Osteoporosis

What Gut Bacteria Have to Do With Bone Density

The evidence here includes a randomised controlled trial in people, which is rare for a bone and gut question. Osteoporosis means bones have lost density and structure, so they break more easily. Bone is living tissue, constantly broken down and rebuilt throughout life. The cells doing the breaking down are immune cells by origin, and that's the opening through which the gut acts.

Bone is rebuilt by immune cells

Two cell types run the process. Osteoblasts lay down new bone. Osteoclasts dissolve old bone.

Osteoclasts develop from the same lineage as macrophages, the immune cells that clear debris and drive inflammation. That makes them responsive to the same inflammatory signals.

When inflammatory signalling rises, osteoclast formation increases and bone is dissolved faster than it's replaced. Density falls.

Gut bacteria are among the strongest influences on background inflammatory signalling in the body, which puts them upstream of that balance.

Work in mice established the connection first. Altering the gut microbiome changed bone mass, apparently by acting on the immune system and on osteoclast formation.

The trial in people

That mouse work led to a specific test.

Researchers recruited women aged 75 to 80 with low bone density but not established osteoporosis. They were randomly assigned to receive either a placebo or Lactobacillus reuteri 6475, a bacterium naturally found in the human gut, at 10 billion colony-forming units daily.

The trial ran for twelve months, double-blind. The main measure was bone density in the lower shin, assessed by a scanner that captures the three-dimensional structure rather than a flat image.

Both groups lost bone, as expected at that age. The women taking the probiotic lost roughly half as much.

The two groups were well matched on body measurements and lifestyle, and side effects were similar between them.

Source: Lactobacillus reuteri reduces bone loss in older women with low bone mineral density: a randomized, placebo-controlled, double-blind, clinical trial

Keeping the result in proportion

A positive trial deserves an honest frame, and the authors gave one.

Bisphosphonates, the standard first-line drug treatment for osteoporosis, produce much larger effects on bone density than this did. Nobody involved suggested a probiotic as a replacement.

The trial also enrolled women with low bone density rather than diagnosed osteoporosis, so it doesn't tell us what happens in established disease. The underlying mechanism in humans hasn't been pinned down either.

Where the result matters is elsewhere. Bisphosphonates aren't recommended for more than three to five years in people at low to moderate fracture risk, which leaves a long stretch of life where there's little to offer. That's the gap this points at.

Introducing bacteria this way also depends on feeding them. A strain arriving in a gut with little fibre and few plant compounds reaching it won't establish, and whatever it was producing stops with it.

Why the response varied between women

A follow-up analysis asked why some women benefited far more than others.

Researchers selected the twenty women with the largest and smallest changes in bone density, then compared their gut bacteria and blood chemistry in detail.

The groups differed. Those who responded well showed a gut community that held together over the year, while the poor responders' communities degraded.

That points at something practical. The strain didn't work in isolation. It worked where the surrounding community was in a state to support it.

Source: One-year supplementation with Lactobacillus reuteri ATCC PTA 6475 counteracts a degradation of gut microbiota in older women with low bone mineral density

What this means for you

Osteoporosis is diagnosed with a bone density scan, and anyone over 50 who has broken a bone from a minor fall should ask about being assessed, since prescribed treatment substantially reduces fracture risk. In addition, the trial above found a specific probiotic strain roughly halving bone loss over a year, with the benefit concentrated in women whose wider gut community held up. A varied, plant-rich diet supports the same gut bacteria this research ties to slower bone loss. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.