Marmalade is made from citrus, but instead of the whole fruit, just the juice and the peel are used. Nothing’s wrong with eating citrus peels, but they do tend to have a bitter flavor, so sugar is added to make marmalade.
Sugar + Sugar = Disaster
Because the fruit juice is already pretty high in sugar, adding more sugar makes marmalade pretty dangerous. It causes a spike in blood sugar and a corresponding spike in insulin to deal with all that sugar. Insulin promotes the storage of nutrients as fat and prevents the breakdown of fat for energy. Diets high in sugar have also been connected with increased inflammation and cortisol (the hormone our bodies produce when under stress), both of which can cause serious health problems if allowed to become chronic.
Mark Sisson has even more reasons to avoid sugar.