BIA and DEXA both report body fat and lean mass. They measure different things, cost different amounts, and tell different stories over time. For most people tracking training and body composition month to month, BIA is the right tool. For a one-off bone density and visceral fat read, DEXA earns its place.

The short answer

If you want frequent, repeatable scans to track training, fat loss, and lean mass changes, use BIA. If you want a medical-grade snapshot of bone mineral density, regional fat, and visceral adipose tissue, use DEXA.

Most adults over 30 do not need both every quarter. They need one consistent method, measured under the same conditions, repeated often enough to see the trend. That is the scan that changes behaviour.

How BIA works

Bioelectrical impedance analysis sends a small electrical current through the body. Fat, muscle, bone, and water conduct that current at different rates. The device estimates body composition from those resistance values, your height, weight, age, and sex.

Modern multi-frequency BIA units (the segmental research-grade models, not bathroom scales) measure each limb and the trunk separately. That gives you arm-by-arm lean mass, trunk fat, and total body water. The scan takes around 60 seconds. At our Christchurch facility we use this as the standard tracking tool across the 1,380+ clients we have run through coaching.

The catch with BIA is hydration. The current is reading water content as a proxy for lean tissue. If you scan dehydrated, post-training, or after a high-carb meal, the number shifts. Standardise the conditions and the noise drops out.

How DEXA works

Dual-energy X-ray absorptiometry passes two low-dose X-ray beams through the body. The differential absorption between the beams separates bone, lean mass, and fat mass. It produces a regional map: arms, legs, trunk, android and gynoid fat, and a visceral fat estimate.

DEXA is the clinical reference standard for bone mineral density. If you are over 50, post-menopausal, on long-term corticosteroids, or have a family history of osteoporosis, this is the scan that matters. Your GP can refer you, and bone density is the metric a DEXA does that nothing else does as well.

For body fat, DEXA is more accurate than BIA in absolute terms. It is also more expensive, less convenient, and involves a small radiation dose (low, but non-zero).

Accuracy versus repeatability

These are not the same thing. Accuracy is how close a single measurement is to the true value. Repeatability is how consistent the method is across repeat scans.

DEXA wins on accuracy. BIA wins on repeatability when conditions are controlled, because you can scan weekly or fortnightly without cost or radiation concerns.

The scan that changes your physique is the one you actually repeat. A perfect DEXA you do once a year tells you less than a standardised BIA you do every two weeks.

For a coaching client tracking a 12-week phase, what matters is the direction and slope of the line, not the absolute fat percentage to one decimal place. BIA, run under the same morning fasted protocol each time, will show you that line clearly.

Cost and frequency in NZ

DEXA scans in New Zealand typically run $90 to $150 per scan through private radiology clinics. Some require a GP referral, others do not. Realistic frequency: once or twice a year.

BIA scans through coaching facilities and gyms range from free (included in coaching) to around $50 standalone. Realistic frequency: every two to four weeks.

Run the maths over a year. Six BIA scans give you a trend line with five data points of change. One DEXA gives you a before-and-after with no granularity in between. If something stalls at week six, the BIA trend tells you. The annual DEXA does not.

Which one you actually need

Match the scan to the question.

  • You are training, eating, and want to see if it is working. BIA, every two to four weeks, fasted, same time of day, same hydration status.
  • You are over 50 and want a bone density baseline. DEXA, once, then repeat every two to five years based on the result and your GP's input.
  • You suspect high visceral fat and want a clinical number. DEXA gives you a VAT estimate. Waist circumference and a BIA trunk reading get you most of the way for free.
  • You are a competitive athlete prepping for a peak. BIA fortnightly through prep, optionally one DEXA at baseline if you want regional lean mass mapped.
  • You are on hormone therapy, GLP-1s, or significant medication changes. BIA monthly to track lean mass retention. We work alongside your GP on the labs side.

The mistake we see often: people pay for a DEXA, see a number they do not like, change nothing systematically, and pay for another one six months later. That is not a measurement problem. That is a coaching problem.

What standardisation actually looks like

A BIA scan is only as useful as the conditions around it. The protocol we use:

  • Morning, fasted, post-toilet, pre-training.
  • No alcohol the prior 24 hours.
  • Consistent hydration the day before (not loaded, not depleted).
  • Same device, same operator where possible.
  • Light clothing, no metal.

Run that protocol every time and the noise floor drops. You can detect a 0.5 kg lean mass change with confidence. Skip the protocol and a 2 kg swing means nothing.

What to do this week

  • Decide what question you are trying to answer: training progress, bone health, or visceral fat.
  • If the answer is training progress, book a BIA scan and lock in the protocol above.
  • If you are over 50 and have never had a bone density scan, talk to your GP about a DEXA referral.
  • Stop scanning weekly on a bathroom scale BIA after meals. The data is noise.
  • Pick one method, repeat it for 12 weeks, then judge whether the trend matches the work.