How many teaspoons of sugar in a tomato?

A 50g raw tomato is a small salad tomato, larger than a cherry tomato and much smaller than a beefsteak. The tomato is not on any of Dr Unwin’s charts, so Table 1 of the 2016 paper by Unwin, Haslam and Livesey is its only source.

The table lists 1.6g of available carbohydrate and a glycaemic index of 54, and prints a glycaemic load of 0.24. Worked from the table’s own inputs, the load is 0.864, and the teaspoon figure in the same row, 0.3, matches 0.864 rather than 0.24. This site records the calculated load and uses the printed teaspoon figure. The mismatch is listed on the sources page, with the same problem in the cauliflower and mushroom rows.

The paper adds a caution for this food. Its GI was not measured in volunteers; it was calculated from the carbohydrate composition of the food, and the paper states that such values are likely to be overestimated. A GI of 54 is therefore best read as an upper estimate rather than a tested value.

Tomatoes are mostly water, and their small carbohydrate content is mainly glucose and fructose. The sugar content rises as they ripen and differs between varieties; small cherry and plum tomatoes are often sweeter than large salad tomatoes.

Tomato products concentrate the fruit. Passata, tomato purée and ketchup hold the carbohydrate of much more tomato per spoonful, and ketchup has sugar added. None of those is in this dataset.

The 2016 paper, describing a leaflet used in Dr Unwin’s practice, names tomatoes with green vegetables and salads as low in carbohydrate.