) with calorie restriction or daily calorie restriction without time limitation, and evaluated body composition by DEXA. All participants adhered to a calorie-restricted diet, and after 12 months, the average weight loss was − 8.0 kg in the TRF group and − 6.3 kg in the daily calorie restriction group. The difference in weight loss between the groups was not statistically significant. Additional analyses of WC, BMI, BF, LM, blood pressure, and metabolic risk factors produced results consistent with the primary outcome, with no significant differences in the occurrence of adverse events between the two groups [29].