Factors influencing energy intake and body weight loss after gastric bypass


Objective: The gastric bypass-induced quantitative and qualitative modifications of energy intake (En In, kcal/day) and their impact on body weight (bw) loss were evaluated. The factors influencing energy intake and body weight loss were also investigated.

Design: Longitudinal study.

Setting: University Hospital of Geneva.

Subjects: Fifty obese women undergoing a Roux-en-Y gastric bypass.

Results: The reduction of EnIn was significantly related to bw loss expressed either in kg or as percentage correction of excess bw (P<0.01 for both), whereas the post-operative modifications of diet composition did not play a role. Age and initial bw significantly influenced bw loss (P<0.0001 and P<0.001, respectively), as shown by multiple regression analysis. Patients were divided into four sub-groups according to their age (under or over 35 y) and initial bw (under or over 120 kg). ANOVA showed that under 35-y-old subjects reduced their EnIn significantly more than their older counterparts having similar bw (P<0.02 and P<0.05); consequently, bw loss, expressed in kg, was significantly (P<0.0001 and P<0.0005) larger in younger patients. Subjects with an initial bw over 120 kg lost significantly (P<0.001 and P<0.02) more weight as compared to patients with a smaller degree of obesity (under 120 kg) and similar age.

Conclusions: Gastric bypass-induced body weight loss is mainly due to the reduction of EnIn, whereas the qualitative modifications of the diet do not play a role. Younger subjects have a greater capacity to reduce EnIn and, therefore, lose more weight. Pre-operative high degree of obesity leads to a larger weight reduction, probably because of a greater energy deficit.


Roux-en-Y gastric bypass is one of the most widely utilized techniques in the surgical therapy of obesity. This procedure induces a substantial body weight loss essentially by reducing energy intake. Several studies have consistently demonstrated that, when compared to other restrictive surgical procedures, Roux-en-Y gastric bypass is the one which induces the largest and more durable weight reduction (Pories et al, 1982; Näslund et al, 1986; Hall et al, 1990); this result is due to a more pronounced reduction of energy intake following the operation (Näslund et al, 1988; Brolin et al, 1994).

Although it is well established that Roux-en-Y gastric bypass yields, in the majority of the cases, a successful result, it is a common observation that the extent of weight reduction is quite variable from one individual to another, when expressed in absolute values, as well as percentage correction of excess body weight. Since gastric bypass-induced body weight loss mainly results from the reduction of energy intake, it is reasonable to expect that also this last parameter is variably modified by the surgical intervention.

As mentioned above, several studies have investigated the differences in food consumption between patients submitted to various restrictive surgical techniques (Näslund et al, 1988; Brolin et al, 1994; Kenler et al, 1990), whereas to our knowledge no study has analyzed the differences in energy intake among a group of patients who underwent the same surgical procedure.

Therefore, the first aim of the present report was to investigate the quantitative modifications in energy intake occurring after Roux-en-Y gastric bypass and their impact on body weight loss. On the other hand, we also attempted to individualize some pre-operative patient characteristics that could influence the variability of the energy intake following the intervention.

It has been reported that, from a qualitative point of view, gastric bypass induces some profound modifications of alimentary habits. Hence sweet eaters would particularly benefit from Roux-en-Y gastric bypass intervention since, by inducing a dumping syndrome, it drastically reduces the intake of rapidly absorbed carbohydrates (Sugerman et al, 1987, 1989). This conclusion, however, has not been shared by other investigators (Lindroos et al, 1996). Therefore, the second aim of our report was to evaluate whether our group of patients significantly decreased sucrose intake and, more generally, whether surgical intervention induced any modifications in the qualitative aspects of food consumption. Finally, we evaluated the impact of diet composition on body weight loss. In fact, it has been suggested that the composition of a hypocaloric diet could influence the extent (Rabast et al, 1981) or the composition (Dwyer, 1992) of body weight loss. In particular, a hypocaloric diet rich in carbohydrates would lead to better results. In contrast with these studies, it has been demonstrated that obese patients submitted to hypoenergetic diets having different compositions but similar energy content showed similar body weight loss (Alford et al, 1990; Golay et al, 1996). These results were obtained in patients that followed hypoenergetic diets with pre-established energy content and composition. In the present study, where patients could freely select their diet composition, we evaluated whether or not the individual differences in the substrates content of the diet had any impact on body weight loss and, consequently, could contribute to its variability.

Material and methods

Fifty obese women (age 38.4±1.4 y, body mass index (BMI) 45.2±0.9 kg/m2), submitted to a Roux-en-Y gastric bypass (Talieh et al, 1997) participated in the study. Before enrollment, patients were informed about the aims and the methods of the study and gave their written consent; the study protocol received the approval of the Ethical Committee of the Surgery Department of the Geneva University Hospital. The parameters presented in this paper were measured at the following timepoints: in basal, pre-operative conditions and 3, 6 and 12 months after surgery. All the measurements were performed after an 8–10 h fasting period.

Patients were placed in a recumbent position with the head in a ventilated hood (Deltatrac; Datex Corp., Helsinki, Finland) to measure, after a 15 min equilibration, VO2 consumption and VCO2 production during a 30 min period (Jéquier & Felber, 1987; Ferrannini, 1988). Glucose and lipid oxidation rates were calculated according to Lusk (1924), while protein oxidation rate was calculated as 6.325 N, where N is urine nitrogen excretion (mg/min). Resting energy expenditure (REE, kcal/day) was calculated from the substrate oxidation rates.

Energy requirements (EnReq, kcal/day) were calculated as:


Patients were instructed to fill in records during the 3 days preceding each visit. Then they were carefully interviewed by a dietitian in order to obtain a more detailed diet recall. Data so obtained were analyzed by a computer program (Prodi 3) to calculate alimentary protein, lipid, carbohydrates and energy intake (EnIn, kcal/day). Patients were also advised to consume a diet having a protein content of at least 60 g/day and an energy content of 1000 kcal/day during the first month, 1200 kcal/day from the first to the fourth month and 1400 kcal/day from the fourth month onwards.

In basal, pre-operative conditions the degree of mis-report in the alimentary history was calculated as:

Mis-report (%)=(EnIn-EnReq)/EnReq×100

EnIn and percentage content of protein, lipid and carbohydrates of the diet were calculated over the 1 y post-operative period using the values measured at 3, 6 and 12 months after surgery.

When investigating the factors influencing energy intake after surgery, the patients were divided in four sub-groups according to their age and initial body weight. The first group comprised subjects under 35 y of age and with a body weight higher than 120 kg (<35 y, >120 kg) the second group comprised subjects under 35 y of age and with a body weight lower than 120 kg (<35 y, <120 kg); the third group comprised subjects with an age over 35 y and a body weight higher than 120 kg (>35 y, >120 kg); the fourth group by subjects with an age over 35 y and a body weight lower than 120 kg (>35 y, <120 kg).

Statistical analysis was performed using factorial or repeated measurements ANOVA and simple or multiple regression analysis (Statview 4.5; Abacus Concepts Inc., Berkeley, CA, USA). Data are reported as mean±s.e.m.


Table 1 shows the profile of body weight and energy intake before and 3, 6 and 12 months after surgery. In Table 1 the post-operative percentage correction of excess body weight is also reported. Analysis of variance for repeated measurements showed a significant (P<0.0001) modification of all these parameters after surgery.

Table 1 Body weight and energy intake profile measured before and 3, 6 and 12 months after surgery, post-operative percentage correction of excess body weight

The mean pre-operative value of EnIn was 2307±107 kcal/day, while the mean EnReq was 2788±49 kcal/day; the degree of mis-report of the alimentary history was therefore evaluated as −17.2±3.6% of the energy requirements.

EnIn was calculated as surface area over the 12 months following surgery; simple regression analysis showed a significant relationship linking this parameter to body weight loss, both when expressed in absolute values (r2=0.13, P<0.01; Figure 1) and when expressed as percentage correction of excess body weight (r2=0.13, P<0.01; Figure 1b).

Figure 1

Relationship between energy intake, calculated per surface area over the 1 y period following surgery, and body weight loss (A) expressed in absolute values and (B) expressed as percentage correction of excess body weight. P<0.01 for both.

Table 2 illustrates diet composition: protein, lipid and carbohydrates are expressed as percentage contribution to the total caloric intake, while sucrose was calculated as percentage of diet carbohydrates. ANOVA for repeated measurements showed a significant decrease of both carbohydrates and sucrose consumption (P<0.01), and an increase in percentage protein (P<0.01); no significant modifications were observed for lipid percentage content of the diet.

Table 2 Diet composition profile measured before and 3, 6 and 12 months after surgery

Diet composition over the year following the operation was evaluated in a multiple regression analysis vs body weight loss (Table 3). When corrected by energy intake, neither the mean percentage content in protein, lipid, carbohydrate nor sucrose showed any significant relationship with weight reduction, expressed in kg. A similar result was observed when expressing weight reduction as percentage correction of excess body weight. In both cases, EnIn remained the only independent variable having a significant (P<0.005) impact on body weight loss.

Table 3 Multiple regression analysis of body weight loss vs diet composition and caloric uptake over 1 y after surgery. Protein, lipid and carbohydrates are calculated as percentage of total energy intake, sucrose is calculated as percentage of carbohydrate content of the diet

Weight reduction was not significantly related to the pre-operative sucrose content of the diet (P=0.72 vs weight loss expressed in kg and P=0.42 vs percentage correction of excess body weight).

As reported in Table 4, when age and initial body weight were taken into account in a multiple regression analysis, EnIn SA was no more significantly related to body weight loss (kg). This indicates that age and initial body weight influenced body weight loss as well as EnIn. For this reason, patients were arbitrarily divided into four sub-groups, according to their age and initial body weight, as indicated in the Methods section. The pre-operative characteristics of the four groups are shown in Table 5 Figure 2a shows the body weight loss at 12 months after surgery. Groups with similar initial body weight were compared first: both sub-groups of younger patients showed a significantly more pronounced weight reduction, in comparison with the respective groups of older subjects (−51.5±2.4 vs −36.9±1.5 kg, P<0.0001 between the groups with an initial bw >120 kg and −40.2±1.8 kg vs −31.3±1.5 kg, P<0.0005 between the groups with and initial bw <120 kg). Both groups of younger patients also showed a significantly lower EnIn than their respective older counterparts (976±49 vs 1275±76 kcal/day, P<0.02 when comparing groups with an initial body weight >120 kg and 1151±76 vs 1365±55 kcal/day, P<0.05, when comparing groups with an initial body weight <120 kg; Figure 2b). When comparing groups with similar age, among under-35-y-old subjects, those with the highest initial body weight lost significantly more weight than the group with an initial body weight lower than 120 kg (−51.5±2.4 vs −40.2±1.8 kg, P<0.001; Figure 2a). A similar result was observed when examinating the two groups of subjects with an age over 35 y (−36.9±1.5 vs −31.3±1.5 kg, P<0.02; Figure 2a). EnIn SA (Figure 2b) was not significantly different in groups with similar age, but different initial body weight.

Table 4 Multiple regression analysis of body weight loss (kg) vs age, initial body weight and energy intake surface area over 1 y after surgery
Table 5 Pre-operative characteristics of the four sub-groups of patients
Figure 2

ANOVA of body weight loss and energy intake measured in the four sub-groups of patients according to their age and initial body weight. *P<0.05 or <0.02; **P<0.001; ***P<0.0001 or <0.0005.


Previous reports have indicated that energy restriction is the primary mechanism responsible for the gastric bypass-induced body weight loss (Kral, 1992); in agreement with this conclusion, our study demonstrates a significant relationship linking the extent of body weight loss to the caloric supply, calculated as the integrated value of energy intake during the first year following the operation. This result also validates the method followed to obtain anamnestic data on food consumption. This is an important point when considering the difficulties in obtaining reliable data on energy intake (Klesges et al, 1995), in particular in obese subjects (Zhang et al, 2000). In our study, the degree of mis-report averages −17% of the evaluated energy requirements, in pre-operative conditions. Data from the literature indicate a large variability in the estimated underreport, which can vary from about 0% (Lissner et al, 1998) to values exceeding −20% (Howatt et al, 1994). This variability depends upon several factors such as sex, age and, in particular, BMI. It has been consistently suggested that under-report increases with increasing BMI (Kretsch et al, 1999) therefore a −17% mis-report represents a reasonable degree of inaccuracy, in a population of morbid obese subjects. Mis-report quite probably occurs also after surgery; however, it is difficult to evaluate its extent, because of the negative energy balance conditions; furthermore, the occurrence of malabsorption and/or vomiting, although limited, could reduce the significance of the relationship between energy intake and body weight loss. Despite these difficulties, our results clearly indicate that post-operative caloric intake significantly influences the extent of gastric bypass-induced weight reduction. It is interesting to observe that, despite a highly standardized surgical procedure, the post-operative energy intake is characterized by a large individual variability, suggesting that some factors, already present before surgery, could influence the post-operative food intake. Indeed, we could demonstrate that age plays a role in determining post-operative food consumption: in fact, younger subjects are the ones who maximally reduce energy intake. Hence, younger subjects realize the largest weight reduction, when compared to their respective older counterparts (P<0.0001 between the groups with initial body weight higher than 120 kg P<0.0005 between the groups with an initial body weight lower than 120 kg). Younger subjects have been reported to achieve a more satisfactory body weight loss (Dixon et al, 2001). However, to our knowledge, no study has related a better capacity of younger patients either in reducing food intake after bariatric surgery or in complying with a hypocaloric diet. We have no explanation for this interesting result. Insulin and leptin, which are involved in the control of food intake, were similar before surgery in the younger and older subjects; after surgery the profile of these hormones was modified according to the decrease in body weight (data not shown). Of course, this does not exclude the possibility that insulin or leptin have different effects on energy intake according to age. It has been reported that, in normal body weight subjects, aging is associated with a lower lipolytic capacity (Toth & Tcherof, 2000), in particular after sympathetic stimulation (Blaak, 2000), and this could explain the increased adipose tissue deposition of older subjects. Furthermore, a blunted lipolytic activity has been described in obese, post-menopausal women submitted to hypocaloric diet (Nickals et al, 1997). These findings suggest that obese, older women have a decreased capacity to supply energy throughout the mobilization of lipids from fat stores. This could condition a larger caloric intake after surgery.

Our results clearly indicate that the pre-operative body weight significantly contributes to determine subsequent body weight loss. In fact, the patients having the highest initial body weight (over 120 kg) showed the largest weight reduction, when compared to the group with lower degree of obesity and similar age (P<0.01 between the under-35-y-old groups and P<0.02 between the over-35-y-old groups). Previous studies (Donnelly et al, 1994; Pasquali et al, 1987) have demonstrated that, when submitting obese patients to the same hypocaloric diet, the heaviest subjects lose significantly more weight. Similarly, after bariatric surgery, patients with the largest degree of obesity (ie over 225% of the ideal body weight) show the greatest absolute weight loss (Bloomston et al, 1997). This should be attributed to the higher energy requirements of these subjects in whom, therefore, the energy deficit is more pronounced. In our study, EnIn was slightly but not significantly lower in the heaviest groups when compared to their respective counterparts. Therefore, in our case also, the difference in body weight reduction should be attributed to the largest relative energy deficit occurring in subjects with the highest initial energy requirements.

As already mentioned, it has been reported that gastric bypass selectively reduces sucrose intake and, therefore, this intervention has been proposed as the elective surgical treatment for sweet-eater obese patients (Sugerman et al, 1987, 1989). However, this conclusion has not been confirmed by other authors who found contrary results (Lindroos et al, 1996). Our data suggest that sucrose intake is significantly reduced, in particular during the early post-operative phase. It is therefore concluded that, at least during the first post-operative year, gastric bypass and dietary assistance lead to an improvement of patients alimentary habits. It should be observed, however, that 12 months after surgery the contribution of sucrose to the total caloric intake tended to return to the pre-operative levels. This suggests that alimentary history should be re-investigated during the period of weight stabilization, in order to evaluate whether alimentary habits are definitely or only transiently modified by surgical intervention. Finally, our results do not show any significant relationship between the amount of sucrose consumed before surgery and the extent of body weight loss or to the degree of correction of the excess body weight (respectively P=0.83 and P=0.49). In agreement with previous reports (Lindroos et al, 1996), this result does not support the hypothesis that gastric bypass leads to better results in sweet eaters.

The evaluation of the qualitative modifications of substrates intake shows a significant (P<0.01) increase in percentage protein contribution to the total energy intake. However, when evaluating the absolute values of protein consumption, it is possible to observe that protein intake is constantly below the recommended values. Therefore, the percentage increase in diet protein should mainly be attributed to the decrease in both lipid and carbohydrate consumption, leading to a relative increase of dietary protein.

As indicated by statistical analysis, diet composition does not influence the extent of weight reduction. In fact, in a multiple regression analysis, the energy intake was the only independent variable significantly related to body weight loss, whereas no significant relationship was observed for the diet content of lipid, carbohydrate or protein. This confirms the results already reported by our group, demonstrating an equivalent weight loss in obese patients submitted to hypoenergetic diets having the same energy content, but different composition (Golay et al, 1996).

In conclusion, our data demonstrate that gastric bypass induces a major quantitative effect on food intake; diet composition is also modified, at least during the first year after surgery, since sucrose intake is significantly decreased. Bypass-induced body weight loss is essentially linked to the quantitative modifications of food intake, whereas diet composition does not play any role in weight reduction. Energy intake is, in turn, influenced by age; younger patients show a higher capacity to reduce caloric supply after surgery. Higher initial body weight, by creating a greater energy deficit, leads to a greater weight loss, when expressed in absolute values (kg).


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This study was partially supported by the Swiss National Funds for Scientifical Research, grant no. 32-61339.00 and by a grant from the Ernst and Lucie Schmidheiny Foundation.

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Bobbioni-Harsch, E., Huber, O., Morel, P. et al. Factors influencing energy intake and body weight loss after gastric bypass. Eur J Clin Nutr 56, 551–556 (2002). https://doi.org/10.1038/sj.ejcn.1601357

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  • obesity
  • gastric bypass
  • energy intake
  • body weight loss
  • age

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