Which method of assessing body composition is described as very accurate but expensive?

The method that is described as very accurate but expensive is DXA, or dual-energy X-ray absorptiometry. This technique uses low-level X-rays to differentiate between bone mass, fat mass, and lean mass in the body. Because of its precision in measuring body composition and its ability to provide detailed and reliable data, DXA is often considered the gold standard for body composition assessment.

While the equipment and procedures involved in DXA can be costly, they offer a level of detail and accuracy that other methods may not provide. For instance, skinfold measures and bioelectrical impedance are useful but can be less accurate due to variations in technique and individual differences. Underwater weighing, while it can be accurate, is less commonly used because it requires specialized equipment and conditions, making it impractical for regular use when compared to DXA. Thus, DXA stands out for its combination of accuracy and reliability, albeit with a higher associated cost.