TOKYO—Japan ran its biggest trade deficit in a single month in eight years in January as high energy costs swelled imports and manufacturers struggled with global supply constraints, causing a decline in car shipments. The growing trade deficit highlights the world’s third-largest economy’s vulnerability to soaring commodity costs and slowing demand from its neighbor China as the economy there struggles to maintain momentum. Imports soared 39.6 percent year-on-year in January to hit a record high in terms of their value in yen, coming to 8.5231 trillion yen ($73.81 billion), Ministry of Finance data showed on Thursday, above a median market forecast for a 37.1 percent increase. That greatly outstripped a 9.6 percent rise in exports in the year to January, bringing the trade balance to a deficit of 2.1911 trillion yen, its biggest in a single month since January 2014. The deficit was much bigger than the median estimate for …