The purpose of this study is to compare the effect of freeze-dried, capsulized FMT and fresh FMT in adults with slow transit constipation.

Constipation is a chronic disease estimated to affect about 10% - 15% of the worldwide general population. Constipation frequency appears to augment with increasing age, particularly after 65 years old.

Recent evidence in the literature and collected in the investigators' laboratory confirm that constipation can be a consequence of intestinal dysbiosis, with an increase of potentially pathogenic microorganisms and a decrease of potentially beneficial microorganisms. These alterations may affect the motility and metabolic environment of colon, especially the production of short chain fatty acids (SCFAs).

A new and under-explored method to manipulate the gastrointestinal microbiota involves fecal microbiota transplantation (FMT). There has been growing interest in the use of fecal microbiota for the treatment of patients with chronic gastrointestinal infections (e.g. CDI) and other extraintestinal conditions (e.g. IBD). Similarly, the investigators suppose that reshaping the gut microbiome with FMT would be effective for patients with slow transit constipation.

Inclusion Criteria:

Chronic constipation according to Rome III criteria, defined as two or fewer spontaneous, complete bowel movements (SCBMs) per week for a minimum of 6 months;

Age ≥ 18 years;

BMI: 18.5-25 kg/m2;

Slow colonic transit confirmed by colonic transit test (colonic transit time (CTT) > 48 hours);

Normal anorectal manometry, with no evidence of dyssynergia and confirmed ability to expel rectal balloon;

No radiographic evidence of functional (i.e. pelvic floor dyssynergia) or anatomical (i.e. significant rectocele and intussusception) impediment to the expulsion of the radio-opaque contrast;

Disease duration > 1 year;

Traditional treatment with dietary modification, laxatives (including osmotic and stimulant laxatives), and biofeedback tried over the past 6 months without success;

Exclusion Criteria:

Bowel constipation due to innate factor (i.e. megacolon) or secondary interventions (i.e. drugs, endocrine, metabolic, neurologic or psychologic disorders);

History or evidence of gastrointestinal diseases (i.e. obstruction, cancer, inflammatory bowel diseases) ;

Previous abdominal surgery, except cholecystectomy, appendicectomy, tubal ligation and cesarean section;

Previous proctological or perianal surgery;

A constipation condition meeting the Rome III criteria for IBS or functional abdominal pain syndrome;

Pregnant or breast-feeding women;

Infection with enteric pathogen;

Usage of probiotics, prebiotics and/or synbiotics within the last month;

Usage of antibiotics and/or PPIs within the last 3 months;

Smoking or alcohol addiction within the last 3 months;

Uncontrolled hepatic, renal, cardiovascular, respiratory or psychiatric disease;

Disease or therapy with drugs (i.e. antidepressants, opioid narcotic analgesics, anticholinergics, calcium antagonists, nitrates, antimuscarinics) that, in the opinion of the investigator, could affect intestinal transit and microbiota.