Here it is shown for the first time that there is a relationship between the degree of post-lipid ingestion rise in plasma concentration of proneurotensin and triglycerides in humans, as demonstrated by significant correlation between the post lipid load AUC of proneurotensin and the corresponding AUC of triglycerides after two different forms of lipid loads, i.e. cream and olive oil. This finding extends evidence of a pivotal role of NT in intestinal fat absorption from prior animal studies [9] to suggest a similar role of NT in humans. The cause of the delayed proneurotensin and triglyceride peaks after olive oil vs cream (Figure 1) is unknown but it is speculated that olive oil, being almost exclusively lipids, results in slower gastric emptying than cream and thus delayed exposure of lipids to intestinal cells secreting proneurotensin.
Previously, high levels of proneurotensin have been shown to predict obesity, diabetes mellitus and CVD, as well as shows relationship with NAFLD [11, 14, 19]. Thus, it is essential to understand mechanisms behind such relationships in order to target the neurotensin system pharmacologically. Although lipid translocation was not measured directly from the intestinal lumen to the blood stream, one can assume that rise in plasma concentration of triglycerides after standardized oral lipid loads more of less exclusively comes from the orally added lipids. Moreover, given the fact that the amount of rise in plasma concentration of proneurotensin after two different oral lipid loads significantly correlated with the rise in plasma triglycerides, it is reasonable to assume that post-lipid ingestion rise of proneurotensin contributes to translocation of the orally added fat from intestinal lumen to the blood stream.
Of note, most of the previous epidemiological studies have examined the associations between fasting level of proneurotensin and cardiometabolic diseases [14, 19, 20] but it is not studied before that proneurotensin release, triggered by fat containing meals have different relationship with the risk of cardiometabolic disease. Studies have shown that high postprandial triglycerides are not only a characteristic feature of diabetes mellitus and insulin resistance but also they are an independent risk factor for future cardiovascular events and might be stronger than the fasting level of triglycerides [21]. Given the link between post-prandial triglyceridemia, CVD risk and current findings of relationship between post-lipid ingestion plasma concentration of proneurotensin and triglycerides, one can speculate that neurotensin contributes to the CVD risk associated with high post-prandial triglycerides. This certainly needs further studies but necessitates discussion on whether proneurotensin levels are modifiable.
The most obvious intervention to reduce postprandial proneurotensin levels is to reduce its trigger, i.e. fat intake. From a pharmacological point of view, the intestinal lipase inhibitor orlistat is of particular interest, which helps in weight reduction by decreasing levels of tumor necrosis factor alpha and interleukin 6 [22] and may reduce the risk of obesity and CVD [23, 24]. Orlistat inhibits both the suggested key effect of neurotensin, i.e. intestinal fat absorption, and intestinal release of neurotensin into the blood stream, the latter being explained by absence of the trigger for neurotensin production and secretion in the enterocyte following orlistat administration, i.e. intracellular fatty acids. It can be speculated in future studies that orlistat therapy can possibly benefit the subjects with high post-prandial proneurotensin and an energy conserving phenotype due to enhanced intestinal fat absorption leading to both reduction in intestinal fat absorption and preventing other potentially harmful effects of high neurotensin like hepatic fat accumulation [9].
Another finding was that glucose significantly decreased after fat ingestion. One explanation for this finding is that neurotensin acts synergistically with glucagon-like peptide-1 (GLP1) and peptide YY (PYY) in the distal small intestines to decrease palatable food intake and to inhibit gastric emptying, thus leading to lower glucose concentration [25]. As a second explanation, it is speculated that glucose production by the liver is reduced after fat ingestion, which could reduce glycemia after lipid ingestion.
LIMITATIONS
This study have some potential limitations. The exact mechanisms by which post-lipid ingestion rise in proneurotensin affects plasma postprandial triglyceride concentrations cannot be explained and no control diets (i.e. protein or carbohydrate) were included, however, the suggested mechanism emanates from the previous animal experimental studies. The study included a rather small number of normal subjects to see the physiological interplay between fat, proneurotensin and triglycerides. To our knowledge, there is no prior study to base the assumption of size of the effect estimate on and therefore a power calculation was not done. Finally, ELISA (Enzyme-Linked ImmunoSorbent Assay) was not performed to compare the mature hormone (neurotensin) and triglycerides in this study, instead the more stable proneurotensin was used as a surrogate.