Bifidobacterium longum, a normal gut bacterium sold as live or heat-killed supplements and tested for digestive complaints and allergies, has strong evidence for specific strains easing irritable bowel symptoms and seasonal nasal and eye allergy. Mood, sleep, stool and blood-marker signals are smaller, less consistent. Nothing links it to longer human life. Harms are mostly short-lived gas; bloodstream infection is documented with a broken gut wall or weak immune defences. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Absolute neutrophil count | Above 1,500 cells/µL; withheld below 500 | Gates safe use of a live organism |
| High-sensitivity C-reactive protein | Below 1.0 mg/L | Flags active inflammation that argues for caution |
| Faecal calprotectin | Below 50 µg/g | Marks gut lining inflammation and barrier compromise |
| Stool frequency and Bristol Stool Form Scale type | Three per week to three per day, types 3–4 | The endpoint that moved fastest in trials |
| Fasting glucose | 4.4–5.0 mmol/L (79–90 mg/dL) | The one metabolic marker with a randomised signal |
| Haemoglobin A1c (HbA1c) | Below 5.4% | Confirms whether any glucose change persists |
| Fasting triglycerides | Below 1.0 mmol/L (89 mg/dL) | Fell in the body-composition trial |
| Total and non-HDL cholesterol | Total below 5.2 mmol/L (200 mg/dL); non-HDL below 3.4 mmol/L (130 mg/dL) | Both fell on a heat-treated preparation |
| Faecal Bifidobacterium abundance | No established target; change tracked from the individual's own baseline | Confirms the organism reached and altered the gut |
Cadence: One- to two-week symptom baseline, with safety blood tests where immune status or gut integrity is in question; symptom records reassessed at 4 weeks, at 12 weeks, then every 6–12 months on continued use; safety blood tests repeated only if immunosuppression, abdominal surgery or unexplained fever intervenes.