Postprandial bone turnover is independent of calories above 250 kcal

Background

Bone resorption is influenced by food consumption and peaks at night with a nadir in the late afternoon, while fasting has been found to inhibit this circadian rhythm. ^1,2 The postprandial mechanisms which result in suppression of bone resorption have not yet been elucidated. Animal and human studies suggest links between gut hormones and bone turnover. ³ Several of these gut hormones have been shown to change in proportion to calorie ingestion. Bone turnover and glucagon-like peptide 2 (GLP-2) have been associated, while peptide YY, glucose-dependent insulinotrophic polypeptide (GIP) and ghrelin have also been implicated. ^3–5 The suppression of bone resorption is more pronounced following calcium containing meals (1200 mg) than those devoid of calcium, possibly through a parathyroid hormone (PTH)-related mechanism. ^6,7 However, the observation that an oral glucose load does not reduce PTH enough to explain the suppressed bone resorption markers denotes that other mechanisms independent of PTH exist. ⁸

Our aim was to investigate the change in C-telopeptide type I collagen (CTX), as a surrogate marker of bone resorption, following ingestion of graded-calorie containing meals with a fixed calcium component. We hypothesized that if both calcium content and volume of meals were to be fixed, then any difference in bone resorption between these meals may be attributed to variations in calorie content, hence suggesting a role for calorie sensitive gut hormones.

Methods

Studies were approved by the local ethics committee, Hammersmith Hospital (03/6499), performed according to the principles of the Declaration of Helsinki and written informed consent was obtained. The exclusion criteria included smoking, pregnancy, medication and abnormal physical examination.

We evaluated the postprandial response to a series of six standard meals (Table 1) in 20 healthy, lean subjects who were all at a stable body mass index of 21.7 ± 0.4 kg/m². The group had a mean age of 28.6 ± 1.6 y (14 women and 6 men). Each subject visited the hospital on three occasions after a 12 h overnight fast and received in random order and in a blinded manner either a 500 mL meal (250, 500 and 1000 kcal) or a 900 mL meal (1000, 2000 and 3000 kcal). The meals contained 500 mg of calcium and had a similar taste and viscosity. Table 1 shows the macronutrient content of the test meals. The 1000 kcal meals given as 500 or 900 mL allowed us to investigate if there was a major volume effect on response. Blood was collected 30 min before and 180 min after the meal. Blood samples were centrifuged, separated and stored at −20°C until assayed in duplicate for CTX using the Serum CrossLaps^® ELISA (Immunodiagnostic Systems Ltd, Boldon, UK; on automated ELISA Triturus). The intra-assay co-efficient of variation (CV) was 3.0% at 0.12 ng/mL and 1.8% at 1.97 ng/mL.

Results

A significant decrease in CTX in response to the multiple meals of between 35.8 ± 5.6% (mean ± standard error of mean [SEM]) and 44.8 ± 3.8% occurred. Table 2 shows the mean CTX values pre- and postmeals according to calorie content of each meal. However, there were no significant differences in the reduction of CTX in relation to meal calorie content or volume using one-way analysis of variance for the analysis of the data (Figure 1).

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Figure 1

Percentage change in CTX levels following meals of different calorie contents; the change reflects the difference between 0 and 180 min

Discussion

We showed that the postprandial reduction in the bone resorption marker CTX is not sensitive to calorie content in meals containing 500 mg of calcium. The timing of sampling premeal and 180 min after a meal was chosen based on previous observations of the nadir of post-prandial bone resorption. ⁵ The findings do not support our hypothesis that a dose-dependent bone resorption response to graded-calorie meals would suggest a role for calorie-sensitive gut hormones, since the postprandial inhibition of CTX was not dose related above 250 kcal if meals contained 500 mg of calcium. Same calorie meals given in volumes of 500 or 900 mL did not show statistically significant differences in CTX response. Our main aim was to study the effect of calorie content without focus on the composition of meals. However, the test meals did vary in their fat content, with less variation in carbohydrate and protein content. We therefore analysed the data again taking into consideration the difference in carbohydrate, protein and fat content of each of the meals and we did not find significant correlation in the data-set with CTX. However, observed differences suggest a non-significant trend to greater CTX suppression with lower protein and higher fat content of meals; this observation would need to be validated in a larger study population.

The interaction between nutrient ingestion and bone turnover has several potential mediators. ¹ Firstly, the postprandial role of PTH in regulating bone turnover is controversial, since evening administration of calcium inhibits PTH, but only partially suppresses the nocturnal bone resorption peak. ⁹ Secondly, calcitonin's effect in the postprandial phase on bone turnover is also controversial. Pharmacological doses of calcitonin inhibit bone resorption, but there is no clear evidence at present linking feeding with postprandial changes in calcitonin secretion. ¹⁰ Thirdly, the central and enteric nervous systems have been proposed to mediate the bone resorption response, similar to leptin. ¹¹ Insulin as a mediator of bone resorption has been negated because hypoglycaemia appeared to be more relevant. ⁸ Finally, both in vitro and in vivo investigations concluded that the responsible factor(s) must have the following criteria: change quickly after nutrient ingestion, show a return to baseline within 4–5 h and be heat labile. ^2,5 This description fits pancreatic and enteric hormones and therefore numerous studies have tried to link gut hormones and bone turnover in both animals and humans. Ghrelin and GLP-2 appeared to be the most likely candidates. ^3,5

Our findings do not support a role for gut–hormone responses to meals above 250 kcal in order to inhibit postprandial bone resorption. We cannot exclude that gut hormone responses below 250 kcal may play a role, especially as (i) oral glucose (75 g following an OGTT) suppresses bone resorption more than intravenous glucose; ² (ii) in vitro evidence that several entero-endocrine hormones regulate bone turnover; ¹² and (iii) the acute suppression of postprandial bone turnover is reversed by octreotide administration. ¹³ As well as being a potent inhibitor of entero-endocrine hormones octreotide also suppresses PTH, insulin, glucagon and pituitary hormones while reducing splanchnic blood flow, portal blood flow, gastro-intestinal motility as well as gastric, pancreatic and small bowel secretions. ¹⁴ A meal of 250 kcal is not sufficient to induce significant satiety or a significant gut hormone response. Thus, it is unclear as to whether gut hormones after such a small meal play a major role in bone resorption. Calcium content of meals may be more important and our study could not exclude a synergistic effect of calcium and calorie content on bone resorption.

Limitations of our work include the inability to analyse the meal composition effect on CTX suppression because of the small number of patients recruited. A study with more subjects may determine the effect of fat and protein content of meals on markers of bone turnover. In conclusion bone resorption, measured by CTX, decreases by a similar degree following meals with calorie contents varying from 250 to 3000 kcal. Changes in CTX are not proportional to the calorie contents or volumes of meals containing 500 mg of calcium. Further studies should now determine whether patients with increased bone resorption would benefit from multiple small meals to slow down the rate of bone loss.

DECLARATIONS

Competing interests: None.

Funding: This research was funded by King's College Hospital.

Ethical approval: The ethics committee of Hammersmith Hospital approved this study (03/6649).

Guarantor: CL.

Contributorship: ME and CL researched literature and conceived the study. ME and RM performed the practical work and ME prepared the first draft of the manuscript. CL was involved in protocol development along with JA and CM, gaining ethical approval, patient recruitment and data analysis. All authors reviewed and edited the manuscript and approved the final version of the manuscript.