In humans, a gut flora similar to an adult's is formed within one to two years of birth.  The gastrointestinal tract of a normal fetus is considered sterile, but microbial colonisation may occur in the fetus  and Lactobacillus and Bifidobacterium species were present in placental biopsies in one study.  During birth and rapidly thereafter, bacteria from the mother and the surrounding environment colonize the infant's gut.  As of 2013, it was unclear whether most of colonizing arise from the mother or not.  Infants born by caesarean section may also be exposed to their mothers' microflora, but the initial exposure is most likely to be from the surrounding environment such as the air, other infants, and the nursing staff, which serve as vectors for transfer.  During the first year of life, the composition of the gut flora is generally simple and it changes a great deal with time and is not the same across individuals. 
Scientists determined exactly how the propionate reduces appetite and also wants to confirm whether inulin-propionate ester (the combination of inulin and propionate) might be more effective than the single compound inulin which scales down the food consumption.
The major challenge associated with this study is to know whether it also reduces the normal food intake . This is a missing piece of the jigsaw in the study, While the results prove that the supplement can decrease activity in brain areas associated with food reward at the same time as reducing the amount of food they eat. This was supported by a concrete evidence as some people are more inclined to gain weight than others, consumption as they may have lower production of propionate.
Another is lack of breast milk, and a third is the increased use of antibiotics. O'Toole says that one study suggests that repeated use of antibiotics tips the microbiota towards one that promotes obesity. In fact there are many studies around the globe that are still in their infancy but which point up connections between the microbiota and diseases and complaints as diverse as irritable bowel syndrome, inflammatory bowel disease, type-two diabetes, Parkinson's, Alzheimer's, autism, depression, cardiovascular disease and colon cancer.