Nov 9 (Reuters) - More U.S. high-yield bond issuers saw downgrades to their credit ratings than upgrades last month, JPMorgan said in a research report.

October saw downgrades to 18 junk bond issuers' ratings accounting for $22.2bn in debt, while just 16 issuers were given upgrades by ratings agencies, the JPMorgan report said.

This is the first time downgrades have surpassed upgrades on U.S. junk-rated borrowers in four months, it added.

Changes to credit ratings are significant for a company, since lower ratings most often result in higher borrowing costs.

Ratings agency Moody's Investors Service has forecast defaults among low-rated U.S. companies will peak at 5.6% in January 2024, before falling to 4.6% by August 2024.

October's ratings actions reflect a broader trend as there have been 293 downgrades totaling $393 billion over the last 12 months, compared to 264 upgrades equaling $479 billion, the JPMorgan report said.

The largest number of junk issuer downgrades this year have been in the healthcare and financial sectors.

While high-grade companies' credit ratings have proven resilient during the Federal Reserve's interest-rate hikes, businesses with significant leverage and floating-rate debt have struggled to keep pace with rising debt-servicing costs.

Persistent inflation and dampened demand have also eroded profit margins. (Reporting by Matt Tracy; Editing by Alexander Smith)