What exactly did the FIT trial test?

FIT tested whether oral alendronate (5-10 mg daily) reduced fractures in postmenopausal women with low bone mineral density. It ran as two parallel randomized controlled trials: FIT-1 for women who already had a vertebral fracture, and FIT-2 for women with low BMD but no prior vertebral fracture. Both compared alendronate against placebo, with all participants receiving calcium and vitamin D supplements.

How large was the fracture reduction in the FIT trial?

In FIT-1, alendronate reduced new vertebral fractures by 47% and hip fractures by 51% over 3 years. In FIT-2, vertebral fractures dropped by 44% over about 4 years. The overall clinical fracture reduction in FIT-2 was not statistically significant for the full study population but was significant in the subgroup with T-scores at or below -2.5.

Who were the participants in the Fracture Intervention Trial?

Participants were postmenopausal women aged 55-81, almost entirely white, recruited from 11 U.S. academic medical centers. All had a femoral neck T-score of -1.6 or lower. FIT-1 enrolled women with at least one existing vertebral compression fracture. FIT-2 enrolled women without any prior vertebral fractures.

Is alendronate the same as Fosamax?

Yes. Alendronate is the generic name; Fosamax is the brand name originally marketed by Merck. The FIT trial used the daily 5-10 mg formulation. Most patients today take the 70 mg once-weekly formulation, which later studies demonstrated produces equivalent bone density gains and fracture protection.

Did the FIT trial find any serious side effects of alendronate?

During the trial period, alendronate was generally well tolerated. Upper gastrointestinal symptoms (heartburn, nausea, abdominal pain) were slightly more common in the alendronate group but did not reach statistical significance in most analyses. The rare long-term complications now associated with bisphosphonates, such as atypical femoral fractures and osteonecrosis of the jaw, were not identified during FIT's follow-up period.

Why did FIT-2 fail to show a significant reduction in overall clinical fractures?

FIT-2 enrolled women with low bone density but no prior fractures, placing them at lower absolute fracture risk. With a lower baseline event rate, the absolute benefit of treatment was smaller and the trial was likely underpowered for this endpoint in the full cohort. When restricted to the pre-specified subgroup with T-scores ≤ -2.5, the clinical fracture reduction did reach significance (36% relative risk reduction).

How did the FIT trial influence osteoporosis treatment guidelines?

FIT provided the definitive evidence that bisphosphonates prevent fractures, not just improve bone density numbers. This led to alendronate becoming the first-line recommended therapy in guidelines from the National Osteoporosis Foundation, the American Association of Clinical Endocrinologists, and the Endocrine Society. The FIT-2 subgroup analysis also helped define the treatment threshold at T-score ≤ -2.5.

What is a bisphosphonate holiday, and how does FIT relate to it?

A bisphosphonate holiday is a planned break from treatment after several years of use. The concept arose partly from the FLEX extension study, which followed FIT participants who had taken alendronate for 5 years. FLEX found that stopping after 5 years did not cause an immediate rebound in fracture risk for moderate-risk patients, supporting the idea that the drug's bone-binding properties provide residual protection after discontinuation.

Can men benefit from alendronate based on the FIT trial data?

FIT enrolled only postmenopausal women, so it does not directly answer this question. Separate trials have since demonstrated that alendronate increases BMD in men with osteoporosis, and the FDA label includes an indication for treatment of osteoporosis in men. The mechanism of action is the same regardless of sex.

How does the FIT trial compare to trials of newer osteoporosis drugs?

FIT set the benchmark. The HORIZON trial for zoledronic acid, FREEDOM trial for denosumab, and FRAME trial for romosozumab all used similar fracture endpoints and morphometric methods. Some newer agents show larger BMD increases or faster onset of effect. But for cost-effectiveness and long-term safety data, alendronate (supported by FIT and its extension studies) remains the standard against which newer drugs are measured.

FIT (Fracture Intervention Trial) Trial: A Plain-English Overview of What It Established

By HealthRX.com Medical Team

Published May 25, 2026Updated May 25, 2026Last reviewed May 25, 2026

Clinical image for FIT (Fracture Intervention Trial) Trial: A Plain-English Overview of What It Established Image: HealthRX.com clinical image

At a glance

| Field | Detail | |---|---| | Trial name | Fracture Intervention Trial (FIT) | | Design | Randomized, double-blind, placebo-controlled | | Total N | 6,459 (FIT-1: 2,027; FIT-2: 4,432) | | Population | Postmenopausal women aged 55-81 with femoral neck T-score ≤ -1.6 | | Intervention | Alendronate 5 mg/day (years 1-2), then 10 mg/day (year 3+) | | Comparator | Matching placebo (all participants received calcium and vitamin D) | | Duration | FIT-1: 3 years; FIT-2: 4 years | | Primary endpoint | New morphometric vertebral fractures | | Key result | 47% relative risk reduction in vertebral fractures (FIT-1); 44% reduction (FIT-2) | | Published | FIT-1: Lancet 1996; FIT-2: JAMA 1998 |

Why This Trial Exists

Before the mid-1990s, doctors treating osteoporosis had limited options and even less certainty. Estrogen replacement was common but carried its own risks. Calcitonin was available but modestly effective. The bisphosphonate class, which had shown promise in earlier dose-finding studies, still lacked the large, definitive trial needed to prove it could prevent the outcome patients actually feared: broken bones.

The Fracture Intervention Trial was designed to answer that question directly. Sponsored by Merck and coordinated across 11 U.S. clinical centers, FIT asked whether oral alendronate (brand name Fosamax) could reduce fractures in postmenopausal women with low bone mineral density (BMD). The trial was split into two parallel arms to address two distinct clinical populations.

FIT-1 (the "Vertebral Fracture Arm") enrolled 2,027 women who already had at least one existing vertebral compression fracture at baseline. These patients were at high risk for further fractures, and FIT-1 was designed with new vertebral fractures as the primary outcome over 3 years.

FIT-2 (the "Clinical Fracture Arm") enrolled 4,432 women with low bone density (femoral neck T-score ≤ -1.6) but no baseline vertebral fracture. This group represented the larger population of women who had osteopenia or early osteoporosis but had not yet broken a bone. FIT-2 ran for approximately 4.2 years.

Who Was in the Trial

All participants were postmenopausal women between ages 55 and 81. Recruitment took place at 11 U.S. centers from May 1992 through May 1993. Eligibility required a femoral neck BMD at least 1.6 standard deviations below the young-adult mean (roughly T-score ≤ -1.6). Women were excluded if they had other metabolic bone diseases, were already on bisphosphonates or fluoride, or had conditions expected to limit their participation over several years.

Both arms included calcium supplementation (500 mg/day) and vitamin D (250 IU/day) for all participants, including the placebo group. This meant the trial measured the benefit of alendronate on top of baseline supplementation, not against nothing.

The average age was approximately 68 years. Roughly 25% of FIT-1 participants had two or more existing vertebral fractures at enrollment. In FIT-2, the average femoral neck T-score was about -2.1, and participants had no radiographic vertebral deformities at baseline.

What They Were Given

The dosing protocol changed during the trial. Initially, all participants in the treatment arm received alendronate 5 mg/day. Partway through the trial, the dose was increased to 10 mg/day based on emerging evidence that the higher dose produced greater BMD gains. The published results reflect this stepped dosing, which later became the standard clinical dose (10 mg daily or 70 mg weekly).

Alendronate had to be taken on an empty stomach with a full glass of water, and participants were instructed not to lie down or eat for at least 30 minutes afterward. This protocol was designed to protect against esophageal irritation, a concern that would become more prominent as real-world use expanded.

How Fractures Were Measured

Vertebral fractures were assessed using lateral spine radiographs taken at baseline and at follow-up visits (every 2 years in FIT-2, at the end of study in FIT-1). Trained readers at a central facility measured vertebral body heights using morphometric analysis. A new vertebral fracture was defined as a reduction of ≥ 20% (and at least 4 mm) in any vertebral height compared to baseline.

This morphometric approach is important to understand. Many vertebral compression fractures are clinically silent. A patient might lose a centimeter of height over a year without ever having a sharp pain event. FIT captured these radiographic fractures systematically, which is why the "new vertebral fracture" numbers are higher than what clinical symptom tracking alone would find.

Clinical (non-vertebral) fractures were also tracked as secondary outcomes, including hip fractures, wrist fractures, and any symptomatic fractures reported by participants and confirmed by radiographic or medical record review.

Results: What the Numbers Actually Show

FIT-1 (Women With Existing Vertebral Fractures, 3 Years)

| Outcome | Alendronate | Placebo | Relative risk reduction | p-value | |---|---|---|---|---| | New morphometric vertebral fractures | 8.0% | 15.0% | 47% | <0.001 | | Clinical vertebral fractures | 2.3% | 5.0% | 55% | <0.001 | | Hip fracture | 1.1% | 2.2% | 51% | 0.047 | | Any clinical fracture | 13.6% | 18.2% | 28% | 0.01 |

The vertebral fracture reduction was the headline finding. But the 51% hip fracture reduction was clinically striking, given that hip fractures carry the highest morbidity and mortality of any osteoporotic fracture. This result achieved statistical significance despite hip fractures being relatively uncommon events in the trial.

FIT-2 (Women Without Existing Vertebral Fractures, ~4 Years)

| Outcome | Alendronate | Placebo | Relative risk reduction | p-value | |---|---|---|---|---| | New morphometric vertebral fractures | 2.1% | 3.8% | 44% | 0.001 | | Clinical fractures (primary) | 12.3% | 13.1% | Not significant | 0.13 | | Hip fracture | 0.7% | 0.7% | Not significant | NS |

FIT-2 told a more nuanced story. Vertebral fractures were significantly reduced. But the primary endpoint of "any clinical fracture" was not significantly different between groups in the overall population. In a pre-specified subgroup analysis of women with baseline T-score ≤ -2.5 (true osteoporosis by WHO criteria), the clinical fracture reduction became significant, with a 36% relative risk reduction.

This subgroup finding shaped how alendronate was ultimately positioned in clinical practice: most effective in women with established osteoporosis rather than milder degrees of low bone density.

Bone Density Changes

Alendronate produced consistent BMD increases across both arms. At the lumbar spine, BMD increased roughly 6.2% over 3 years compared to placebo. At the femoral neck, the increase was approximately 4.1%. These gains appeared to stabilize rather than continue climbing indefinitely, consistent with the mechanism of bisphosphonates (they slow resorption rather than build new bone).

What Makes This Trial Credible

Several methodological strengths set FIT apart from smaller or less rigorous studies that preceded it.

Large sample size with long follow-up. Over 6,400 women followed for 3-4 years gave FIT the statistical power to detect differences in relatively uncommon events like hip fractures.

Hard clinical endpoints. The trial measured actual fractures, not just BMD changes. Earlier studies had sometimes used BMD as a surrogate endpoint, which left open the question of whether better numbers on a DEXA scan translated to fewer broken bones. FIT closed that gap.

Central radiograph reading. All spine films were read at a single facility using standardized morphometric criteria, reducing inter-reader variability.

Calcium and vitamin D for everyone. By supplementing all participants, the trial isolated the effect of alendronate specifically, avoiding the confounder of some participants being vitamin D deficient and others not.

Limitations the Authors Acknowledged

The FIT investigators were transparent about several constraints. The study population was almost entirely white postmenopausal women, limiting generalizability to men, premenopausal women, and other racial groups. Recruitment through academic centers may have selected for healthier, more compliant patients than the broader osteoporosis population.

The mid-trial dose change from 5 mg to 10 mg complicates interpretation. We cannot clearly separate the effects of the two doses, although both showed benefit over placebo.

FIT-2's failure to reach significance on its primary endpoint (any clinical fracture in the full cohort) was an important negative finding. It meant that for women with low bone density but no prior fractures, the absolute benefit of alendronate was smaller, and treatment decisions needed to weigh cost and inconvenience against a more modest expected gain.

Long-term safety data were unavailable at the time of publication. Questions about atypical femoral fractures and osteonecrosis of the jaw, which later emerged as rare complications of prolonged bisphosphonate use, were not yet on the radar when FIT was published.

What Changed Because of This Trial

FIT's results reshaped osteoporosis treatment in several concrete ways.

The FDA approved alendronate (Fosamax) for treatment and prevention of postmenopausal osteoporosis, and FIT was the primary evidence base for that approval. The trial data supported the current FDA labeling that cites fracture reduction as the core clinical benefit.

Professional societies, including the National Osteoporosis Foundation (now the Bone Health and Osteoporosis Foundation), incorporated FIT's findings into treatment guidelines that recommend bisphosphonates as first-line pharmacotherapy for patients meeting treatment thresholds.

The FIT-2 subgroup analysis contributed to the clinical practice of reserving pharmacotherapy primarily for women meeting WHO criteria for osteoporosis (T-score ≤ -2.5) or those with prior fracture history, rather than treating all women with any degree of low bone density.

FIT also established the methodological template for subsequent bisphosphonate trials. The VERT trial for risedronate, the HORIZON trial for zoledronic acid, and the FLEX extension study all borrowed elements of FIT's design, endpoints, and morphometric fracture assessment approach.

Where the Evidence Stands Now

Nearly three decades after FIT, alendronate remains one of the most prescribed osteoporosis drugs worldwide. Generic formulations (typically the 70 mg weekly dose, which showed equivalent efficacy to daily dosing in later studies) cost as little as a few dollars per month, making alendronate one of the most cost-effective interventions in all of medicine.

The debate has shifted from "does alendronate work?" to "how long should patients stay on it?" The FLEX extension study (a follow-up of FIT participants) showed that stopping alendronate after 5 years led to gradual bone loss but no immediate spike in fracture risk for most patients. This gave rise to the concept of a "bisphosphonate holiday," now endorsed by the American Association of Clinical Endocrinologists for patients at moderate risk after 5 years of treatment.

For patients at very high fracture risk, newer agents like denosumab (Prolia), romosozumab (Evenity), and teriparatide (Forteo) have expanded the toolkit. But FIT is the reason those trials were designed the way they were, and alendronate often remains the drug patients are transitioned to after completing courses of those newer, more expensive therapies.

Frequently asked questions

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References

Black DM, Cummings SR, Karpf DB, et al. Randomised trial of effect of alendronate on risk of fracture in women with existing vertebral fractures. Lancet. 1996;348(9041):1535-1541.
Cummings SR, Black DM, Thompson DE, et al. Effect of alendronate on risk of fracture in women with low bone density but without vertebral fractures: results from the Fracture Intervention Trial. JAMA. 1998;280(24):2077-2082.
Black DM, Schwartz AV, Ensrud KE, et al. Effects of continuing or stopping alendronate after 5 years of treatment: the Fracture Intervention Trial Long-term Extension (FLEX). JAMA. 2006;296(24):2927-2938.
Camacho PM, Petak SM, Binkley N, et al. American Association of Clinical Endocrinologists/American College of Endocrinology clinical practice guidelines for the diagnosis and treatment of postmenopausal osteoporosis. Endocr Pract. 2020;26(Suppl 1):1-46.
FDA. Fosamax (alendronate sodium) prescribing information. accessdata.fda.gov.
Cosman F, de Beur SJ, LeBoff MS, et al. Clinician's guide to prevention and treatment of osteoporosis. Osteoporos Int. 2014;25(10):2359-2381.