Sugar is controversial now, yet lots of people stand by it. "What is the big deal?" they say. As with epidemiology it is always difficult to prove direct effects in humans. People are complicated, they have different genes, different lifestyle habits. Some smoke, some run marathons, some live in cities with smog, some are in the country by animal waste. All this makes it difficult to find a direct effect between nutrition and health.
In the case of blood sugar, we need to look at what we know about how the body works. First, when blood sugar is high your pancreas releases insulin so the excess sugar (glucose) can be absorbed by fat cells in your body. Conversely, when your blood sugar is low, the pancreas releases glucagon which signals the liver to release glucose in the blood. Both insulin and glucagen are hormonal signals, which send messages to your body. They are bound to have more than one function, and indeed as we have seen earlier, insluin is also important in making you feel hungry. This is part of the reason why a person usually feels hungry after eating high sugar foods.
So how does all this relate to health and aging? Well first off, if your blood sugar is consistently high from eating high sugar foods then your body eventually becomes less and less responsive to the insulin, precipitating a condition known as type II diabetes.
Further, caloric restriction is probably the most famous intervention known to extend healthy years of life and "preserve youth". While it is still not known how caloric restriction does this, one of the main theories is that by starving the body, insulin sensitivity increases. In other words, it takes less insulin to keep blood sugar in line, and this in turn slows down the aging of your body. Could you ask for a better reason to eat lettuce instead of candy?