<p>A large amount of non-protein nitrogen, in the form of urea and ammonium, is present in human breastmilk; however its physiological role in the infant is as yet not fully understood. It has been hypothesized that an insufficient enteric metabolism of urea could play a role in the sudden infant death syndrome (SIDS). This thesis was undertaken to study the enteric metabolism of non- protein nitrogen in healthy infants, in comparison with those who had succumbed to SIDS.</p><p>Aerobic and anaerobic faecal microflora, were studied in healthy infants from birth to 6 months of age. They were found to appear in faeces within 3 days of birth and were present throughout the first 6 months of life. The effect of nitrate, nitrite and nitric oxide on faecal urease activity was investigated and found to be inhibitory in action. The sigmoid faecal urease activity and sigmoid faecal urea content of SIDS infants were compared to those of control infants; significantly lower sigmoid faecal urease activity and greater sigmoid faecal urea content were found in the SIDS infants. The total number of SIDS cases occurring in Sweden during the period 1990 through 1996 was analysed regarding geographical and seasonal distribution, in relation to the nitrate concentration in drinking water and changes in the groundwater level. The northernmost parts of the country had its highest incidence when the rest of the country had its lowest incidence, and the occurrence of individual deaths was associated with the recharge of groundwater, which is known to increase its nitrate content. The effect of ingested ammonium on carbon dioxide production was determined in healthy infants using the doubly labelled water technique. No change in carbon dioxide production was observed.</p><p>An insufficient enteric metabolism of urea in infants and peak or greatly varying nitrate concentrations in drinking water are associated with the occurrence of SIDS. Ingested ammonium supplements in the given doses did not influence carbon dioxide production in healthy infants.</p>

A large amount of non-protein nitrogen, in the form of urea and ammonium, is present in human breastmilk; however its physiological role in the infant is as yet not fully understood. It has been hypothesized that an insufficient enteric metabolism of urea could play a role in the sudden infant death syndrome (SIDS). This thesis was undertaken to study the enteric metabolism of non- protein nitrogen in healthy infants, in comparison with those who had succumbed to SIDS. Aerobic and anaerobic faecal microflora, were studied in healthy infants from birth to 6 months of age. They were found to appear in faeces within 3 days of birth and were present throughout the first 6 months of life. The effect of nitrate, nitrite and nitric oxide on faecal urease activity was investigated and found to be inhibitory in action. The sigmoid faecal urease activity and sigmoid faecal urea content of SIDS infants were compared to those of control infants; significantly lower sigmoid faecal urease activity and greater sigmoid faecal urea content were found in the SIDS infants. The total number of SIDS cases occurring in Sweden during the period 1990 through 1996 was analysed regarding geographical and seasonal distribution, in relation to the nitrate concentration in drinking water and changes in the groundwater level. The northernmost parts of the country had its highest incidence when the rest of the country had its lowest incidence, and the occurrence of individual deaths was associated with the recharge of groundwater, which is known to increase its nitrate content. The effect of ingested ammonium on carbon dioxide production was determined in healthy infants using the doubly labelled water technique. No change in carbon dioxide production was observed. An insufficient enteric metabolism of urea in infants and peak or greatly varying nitrate concentrations in drinking water are associated with the occurrence of SIDS. Ingested ammonium supplements in the given doses did not influence carbon dioxide production in healthy infants.