Obesity is defined as a chronic and inflammatory disease that develops due to excess fat accumulation in the body, which can disrupt health and cause disease (1). The body mass index (BMI) used in the classification of obesity is calculated as the body weight in kilograms divided by the height in meters squared (2). According to BMI values, individuals over 25 kg/m² are classified as having excess body weight, and individuals over 30 kg/m² are classified as obese (1). Excess energy taken from food accumulates in many different parts of the body such as the liver and muscles and can cause diseases (3). The main ones of these diseases are metabolic diseases, heart diseases, psychosocial, central nervous system, reproductive system and pulmonary diseases (4). There are many different treatment options for obesity. These are grouped under 5 different headings: nutritional therapy, physical activity, lifestyle and behavioral changes, drug therapy and surgery (5). Treatment should always start with medical nutrition therapy, which is shown as the safest method. However, surgery should be considered in individuals who do not respond and meet the treatment criteria (6). The criteria that individuals who are considered suitable for surgical procedures will have are BMI > 40 kg/m² or BMI 35–39.9 kg/m² and obesity-related (prediabetes, diabetes, dyslipidemia, obstructive sleep apnea, asthma, polycystic ovary syndrome, infertility, sexual dysfunction, hypertension, atherosclerosis, impaired kidney function) having at least one concomitant disease or BMI 30–34.9 kg/m² and concomitant diabetes and it is indicated in the form of metabolic syndrome (7).

The most basic purpose of surgical procedures is to reduce body weight and the health risks associated with obesity (8). There are 3 main procedures of bariatric surgery (BS): restrictive, absorbent and both restrictive and absorbent (9). Sleeve gastrectomy (SG), one of the surgical methods, is the vertical division of the stomach volume, leaving approximately one-fourth of it. In this process, the pyloric structure, gastric functions and digestive processes are not damaged (4). Reducing visceral fat in the body, accordingly, it is aimed to increase insulin sensitivity, reduce the amount of free fatty acids and interleukin-6 level (10). It has also been observed that their quality of life and psychosocial status have improved (11). However, there are also disadvantages. The most common complications are; leakage, hemorrhage, kidney stones, cholecystectomy, insufficient weight loss, liver or spleen injury, portal vein thrombosis, venous thromboembolism, breathing difficulties, abscess, stomach stenosis, venous thromboembolism, choledocholithiasis, pneumonia, sepsis, infection, minor complications and deaths. It can also cause macro and micronutrient deficiencies (12). It has also been observed that individuals regain weight after surgery due to the main reasons such as increasing the amount of energy consumed over time, choosing high-energy beverages, having five or more meals, insufficient activity level, advancing age, drug use or hormonal changes, increase in stomach volume, and high-carbohydrate diet (13).

When observational studies were examined, it was stated that the energy consumed should be similar to the general nutrition recommendations. It should be planned that 35–48% of the total energy should be met from carbohydrates, 37–42% from fat. Adequate and balanced nutrition recommendations should be determined by experienced dietitians during the day, the importance of high protein foods should be explained, and protein supplementation recommendations should be made if necessary (14). The most common macronutrient deficiency in all BS methods, especially in absorption-disrupting procedures, has been identified as protein (15). Protein deficiency is observed within the first month after surgical procedures, and the sensitivity to protein foods develops at the same rate (16). Protein deficiency is seen within the first month after surgical procedures, and sensitivity to protein foods develops at the same rate (16). This situation can occur in 3 main ways. The first is an aversion or feeling of nausea that develops against protein-rich foods due to changes in taste and smell. The second is the decrease in the amount of food taken due to the removal of a large part of the stomach volume. Finally, it is a decrease in the secretion of digestive enzymes, especially stomach acid and pepsinogen, and the passage of food through the digestive lumen very quickly (15, 17, 18). In addition, most patients after bariatric surgery are subjected to a solid liquid diet in the early period after surgery. They cannot consume a large amount of food at one meal or get solid protein in the first months. This indicates that there is a higher risk of developing protein malnutrition (19). It has been observed that inadequate protein intake can cause problems such as decreased skeletal muscle mass, low serum albumin level, endocrine imbalances, acceleration of the aging process, anemia, low immune system, bone and calcium losses, decrease in metabolic rate and failure to reach the target weight (20). A small amount of muscle mass loss can be expected in weight losses, but maintaining metabolic balance and muscle integrity is of vital importance (21).

Protein is an essential nitrogenous element necessary for all living life. It forms the building block of cells and enzymes that catalyze the metabolic reactions that take place (22). Protein is an essential nitrogenous element necessary for all living organisms. It is the building block of cells and enzymes that catalyze metabolic reactions (22). It is recommended as 0.8 g/kg/day for adults. The amount of intake can be changed during pregnancy and lactation, in growing children and in pathological conditions (23). Current guidelines recommend that patients consume 60–80 g protein per day or 1.2 g/kg of ideal body weight (24, 25). However, it has been reported that 45% of BS patients have problems in complying with these guidelines (26). A significant prevalence of lean body mass loss has been described in BS patients. Patients have been found to lose approximately 22% of their lean body mass within the first year after Laparoscopic Roux-en-Y gastric Bypass (RYGB) (27, 28). Moderately high protein diet intake has been found to preserve muscle mass and basal metabolic rate. However, it has also been found to support weight loss (29, 30). Considering these considerations, bariatric guidelines recommend the adoption of a relatively high protein intake in the first months after surgery, when the risk of energy restriction and associated loss of lean mass is higher (31, 32). During this period, the use of high-quality protein sources with high leucine content is of vital importance for many tissues and organs, especially muscle mass (33). When the literature was reviewed, it was observed that the number of studies on this subject was insufficient. The aim of the study was to evaluate the effect of protein supplementation recommended in addition to medical nutrition therapy on the muscle mass of patients after BS. To emphasize the importance of more comprehensive nutrition education before supplementation is considered.

Our study, which aimed to evaluate the success of protein supplements recommended in addition to postoperative nutrition education and their effect on muscle mass, was conducted with 60 individuals aged 19–60 years who underwent sleeve gastrectomy (SG) surgery and had similar characteristics. The groups were divided into two groups as protein supplement users (BSD + PS) and non-users (BSD). There was no significant difference between the groups in terms of muscle mass in the postoperative period. Although supplementation increased protein intake, we found that it caused compliance difficulties due to different reasons. In addition, we observed that not all individuals were able to complete the targeted daily intake. We determined that more comprehensive, strictly monitored and nutrition education prepared by expert dietitians is required before protein supplementation is considered.

High weight loss in postoperative processes may lead to undesirable loss of muscle mass (43). Since it has a great effect on the regulation of metabolic balance, preservation of skeletal muscle integrity and functional capacity according to age, precautions should be taken (21). Surgery can realize the expected weight loss with new arrangements in the anatomical structure (44). However, the intake, digestion and absorption of many nutrients, especially amino acids, change due to these new arrangements and cause nutritional deficiencies. One of the major reasons for this has been defined as a decrease in gastric volume and inadequate release of digestive enzymes, especially hydrochloric acid (15). It has also been observed that taste and odor sensitivity may develop in the postoperative period, especially in the first month (13).

In a study by Andreu et al. examining the effect of protein intake on muscle mass and blood parameters, it was found that supplement use despite recommendations was 63.4, 50.5, and 33.7% at 4th, 8th, and 12th months, respectively. In addition, 45, 35, and 37% of the targeted protein intake was <60 g/day at 4th, 8th, and 12th months, respectively. Male gender and weight loss were significantly associated with muscle mass independent of protein intake. It was emphasized that compliance with targeted protein supplements was poor (14). Another study supporting this situation was revealed in a study conducted by Bertoni et al. (45). Assessed protein intake in the first three months after sleeve gastrectomy (SG) in 47 patients with severe obesity. It found that protein intake from foods was insufficient, averaging 30.0 g/day in the first month and increasing to 34.9 g/day (p = 0.003) by the third month, both below the recommended 60 g/day. The use of protein supplementation significantly increased total protein intake to 42.3 g/day (p < 0.001) in the first month and 39.6 g/day (p = 0.002) in the third month, but compliance with supplementation was low, dropping from 63.8 to 21.3%. Overall, the study concluded that despite dietary guidance and supplementation, protein intake remained inadequate in the early post-operative period (45). In our study, there was a significant increase in the protein intake of both groups at the end of 3 months, but the recommended protein intake was not reached. Literature data support our study.

In a 2016 study by Schollenberger et al. (39) researchers examined the effect of 15 g/day protein supplementation in addition to standard nutritional therapy on body weight, fat mass loss, and muscle mass preservation after surgery. The results showed that protein intake amounts were significantly higher in the protein supplementation group compared to the control group (p < 0.001). At the 6-month follow-up, fat mass loss was found to be significantly greater in the protein supplementation group (79%) compared to the control group (73%) (p = 0.02). Additionally, muscle mass loss was less significant in the protein supplementation group (21%) compared to the control group (27%) (p = 0.05). However, when analyzing overall body weight loss, the results were similar between the protein supplementation group (25–7.2%) and the control group (20.9–3.9%) (p > 0.05).

In a study examining the effects of high protein (2 g/kg/day) and standard protein (1 g/kg/day) diets on anthropometric measurements after surgery, the high protein group showed a significant decrease in fat mass from the 3rd month (p < 0.01), while muscle mass and basal metabolic rate were preserved (p < 0.01). Another study also found that protein supplementation prevented muscle loss. In the group receiving 1.2 g/kg/day protein supplementation in the 1st month, muscle mass at the end of the 6^th month was similar to the control group (46). In our study, we also found a significant decrease in muscle loss over time with protein supplementation, but no difference between the groups (Group: F = 3.297; p = 0.075, time: F = 202.784; p < 0.001, group*time; F = 0.317; p = 0.743).

Supplements may be considered to have an indirect effect by helping to support reaching the targeted intake, but the expected adequate protein intake (<60 g/day) could not be reached in nutritional therapy due to reasons such as the 3-month follow-up of the study due to pandemic conditions, the possibility of food intolerance in individuals, gastrointestinal complaints caused by some foods, shrinking stomach volume and changing hormonal systems.

In the study conducted by Dagan et al. (47) in 2016, when the average energy intake of all individuals followed up was examined, it was found to be 2117.6 ± 920.9 kcal in the preoperative period, 838.9 ± 348.5 kcal in the 3rd month, 1105.9 ± 453.7 kcal in the 6th month and 1296.5 ± 496.5 kcal in the 12th month. When the daily carbohydrate intake values were examined, it was found to be 222.2 ± 125.8 g in the preoperative period, 90.8 ± 47 g in the 3rd month, 121.1 ± 67.2 g in the 6th month and 142.3 ± 69.8 g in the 12th month. When the average daily fat intake was examined, it was found to be 90.4 ± 43.9 g in the preoperative period, 32.7 ± 16.9 g in the 3rd month, 43.9 ± 19.4 g in the 6th month and 52.4 ± 22.6 g in the 12th month. When the daily protein intake was examined, it was found to be 93.5 g in the preoperative period, 41.4 ± 18.5 g in the 3rd month, 51.7 ± 18.9 g in the 6th month and 58.1 ± 22.9 g in the 12th month.

In a study conducted by Giusti et al. (48) in 2015, the average daily energy intake of 16 women decreased from 2072 ± 108 kcal preoperatively to 681 ± 58 kcal in the 1st month, then gradually increased to 1.448 ± 57 kcal by the 36th month. Average daily protein intake declined from 87 ± 4 g preoperatively to 29 ± 2 g in the 1st month, later reaching 57 ± 3 g at 36 months. Carbohydrate intake dropped from 231 ± 11 g preoperatively to 76 ± 8 g in the 1st month, then increased to 144 ± 6 g by 36 months. Similarly, fat intake decreased from 89.7 ± 7 g preoperatively to 29 ± 3 g in the 1st month, before rising to 76 ± 5 g at 36 months. Overall, significant decreases in energy, protein, carbohydrate and fat intake were observed in the early postoperative period, with gradual increases over the long-term.

In the study conducted by Gobato et al.’s (49) in 2014, study found that pre-operative daily energy intake was 1812.2 ± 767.52 kcal, decreasing to 1610.3 ± 322.25 kcal by the 6th month. Daily protein intake also decreased from 96.2 ± 36.13 g pre-operatively to 47.1 ± 17.70 g at 6 months. The average daily energy intake was higher at 2555.7 ± 806.24 kcal in the BSD + PS group and 2457.8 ± 913.92 kcal in the BSD group, compared to other studies. Daily zinc intake decreased from 10.0 ± 4.01 mg pre-operatively to 6.9 ± 3.25 mg at 6 months. Intakes of vitamins and minerals like vitamin B12, iron, calcium, magnesium, and folic acid also decreased significantly from pre-operative to 6-month levels. In our study, it was determined that the iron, calcium, zinc, magnesium and potassium mineral intake averages were below the targeted values in all periods, and similarly, vitamins, especially vitamin A, vitamin C, thiamine and riboflavin, fell below the targeted values in the 1st and 3rd months after surgery.

The existing literature supports that protein intake is inadequate in the postoperative period and adherence to protein supplementation is low. The most important limitations of our study are the small sample size and the relatively short follow-up period. We focused our attention on the first period after surgery, when the protein intake issue is more important and weight loss is faster. In order to better evaluate the clinical impact of our results, they can be supported by further studies with longer follow-up and body composition data. However, our sample was mostly limited to female gender and adult patients, and the physical activity levels recorded in the first three months after surgery were very low. In our study, we evaluated only patients treated with SG, and we can assume that postoperative protein intake may vary in different types of bariatric surgery.

In post-bariatric surgery patients, muscle mass preservation is highly related to protein intake. However, in our prospective study, the targeted daily minimum protein intake level could not be reached even with protein supplements. No effect of daily protein intake with protein supplements on the decrease in muscle mass loss or increase in fat mass loss was detected during the 3-month follow-up period. In addition, undesirable reasons such as the fact that protein supplements consumed by individuals may cause gastrointestinal complaints, the feeling of boredom caused by daily use of these flavors, and the inability to maintain consumption discipline made it difficult to comply with the targeted protein intake recommendations. For this reason, it was observed that the targeted daily intake amounts of other food groups could not be reached. In addition to the protein malnutrition experienced by post-bariatric surgery patients, vitamin and mineral losses were found to be significantly low. This study also raises awareness among existing healthcare providers who need to encourage adequate protein intake in post-bariatric surgery patients. It was concluded that new postoperative diet models that include more rigorous and intensive training programs that take into account all food groups, especially protein, are needed before considering supplements to minimize muscle mass losses. Studies investigating the quantity (g/day) and quality (whey, casein or soy) of protein supplements or high-protein diets in these models in larger study populations are needed.