Modern Britain was built on sugar; there is hardly a manufacturing town on these shores that was not in some way connected with the "Africa trade". The glittering prosperity of slave ports such as Bristol and Liverpool was derived in large part from commerce with Africa. In the heyday of the British slave trade, from 1700 to 1808, West Indians (as white sugar barons were then known) became conspicuous by their new wealth. Often they cast Barbados or Jamaica aside like a sucked orange in order to fritter their profits in England. A popular melodrama of 1771, Richard Cumberland's The West Indian, satirised planters as drunken layabouts in ostentatiously buckled shoes and hats.

Behind the great wealth of planters lay the "triangle merchants" who motivated the slave trade between Britain, Africa and the West Indies. A typical "triangle voyage" carried trading goods from England to Africa, then slaves from Africa to the Caribbean, and finally sugar, coffee, cotton and rum on the home stretch to England. It was one of the most nearly perfect commercial systems in modern times, a loop of supply and demand. The Atlantic crossing from Africa  the feared Middle Passage  foreshadowed the greater cruelties of the plantations in the British Caribbean, as Africans were herded on to slave ships and branded with irons.

In Barbados, where Andrea Stuart was born in the early 1960s, slaves were flogged virtually into the grave in order to speed up cane-cutting and crushing. For three centuries the Caribbean island's slave-grown sugar satisfied the British craving for tea (that "blood-sweetened beverage", the abolitionist poet Southey called it), as well as cakes and other confections. Barbados was the first colony in the English Americas to import slave labour across the Middle Passage, says Stuart. The earliest slaves to arrive in Barbados were not Africans, however, but white convicts and other "undesirables" shipped out from England in the 1600s.

Among the first English planters to settle in Barbados in the 17th century was a blacksmith called George Ashby, who happens to be a maternal descendent of Stuart's. As a tough "frontiersman", says Stuart, Ashby had no likely scruple about the brutality of the white slave trade. For most of its existence as a slave colony, Barbados was run by boorish, limited men (family black sheep, army deserters) who had gone to the Caribbean to acquire a fortune and a social status they would have been denied at home.

In Sugar in the Blood, a diligently researched hybrid of family memoir and history, Stuart chronicles her locally famous Ashby ancestors and their place in Barbadian slave society. The difficulties in writing such a book are evident from the start. In the absence of surviving documentation, Stuart is constrained to follow the "would have" school of biography. "As George Ashby saw his home for the first time", runs a typical sentence, "he would have had no idea what a tumultuous story he had been catapulted into." To compensate for the dearth of written evidence, Stuart brocades her family history with descriptive passages of "talcum-fine white sands" and "rustling" cane fields. The book has been underway for some time, it seems; but a writer's most personal book is not always her best. The early chapters are slow to catch fire; clichés cling like grime to the prose ("the winds of change", "miasma of fear").

Stuart is on more solid ground in the book's second half, when "identifiable" slave forebears begin to emerge. Like most former slave colonies, Barbados is an island of bewildering mixed bloods and ethnicities. Chinese, British, Jewish and aboriginal Taíno Indian have all intermarried to form an indecipherable blend of Caribbean peoples. Typically for the West Indies, Stuart's white planter ancestors had mixed with freed African slave women, and had illegitimate children by them. Stuart herself is a mixed race descendent of these slave-era "miscegenations". The British empire gave slave-owners like the Ashbys the licence to abuse their captive women, Stuart suggests, and indulge a predatory nature.

In spite of its brutal past, post-slavery Barbados was in some ways a more "modern" society than post-war Britain, where Stuart migrated as a teenager with her parents in 1976. Typically, a hostile reception awaited them. "We landed in a country that was a cauldron of bitter rhetoric about migration which left us in no doubt how unwelcome we were." Stuart's father, by profession a doctor, loved his adoptive nation and was especially aggrieved by British displays of prejudice. Calls for racial purity puzzled many newcomers, as racial mixing was not new to them. Barbados remains an island both parochial and international in its collision of cultures.

Throughout, Sugar in the Blood is enlivened with descriptions of contemporary Barbados, with its Anglican churches and Jacobean great houses. The island was reputedly named after the hairy tendrils of its banyan trees: barba dos is Portuguese for "double-bearded". Along the way, unfortunately, Stuart repeatedly compares the Atlantic slave trade to Hitler's war against the Jews. ("For the slave ship, like the gas chamber, was a diabolic innovation.") The comparison is historically inept, as well as potentially disrespectful to those murdered by the Nazis. An absorbing if uneven work, Sugar in the Blood provides testimony to the high human drama of Caribbean slave trafficking and the misery endured by millions in the pursuit of sweetness.

 Ian Thomson's The Dead Yard: A Story of Modern Jamaica is published by Faber.