Metabolic adaptation to fasting is an essential mechanism developed by mammals in order to survive. The transition from the fed to the fasted state is tightly regulated. This metabolic shift includes reducing glucose oxidation and storage, and increasing the supply of free fatty acids (FFA) and ketone bodies (KB) to peripheral tissues. Glucose is spared for obligate glucose-consuming cells (such as some neurons, erythrocytes, kidney cells) by FFA's effects on membrane glucose transporters in peripheral tissues, upregulation of lipolytic enzymes and downregulation of glycolytic enzymes. Overall, if extended, fasting will produce a state of peripheral insulin resistance, which implies that skeletal muscle will become desensitized to insulin's effect, thereby reducing glucose transport into cells. However, this scenario differs from that observed during pathological insulin resistance, in which skeletal muscle, liver and adipose tissue are insulin resistant, so there is no control of glucose and FFA levels, leading to glucotoxicity and lipotoxicity.
Healthy humans should be able to fast without problems (metabolic flexibility). If everything is working as supposed to, there should be no problem when switching to a predominantely lipolytic/ketogenic metabolism. How hard the transition to a fasting state is may be a marker of the functioning of intermediate metabolism.
Crawford et al. (1) tested a basic hypothesis, based on previous findings:
- Germ-free (GF) mice are leaner than conventionally raised (CONV-R) mice, even when GF mice consume more food (2).
- Transplantation of the gut microbiota from obese mice to GF recipients causes a greater increase in adiposity than does a microbiota from lean mice (3).
The gut microbiota regulates ketone body metabolism during fasting
Compared to CONV-D*, GF mice showed 37% lower levels of serum betahydroxybutyrate (BHB) in the fasted state, with no differences in the fed state. Moreover, levels of insulin, glucose, FFA and triglycerides where unchanged. CONV-D mice had increased hepatic triglyceride stores compared to GF mice, difference which was enhanced dramatically with fasting. As expression of Pnpla2 was increased similarly in both CONV-D and GF mice, fasting-induced fatty acid mobilization was not impaired by the absence of gut microbiota. PPARa expression was higher in CONV-D than in GF mice, and the fasting-induced ketogenesis was impaired in CONV-D PPARa-/- mice.
The liver produces BHB for utilization in peripheral tissues. It lacks the key enzyme 3-oxoacid CoA transferase, so is incapable of oxidizing ketone bodies. When fasted, GF mice had 50% less concentration of liver BHB compared to CONV-D mice. Expression of Fgf21 and Hmgcs2, both targets of PPARa which stimulate ketogenesis, was lower in fasted GF mice compared to CONV-D mice.
These results suggest that the gut microbiota stimulates ketogenesis during fasting by a PPARa-dependent mechanism. Additionally, it promotes hepatic triacylglycerol synthesis and storage.
There are two possible ways of generating acetyl CoA in the liver during a fast: from acetate produced in the gut and from oxidation of fatty acids from adipose tissue. In GF mice, the only source is the latter. Cecal acetate levels were very low in fed GF mice and 20-fold greater in CONV-D mice. During fasting, these levels were reduced in CONV-D, but remained significantly higher compared to GF mice, which showed no reduction.
One unexpected result was the change in the microbiota composition induced by fasting. It was found that there was a significant increase in the proportion of Bacteroidetes and a significant reduction in the proportion of Firmicutes. Previous studies have found a correlation between a reduction in Bacteroidetes and an increase in Firmicutes with obesity (the high Firmicutes/low Bacteroidetes hypothesis) (5, 6).
The gut microbiota influences myocardial metabolism
Analysis of the myocardial transcriptome of CONV-D and GF mice revealed an enrichment in the ketone body metabolic pathway in both groups, compared to their PPARa -/- counterparts. Expression of genes involved in fatty acid and ketone body metabolism were increased with fasting in both groups, but Oxct1 gene expression was higher in CONV-D mice. Conversely, an increased Glut1 expression was only observed in GF mice.
The rate of glucose oxidation was significantly increased in isolated working hearts of GF mice, without alteration of fatty acid oxidation. In the absence of BHB, glucose utilization was also significantly greater. Glycogen levels were reduced in the myocardium of fasted GF mice compared to CONV-D mice, and there were no significant differences in heart rate, cardiac hydraulic work, mitochondrial morphology or number, or mitochondrial respiration.
The shift towards a ketogenic metabolism in the myocardium is one hallmark of adaptation to fasting, as BHB is more energy efficient than glucose. So in periods of deprivation of nutrients, the myocardium maintains its normal functioning by using BHB instead of relying in glucose. This adaptation is impaired in GF mice.
The gut microbiota affects myocardial mass
Heart weight was reduced in fasted GF mice compared to CONV-D mice. Training elicits an hypertrophic response in the heart. However, this response was blunted in the absence of gut microbiota, as the hearts of trained GF mice remained smaller than the hearts of trained CONV-D mice. This correlated with alterations in a subset of pathways, which included ketone body metabolism. Administration of a ketogenic diet rescued heart weight in GF mice and shifted the myocardial transcriptome toward ketone body metabolism.
These results suggest that the gut microbiota is an important component for cardiovascular health, and that ketone bodies represent an essential substrate for the heart.
This is one of the most interesting studies that I have read lately. It provides a new template for the relationship between metabolism and gut microbiota, and shows the importance of gut bacteria for the normal response to fasting. I have summarized the findings of the study in the following figure (my interpretation):
* CONV-D (conventionalized) mice were transplanted with distal gut microbiota from CARB-fed CONV-R lean mice.