TyG index

Insulin is the hormone that sends glucose into your cells after a meal and tells your liver there is enough. If your cells respond less well to it, which is called insulin resistance, more glucose stays in your blood and your liver, which also misses the signal, makes more triglycerides. Those two consequences can both be measured, fasting, from the same tube, and the TyG index puts them together: the logarithm of your triglycerides times your glucose, divided by two, with both in mg/dL. The index is not a test but a calculation, and no extra tube of blood goes into it. What gives it weight is what it was tested against. It was compared with HOMA-IR, the usual estimate from glucose and insulin, and then with the clamp test, the reference method in which insulin and glucose are given by infusion to measure exactly how sensitive your cells are. The link with that measurement was strong, with a correlation of -0.681, and stayed the same in men and women, with and without excess weight, with and without diabetes. The index appears on no Dutch report; you calculate it from your fasting triglycerides and glucose, after converting to mg/dL, where triglycerides in mmol/L go times 88.6 and glucose times 18. The result has no unit. There is no normal, but there are two boundaries from research: 4.65 against HOMA-IR and 4.68 against the clamp test, both from a Mexican study group. Below that is healthy, and lower is better.

Insulin resistance is the forerunner of type 2 diabetes and precedes a fasting glucose that becomes too high by years, because the pancreas compensates the loss of sensitivity for a long time with more insulin. Anyone who sees it early can reverse it with losing weight, exercise and less sugar. The direct measure, an insulin test, is expensive and in the Netherlands can be drawn at only a handful of places; the TyG index pulls the same pattern out of two numbers you already have. The second reason is that the index shows the gain of lifestyle on two sides. Less sugar and alcohol, losing weight and exercise lower the triglycerides within weeks and the fasting glucose within months, and the index falls on both fronts. In large population studies a higher index is also linked to more cardiovascular disease, fatty liver and kidney damage, also in people without diabetes. The honest limitation is that the index is an approximation and not a diagnosis. The researchers themselves call it a surrogate: it estimates something you would measure with the clamp test and does not replace that measurement. The boundaries come from one study group and were not established in a Dutch population, and the index is only as good as the draw: two non-fasting numbers give an index that measures something else.

A low index is favourable and means your cells respond well to insulin. The most common explanation first: a healthy insulin sensitivity, with a healthy weight, little sugar and regular exercise; falling fasting triglycerides, the most mobile of the two input values, which respond within weeks to less sugar and alcohol; and a falling fasting glucose, which follows more slowly. Below 4.65 nothing needs to happen. A high index means your triglycerides and glucose both sit high. Usually it is the first and rarely the last: not drawn fasting, because both glucose and triglycerides rise after a meal and then lift both sides of the sum at once; insulin resistance, what the index was made for, usually through belly fat, little exercise, a lot of sugar and lack of sleep; and rising fasting triglycerides on their own, through alcohol or a lot of fast sugars, even when the glucose is still normal. Above 4.68 the index fits insulin resistance, and then your HbA1c is the next number to set beside it. Read the index beside the fasting triglycerides and glucose themselves, because the same result belongs to several combinations, beside your HbA1c, which shows whether the blood sugar over months is already moving along, beside your HDL, which falls with insulin resistance, and beside your waist size and weight, which usually give away the cause already.