To estimate dietary fiber intake, researchers have previously depended on study participants’ self-reports of their dietary habits. Now scientists from the German Institute of Human Nutrition (DIfE) have found a new way of objectively evaluating and quantifying the individual intake of dietary fiber based on the blood plasma concentrations of certain fatty acids. In the future, the discovery may help to improve the informative value of dietary studies as well as to make individual diet recommendations more accurate.

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.

Background Information:
Dietary fibers are only found in plant foods. These are various carbohydrates (incl. lignin), which escape the enzymatic digestion in the small intestine and are partially or completely degraded by gut bacteria in the colon. In principle, a distinction is made between soluble and insoluble dietary fibers. Insoluble dietary fibers are primarily scaffold and supporting substances of the plants. They include lignin, cellulose and hemicellulose. The insoluble dietary fibers are only partially degraded by the gut bacteria and, among other properties, are able to bind different amounts of water in the colon. Through the binding of the water, the chyme swells and becomes softer, promotes the movement of the colon, and thus contributes to an increase of the weight of the stool and a shortened transit time. The soluble dietary fibers include pectins, resistant starches, plant gums, mucus, beta glucans and gels from sea algae (e.g. agar agar, carrageenan) and also inulin. The latter is a mixture of multiple sugars, consisting of up to 100 fructose building blocks and a terminal glucose residue. Inulin naturally occurs in some plants such as Jerusalem artichokes and artichokes. The soluble dietary fibers are largely degraded by the bacteria living in the intestine to form short-chain fatty acids such as propionate and gases. The fatty acids formed by the bacteria serve, on the one hand, as a nutritional basis for the bacteria themselves, which assures their survival and multiplication. On the other hand, they are responsible, together with the resulting gases, for making the stool more voluminous and softer. 
 
The German Institute of Human Nutrition Potsdam-Rehbrücke (DIfE) is a member of the Leibniz Association. It investigates the causes of diet-related diseases in order to develop new strategies for prevention and therapy and to provide dietary recommendations. Its research focus includes the causes and consequences of the metabolic syndrome, which is a combination of obesity, high blood pressure, insulin resistance and lipid metabolism disorder, as well as the role of diet in healthy aging and the biological basis of food choices and eating habits. In addition, the DIfE is a partner of the German Center for Diabetes Research (DZD) which was founded in 2009 and has since been funded by the Federal Ministry of Education and Research (BMBF).

The Leibniz Association is the umbrella organization for 91 independent research institutions whose spectrum encompasses the natural, engineering and environmental sciences, economics, the spatial and social sciences as well as the humanities. Leibniz Institutes address issues of social, economic and ecological relevance. They conduct knowledge-driven and applied basic research, also in the overarching Leibniz research networks; they comprise and maintain scientific infrastructure and provide research-based services. The Leibniz Association sets priorities in knowledge transfer, in particular with the Leibniz research museums. It provides advice and information for policymakers, academia, business and industry, and the general public.  Leibniz institutions maintain close cooperative relationships with universities – inter alia in the form of “Leibniz Science Campi”, as well as with industry and other partners in Germany and abroad.  They are subject to a transparent and independent evaluation procedure. Due to the importance of the Leibniz Institutes for Germany as a whole, they are funded jointly by the federal government and state governments, employing some 18,600 individuals, including 9,500 researchers. The overall budget of the institutes amounts to more than EUR 1.7 billion.

Contact:
Prof. Dr. Susanne Klaus
Department of Physiology of Energy Metabolism
German Institute of Human Nutrition Potsdam-Rehbrücke (DIfE)
Arthur-Scheunert-Allee 114-116
14558 Nuthetal/Germany
Phone: +49 (0)33200 88-2326
email: Klaus(at)dife.de 

Karolin Weitkunat
Department of Physiology of Energy Metabolism
German Institute of Human Nutrition Potsdam-Rehbrücke (DIfE)
Arthur-Scheunert-Allee 114-116
14558 Nuthetal/ Germany
Phone:  +49 (0)33200 88-2332
email: Karolin.Weitkunat(at)dife.de 

Dr. Sara Schumann
Department of Physiology of Energy Metabolism
German Institute of Human Nutrition Potsdam-Rehbrücke (DIfE)
Arthur-Scheunert-Allee 114-116
14558 Nuthetal/ Germany
Phone:  +49 (0)33200 88-2332
email: Sara.Schumann(at)dife.de 

Media Contact:
Dr. Gisela Olias
Coordinator for Press and Public Relations
German Institute of Human Nutrition
Potsdam-Rehbrücke (DIfE)
Phone:  +49 33200 88-2278/-2335
email: olias(at)dife.de   
or presse(at)dife.de