The present study showed that both diets improve the metabolic profile and body composition, but the MRT causes more significant weight loss than the ULC diet.
In line with the present study results, in a study by Tsong-Ming Lu et al. (15), body weight and body fat levels were significantly reduced after eight weeks of a meal replacement diet. There was no control group in this study, and only pre-diet and post-diet evaluation was performed. Also, two meals were replaced with meal replacements, and calorie intake was limited so that women received less than 1,200 kcal per day and men less than 1,500 kcal per day. Also, in a 26-week study by Lewis et al.(16)and a 12-week study by Haywood et al.(17), consistent with our findings, MRT caused more significant weight loss. Although further weight loss occurred in the intervention group in the studies mentioned above, there was a difference in daily calorie intake and the distribution of macronutrients between the two study groups. For example, in the study by Haywood et al.(17), one group received only dietary recommendations; one group received a typical diet with a reduction of 500 kcal of the total daily requirement, which was 30% calories from protein, 30% calories from fat, and 40% total calories from carbohydrates, and the third group received a Very low calorie diets (VLCD) using meal replacement, which in this study, did not receive the same calories in the three groups.
Also, in a study by Lewis et al. (16), a group received a diet of 800 to 1200 kcal with 40% protein, 40% carbohydrates, and 20% fat in meal replacements based on BMI, while the control group received a diet with an emphasis on reducing fat intake to 25 to 30% of total calories and reducing 500 to 750 kcal of the total daily requirement. In this study, the caloric intake was different in the two groups. These differences in calories and macronutrient distribution between the two groups may have affected the results of these studies.
In a study by Chaiyasoot et al.(18), after 12 weeks of dieting, the MR group achieved more significant weight loss than the control group. In Metzne et al.(19)study, both groups received an energy-restricted diet of approximately 1200 kcal/d. The dietary intervention resulted in significant weight loss in both groups, without a significant difference between the two groups; however, the MR group's weight loss was higher than the Control group. In our study, similar to Metzne's study, the caloric intake between the two groups was the same, but the results of the two studies were different.
Differences in the results of studies may be due to differences in the duration of interventions and differences in the type and distribution of macronutrients in diets. For example, although meal replacements have been used in these studies, the type of replacements, calories, and distribution of macronutrients have varied.
In our study, hunger feeling was less common in the MRT group than in the ULC group. This may be one of the main reasons for more significant weight loss in this group, leading to greater adherence to the diet. Although no proper tools were used in this study to assess satiety or huger feeling, consuming more protein (10gr/day more in MR group(20)(21) and compounds in meal replacements in this group may increase satiety and thus increase adherence to the MRT diet.
In our study, the 24-hour dietary recalls were collected from participants, but these energy intake estimation tools are not quantitatively precise, although there was no significant difference in calorie intakes. In this nutritional assessment, the intake is often misreported, such as not remembering to consume specific foods or incorrectly estimating portion sizes. So, the probability of misreporting food recalls may affect study results (22)(23). Another reason may be the meal replacement ingredients. For example, each meal replacement sachet contains 420mg of calcium, which can help regulate weight, reduce body fat, and increase energy expenditure (EE), according to previous studies (24). We compared the macronutrient intakes between the groups, and it is recommended to compare micronutrients in future studies.
Although non-significant compared to ULC, in our study, MRT caused a more significant reduction in TBFM. No significant difference was detected between the two groups in the study by Chaiyasoot et al.(18)in FM. However, in fat-free mass (FFM), there was a significant decrease in the MR group, but not in the control group, but in the study of Lewis et al.(16) MRT caused a more significant reduction in both TBFM and FFM, and in the study by Haywood et al.(17) MRT caused a higher decrease in fat mass and an increase in muscle mass. Differences in the results of studies may be due to differences in the duration of interventions and differences in the type and distribution of macronutrients in diets.
The present study's findings showed that after eight weeks of dieting, RMR decreased in both groups, but this decrease was not significant in the MRT group or comparison between the two groups. In the study by Smith-Ryan et al.(25), which used High-Fat Breakfast Meal Replacement, RMR also decreased, but the reduction was not significant. In Alex E. Mohr et al. study, In 2020 (26), consumption of an MR meal increases postprandial thermogenesis compared to a whole food meal.
Given that in our study, the reduction in FFM was equal in both groups, the lower decline in RMR in the MRT group could not be explained by maintaining FFM, but changes in sympathetic activity or other metabolic factors may affect the IC results. It should also be borne in mind that FFM is composed of different components, and that their changes may have been different between the two groups. For example, differences in organ changes with a high metabolism (e.g., skeletal muscle) or changes in total body water can affect FFM (27). Also, studies have shown that the weight of internal organs is sensitive to nutritional changes and can cause resting energy expenditure (REE) changes (28). In addition, meal replacement sachet contains vitamins and minerals required by the body, and differences in nutrient intake between the two groups may have affected the results and could explain the lower reduction in RMR in the MRT group. This explanation is based on the findings of previous studies. In a systematic review by Liu et al.(29)and a study by Carsten et al.(30), compounds such as vitamin D and vitamin A could alter the metabolism rate by acting on brown adipose tissue (BAT) thermogenesis. For example, vitamin A can increase the thermogenesis in BAT by increasing mitochondrial biogenesis and increasing the proliferation of the UCP1 gene, which may be one of the possible mechanisms for increasing RMR or preventing its decrease (29). Calcium can also be effective in weight regulation, which may be through thermogenesis and increasing energy consumption(31). Polyunsaturated fatty acids (PUFAs), such as linoleic acid, can produce more heat and higher EE than saturated fats by activating the transient receptor potential vanilloid1 (TRPV1) channel (29). In our study, saturated fat consumption was lower in the MRT group. Also, in this group, with each meal replacement sachet, 1.2 grams of linoleic acid was received, which can be one of the reasons for the difference in RMR changes between the two groups.
In the present study, after eight weeks of dieting, the lipid profile, fasting plasma glucose, and insulin decreased significantly in both groups, but there was no significant difference between the two groups. Consistent with our study, in a study by Lu et al.(15), the lipid profile improved after eight weeks of meal replacement, and serum insulin and HOMA-IR insulin resistance were significantly reduced. In the study of Metzner et al.(19), lipid profiles improved in both groups, and there was no significant difference between the two groups. In the study of Gómez et al.(32), one group replaced two meals a day, and the other group replaced one meal a day with meal replacements, no significant difference was observed in lipid profile changes between the two groups, and these results were consistent with our findings.
Contrary to our findings, in Chaiyasoot et al.(18)study, FPG was significantly reduced in the MR group after 12 weeks compared to baseline but not in the control group.
In our study, the level of satisfaction with the diet did not differ significantly between the two groups. However, gastrointestinal symptoms (nausea and diarrhea) were further reported in the intervention group. In the Davenport study (33), which was performed on two different meal replacements, The rate of bloating was higher in the group receiving Optifast. Also, in the Lewis study(16), 11 percent of those who found meal replacement experienced diarrhea, and 11.6 percent experienced nausea, while in the control group, this rate was 4 and 2 percent, respectively.
Our study evaluated several outcomes, including metabolic profile, RMR, dietary tolerability, physical symptoms, and weight changes. Furthermore, the study only included women, controlling one of the most important confounding factors. However, our study did not consider other variables affecting dieting success, such as sleep patterns and mental status. Due to the COVID-19 pandemic, there were many challenges in face-to-face and indirect calorimetry. In the last visit, indirect calorimetry was not performed on some people due to suspicious symptoms or unwillingness of the participant. We also did not calculate specific calorie requirements based on the individual needs, and although we used 24-hour dietary recall, reporting was poor. It would have been better to use a more valid method to assess the side effects of the diet. Finally, randomization and blinding were not possible due to the study design.
We suggest that in future studies:
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The intake of micronutrients, caffeine, and fatty acids be examined. (We propose that future research look into micronutrients, caffeine, and fatty acids.)
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Due to the possible effect of BMI on weight loss (34)and diet adherence, It is best to do further studies on people with a closer BMI range.
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To check food intake and adherence to the diet during the study, instead of a 24-hours recall, it is better to use a more reliable method) Web-based programs, mobile applications, and wearable devices)(35).
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If the study is performed during a coronavirus epidemic, consider more drop in outing rate.