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Energy expenditure and bypass surgery

Energy expenditure increases after gastric bypass surgery

Until now this reduced caloric intake does not provide the entire explanation for the success of gastric bypass patients at keeping the pounds off over the long term
The researchers hope that the process can one day be augmented by means of drugs such that surgery does not have to be performed as often

Researchers at Sahlgrenska Academy (Sahlgrenska University hospital), Gothenburg, Sweden, have found part of the explanation as to why gastric bypass surgery often leads to a sustained weight loss - the operation enhances energy expenditure such that eating actually helps the person lose weight. Although it is known that gastric bypass surgery reroutes food past the stomach and duodenum directly to the small intestine and patients become satiated faster and tend to eat more nutritiously, until now this reduced caloric intake does not provide the entire explanation for the success of gastric bypass patients at keeping the pounds off over the long term. The paper, ‘Roux-en-y gastric bypass surgery increases respiratory quotient and energy expenditure during food intake' is published online at PLOSone.

Malin Werling

“Parts of the small intestine become more active and require additional nutrition after a gastric bypass,” said Sahlgrenska researcher Malin Werling, who was also a co-author of the study. “As a result, the blood absorbs fewer nutrients to store as fat. You might say that people burn calories by eating.”

The study was designed to reveal the impact of Roux-en-Y gastric bypass (RYGB) on each major component constituting total energy expenditure (EE) and characterise meal associated thermogenesis (MAT) before and after RYGB using a state-of-the-art metabolic chamber. The primary aim of the study was to investigate meal-associated thermogenesis at weight stability before and after RYGB within the same individual during 24 hours of the standardised conditions in the metabolic chamber. Secondary aims were to examine the surgical effect on 24h EE, basal metabolic rate (BMR) and spontaneous physical activity and the associated non-exercise activity thermogenesis (NEAT) during the weight loss phase as well as at weight stability.

The study examined patients by means of a metabolic chamber before and up to two years after surgery. Subjects spent 24 hours in the chamber, which was furnished like a small hotel room, in order for researchers to study their metabolism in detail. The metabolic chamber assesses whole body thermogenesis being a compound variable of BMR, muscular activity thermogenesis and MAT. Thus in order to accurately calculate MAT, the BMR and activity thermogenesis must be determined (BMR is defined as resting EE in the awake state).

Six obese, female patients were included from the waiting list for RYGB. Patients were not prescribed any pre-surgical weight loss diet but were weight stable for at least three months prior to the study. They received similar multivitamin and mineral supplementations for one month before start of investigations. Patients were free from comorbidities, medication and were non-smokers. Study participants were invited to study visits during the same phase of the menstrual period. At arrival on each study visit, fasting blood samples were collected for plasma levels of free fatty acids, total cholesterols, high density lipoprotein (HDL), low density lipoprotein (LDL), iron, glycated haemoglobin (HbA1c), thyroid stimulating hormone (TSH), free thyroxin (fT4) and total triiodothyronine (T3), follicle stimulating hormone (FSH) and creatinine.

The 24h EE was assessed four times: preoperatively (visit 1; weight stability), ten days postoperatively (visit 2; start of weight loss), three months postoperatively (visit 3; weight loss phase), as well as after a self-reported three months period of weight stability after the surgical procedure (visit 4; weight stability). In total, patients spent 25 hours in the chamber at each visit. Thirty minutes in the beginning and 30 minutes at the end were excluded from analysis to allow for acclimatisation, settling after entrance and preparation before exiting.

At arrival in the morning on each study visit weight and height were measured in light underwear. Body mass index (BMI) was calculated and Dual Energy X-Ray Absorptiometry (DEXA) (LUNAR Radiation, Madison, WI, USA) was used to assess total, adipose and lean tissue. The DEXA values were used in the analysis of energy expenditure and in adjustments relating recorded values to total or lean tissue.

Due to adaptation of the gut and small intestine after RYGB patients have a dramatically reduced food intake during the first months after operation. The test meals served at visit 2 and 3 during the weight loss phase, therefore contained a lower amount of calories compared to during visit 1 and 4 at weight stability.


The results revealed that the median BMI decreased significant from 41.4 preoperatively to 28 as measured 20 months after surgery (p<0.05). Lean tissue decreased significantly by 6.4kg while the proportion lean tissue in relation to total mass increased significantly by 18.1% (both p<0.05). Adipose tissue decreased by 34 kg and the proportion adipose tissue in relation to total tissue decreased by 19.6% (both p<0.05), as shown in Table 1.

Table 1: Body composition in six females before and after gastric bypass surgery

Gastric bypass surgery was found to have improved lipid profiles by increasing HDL (high density lipoproteins) and decreasing LDL (low density lipoproteins). Free fatty acids, iron, free T4, TSH, FSH and creatinine did not change and were within the reference range both before and after surgery. Haemoglobin, HbA1c and Total T3 decreased after surgery but remained within reference range. The area under the curve representing the postprandial profile for GLP-1, glucagon and oxyntomodulin were all elevated (all p<0.05). Postprandial levels of GIP were significantly lower after the operation (p<0.05).

BMR decreased after surgery, but there was no difference between visits after adjustment for total tissue weight, nor for lean tissue weight. Absolute 24h EE decreased 20 months after surgery from 1.6 to 1.2 kcal/min (p<0.05), while 24h EE adjusted for total tissue weight increased from 12.9 cal/min/kg to 14.7 cal/min/kg (p<0.05), as shown in Table 2.

Table 2: Energy expenditure (EE) and respiratory quotient (RQ) in sex females before and after gastric bypass surgery

NEAT was assessed during 1 h before dinner. Generally the physical activity level of the participants was low and none of them exercised in the chamber. As expected pre-prandial EE decreased with time after surgery and NEAT did not change significantly between visits.

Despite the significant weight loss, MAT in absolute terms did not change between pre- and postoperative weight stability recordings (ie visit 1 and 4). However, relative to 24h EE or when adjusted for total tissue, MAT was significantly higher after surgery during the first postprandial hour, as shown in Figure 1. MAT%, both as absolute values and adjusted for total tissue was very variable both at visit 2 and 3 but showed significantly higher values at visit 2. MAT% adjusted for total tissue was significantly increased at visit 4 compared to preoperative analyses.


Figure 1: Six female subjects assessed for energy expenditure during one hour after a 600 kcal standard meal; from 18:20 to 19:20

Respiratory quotient (RQ) of the entire 24h period was lower immediately after surgery while patients were in a negative energy balance and losing weight (visits 2 & 3), but had increased significantly above the preoperative values at weight stability (visit 4). An identical pattern was observed for RQ during the first hour after dinner, whereas the RQ values recorded during BMR did not differ between the study visits.

“In the present study we found that weight adjusted EE after meals increased following RYGB,” the authors write. “Moreover the contribution of MAT as a component of total EE also increased after surgery. In absolute terms MAT did not change despite markedly reduced adipose and lean tissue mass. The physiological processes behind MAT are not completely clear but the contribution of striated muscle is minimal making adjustments of MAT for lean tissue mass not optimal. Correcting MAT for total tissue may be more appropriate as elements of the intestinal digestion/absorption and/or distant metabolic effects elicited by the gut are most probably generating MAT, thus being in the context of total tissue mass.”

“RYGB surgery was associated with decreased basal metabolic rate, unchanged non-exercise activity thermogenesis but with an increase in energy expenditure after food intake when corrected for body composition,” the researchers conclude. “Enhanced postprandial RQ after RYGB suggests increased glucose metabolism, presumably in the small bowel.”

They said that further studies are needed to be able to compare gender differences regarding energy metabolism after bariatric surgery. Additional studies will trace the specific mechanisms that generate the greater meal associated energy requirements after such operations. The researchers hope that the process can one day be augmented by means of drugs such that surgery does not have to be performed as often.

Co-authors of the study were Lars Fändriks, Torsten Olbers, Marco Bueter, Lars Sjöström, Hans Lönroth, Ville Wallenius, Kaj Stenlöf and Carel W le Roux.

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