The trillions of bacteria that inhabit our gastrointestinal tract play an intricate and increasingly appreciated role in human health. Deemed by some as the “forgotten organ” (1), new connections between these microorganisms and our physiology are being discovered all the time.
Bone health: The basics
Bone is not a static structure and is constantly being remodeled. Mature bone tissue is regularly removed from the skeleton by cells called osteoclasts (a process called resorption), and new bone tissue is formed by cells called osteoblasts (a process called ossification). Remodeling helps to reshape bone following fractures and also responds to mechanical loads like exercise (2). An imbalance in bone resorption and formation can result in bone diseases like osteoporosis and arthritis (3).
There are numerous measures of bone health. In humans, researchers often use a DEXA scan to determine bone mineral density. In rodent models, scientists can also measure bone mass, length, volume, and mineral composition to get a more complete assessment of bone structure. In general, higher bone mineral density is associated with reduced risk of fracture and bone disease. However, increased bone mineral density, bone mass, and bone length do not always suggest better bone health (4).
The association between gut microbes and bone
One of the key ways that researchers study the effects of the microbiota on an organ system is using mice that don’t have one at all. Called “germ-free” (GF), these mice are born and raised in sterile incubators. Studies of skeletal health in GF mice have found mixed results. In one type of mice, GF mice had reduced bone mass compared with conventionally raised animals, which have a full consortium of microbes (5). In another type of mouse, the GF environment resulted in increased bone mass, while the conventional environment increased measures of bone turnover (6). Although it is difficult to reconcile these contradictory findings, the key feature of both studies was that the mere presence or absence of a microbiota significantly changed the structure of the bone.
Source: Another reason to protect the gut: Your microbiome affects your bones