The researchers led by Karolin Weitkunat, Sara Schumann and Susanne Klaus have now published their results in "The American Journal of Clinical Nutrition" (Weitkunat et al. 2017; doi:10.3945/ajcn.117.152702).
What was previously known?
The results of many long-term observational studies suggest that people reduce their type 2 diabetes risk when they consume an adequate amount of dietary fiber. However, few people ingest the amount of 30 g per day recommended by the German Nutrition Society (DGE). Furthermore, studies suggest that people who have high concentrations of C15 and C17 fatty acids in their blood also have a decreased risk of diabetes. Until now, scientists had assumed that humans cannot produce these fatty acids themselves and must absorb them with food, for example from dairy products.
Studies on animals, however, suggest that at least the liver cells of rodents have the ability to produce precursor (propionate) C15 and C17 fatty acids and deliver them into the blood. Propionate is the salt of a short-chain fatty acid, which is produced by microbial decomposition of soluble dietary fibers in the gut.
What did the scientists investigate?
Ultimately, all this data prompted the DifE scientists to hypothesize that also in humans there could be a direct association between dietary fiber intake, fatty acid levels in the blood and diabetes risk. However, since direct evidence for the human endogenous synthesis of C15 and C17 fatty acids was still lacking, the researchers conducted a dietary study with 16 healthy female and male subjects. For seven days, each of the ten women and six men alternately received either 30 g of cellulose, 30 g of inulin or 6 g of propionate in addition to their normal diet. Before and after each supplementation the researchers determined the fatty acid levels of the study participants. While cellulose belongs to the group of insoluble dietary fibers which does not contribute to microbial propionate formation in the gut, inulin is one of the soluble dietary fibers. In addition to the dietary study, the researchers conducted cell culture experiments to more accurately analyze the propionate metabolism in human liver cells.
What are the researchers’ conclusions?
As the researchers noted, the intake of cellulose did not affect the plasma levels of the C15 and C17 fatty acids. On the other hand, the levels of C15 fatty acid increased by 17 percent after ingestion of inulin and by 13 percent after the intake of propionate. The plasma levels of the C17 fatty acid increased in parallel by an average of 11 and 13 percent, respectively. Also in the cell culture experiments, the addition of propionate to the nutrient medium stimulated the production of the two fatty acids in the liver cells.
“In summary, our results provide new insight into the metabolic mechanisms associated with the consumption of dietary fiber. For the first time, we were able to show that humans are also able to produce C15 and C17 fatty acids from the precursor propionate. In addition, the more propionate released into the liver cells, the more fatty acid synthesis shifts to the two fatty acids," said Karolin Weitkunat, who shares the first authorship with Sara Schumann. “The microbial degradation of soluble fiber in the gut is crucial for the amount of propionate available in the liver," the scientist went on to say. "Various studies indicate that propionate improves the insulin sensitivity of the body cells. Therefore, there is much evidence that there is a causal, biological link between increased intake of soluble dietary fiber, increased plasma levels of C15 and C17 fatty acids, and decreased type 2 diabetes risk," Sara Schumann added.
How can these new findings be used?
"Our research results support the results of the observational studies. They also suggest using the plasma levels of the C15 and C17 fatty acids as biomarkers in the future, in order to quantify the intake of soluble dietary fiber for the first time independently of the study participants’ self-reports, which are frequently incorrect. This could help increase the informative value of future dietary studies and thus contribute to a greater acceptance of their results," said Susanne Klaus, who heads the Department of Physiology of Energy Metabolism at DIfE. Last but not least, through their research findings the scientists hope to raise awareness for the importance of sufficient fiber intake because particularly in the Western industrial nations people still consume too little fiber. Adequate dietary fiber intake could considerably help prevent cardiovascular diseases, type 2 diabetes as well as colon cancer.