One of every four Americans under 65 is without health insurance coverage at least part of the year or has a private policy that fails to cover major health costs, according to a study released yesterday by the Department of Health and Human Services.

Although the nation is spending more money on health care than ever before, the study found that anywhere from 49.1 million to 76.7 million Americans under 65 either lack full-year insurance or have inadequate policies, depending upon how the adequacy of a private health policy is measured.

The study, probably the most detailed look to date at the nation's health insurance coverage patterns, was conducted by Pamela J. Farley of the HHS National Center for Health Services Research.

Three alternative measurements of the adequacy of health plans were used.

One alternative assumed a moderate illness with costs of a few thousand dollars, the type likely to befall about one person in 20. A policy was considered adequate if it covered the bulk of the charges, so that the family did not have to spend more than 10 percent of its income.

The second alternative, which the author said is probably the most realistic, assumed a more serious and costly illness with a one in 100 risk of occurrence; the policy was considered inadequate if family outlays would have exceeded 10 percent of income.

The third alternate measure of adequacy assumed a serious illness with catastrophic costs, of which the risk is small. The policy was considered inadequate if it did not include a catastrophic protection feature.

The study found:

* Using the first definition, about 23.5 percent of the population under 65 -- 49.1 million people this year -- either lack coverage or have inadequate coverage. This consists of 9 percent without any private or government health protection such as Medicare or Medicaid, 9.4 percent who were uncovered for part of the year (these two categories include 38.5 million people) and 5.1 percent whose private coverage is inadequate (10.6 million).

* Under the second definition, about 26.7 percent of the under-65 population -- 55.8 million people -- lack coverage, have coverage only part of the year or have inadequate coverage. This consists of the 38.5 million without any coverage or without full-year coverage, plus 8.3 percent (about 17.4 million people) whose private insurance does not meet the adequacy test.

* Using the third definition, 36.7 percent of the under-65 population, or 76.7 million people, lack coverage or are underinsured. This consists of the 38.5 million plus 18.3 percent without catastrophic coverage (38.2 million).

The study shows that there are substantial differences among various groups in coverage.

Using the second definition, for example, Farley's figures show that 39 percent of female-headed households have no coverage or inadequate coverage, compared with only 25 percent of male-headed households.

About 25 percent of households headed by whites have no coverage or inadequate coverage, while blacks and Hispanics are about 8 points higher.