Diet and inflammation : the role of nitrate and conjugated linoleic acid

Abstract

A diet rich in vegetables and unsaturated fatty acids is associated with a lower risk of major diseases including cardiovascular disease, type 2 diabetes and chronic inflammation. Yet, despite extensive research, the active component(s) responsible for these effects has not been pinpointed and studies with single nutrients have been largely unsuccessful. Recent research from our laboratory and elsewhere suggests that the inorganic anion nitrate (NO3-), especially abundant in green leafy vegetables, is converted in our bodies to nitrite (NO2-) and then further to nitric oxide (NO). The latter is a central signalling molecule with a number of beneficial effects in the cardiovascular- and gastrointestinal systems. Ingested nitrate is absorbed to the blood and mixed with nitrate from endogenous sources formed by NO synthases (NOS). Circulating nitrate is actively transported and accumulated in the salivary glands and excreted with saliva. Oral commensal bacteria play a surprisingly important role in nitrate bioactivation by reducing salivary nitrate to the more reactive nitrite anion. The nitrate-nitrite-NO pathway is now emerging as a significant source of NO, in addition to classical endogenous formation of this gas by NOS.

A diet rich in vegetables and unsaturated fatty acids is associated with a lower risk of major diseases including cardiovascular disease, type 2 diabetes and chronic inflammation. Yet, despite extensive research, the active component(s) responsible for these effects has not been pinpointed and studies with single nutrients have been largely unsuccessful. Recent research from our laboratory and elsewhere suggests that the inorganic anion nitrate (NO3-), especially abundant in green leafy vegetables, is converted in our bodies to nitrite (NO2-) and then further to nitric oxide (NO). The latter is a central signalling molecule with a number of beneficial effects in the cardiovascular- and gastrointestinal systems. Ingested nitrate is absorbed to the blood and mixed with nitrate from endogenous sources formed by NO synthases (NOS). Circulating nitrate is actively transported and accumulated in the salivary glands and excreted with saliva. Oral commensal bacteria play a surprisingly important role in nitrate bioactivation by reducing salivary nitrate to the more reactive nitrite anion. The nitrate-nitrite-NO pathway is now emerging as a significant source of NO, in addition to classical endogenous formation of this gas by NOS.

The results show that nitrite and dietary nitrate can reduce leukocyte recruitment during acute inflammation in the microcirculation. Dietary nitrate also prevented NSAID-provoked small intestinal inflammation in a process dependent on oral nitrate-reducing bacteria. Although strong anti-inflammatory effects were observed with dietary nitrate, the ability to clear an infection was not impaired. Dietary nitrate, nitrite and CLA were further demonstrated to alleviate inflammation in a mouse model of colitis. The protective effect seen with CLA involved upregulation of trefoil factor 3 (TFF3) expression through activation of peroxisome proliferator-activated receptor gamma (PPARγ) in the colon mucosa. Furthermore, dietary nitrite also had therapeutic effects in already established colonic inflammation, possibly mediated by maintaining the colonic mucus layer and promoting healing of colon epithelial cells.

Finally, we demonstrate that the firmly adherent gastric mucus layer, normally present in conventional mice, was almost absent in germ free mice. In addition, a reduced gastric mucus layer was also observed in mice treated with broad spectrum antibiotics. While treatment with nitrate increased the mucus thickness further in conventional mice it had no effects in germ free mice, again demonstrating the essential role of oral bacteria in bioactivation of nitrate. Remarkably however, when the germ free mice were fed a low dose of nitrite, resembling what would normally be generated in saliva by bacteria, the gastric mucus thickness increased dramatically. This suggests that commensal oral bacteria modulate gastric homeostasis via physiological recycling of nitrate, originally derived from NOS.

In conclusion, the studies in this thesis demonstrate that dietary nitrate, nitrite and CLA play a direct role in the regulation of inflammatory responses, both locally in the microcirculation and in the gastrointestinal tract. These results may have implications for future dietary recommendations in prevention and treatment of inflammatory disorders.