Blood sugar goes up when the sugar going into the bloodstream is greater than the amount of sugar being taken out of the bloodstream.
The reasons that the body has trouble taking sugar out of the bloodstream are, essentially, the reasons that people have diabetes.
Some treatments of diabetes, however, focus on the rate at which sugar is added to the blood stream. This is done in two ways: with medications that change the speed at which the body releases sugar after meals (these medications include metformin and symlin, among others), and with dietary means. Dietary means typically focus on changing what types of foods are eaten to consume those foods that are converted to sugars the slowest.
Generally, there is a ranking in terms of how fast it takes the body to go from eating a food to adding sugar to the blood stream. Most people believe that some foods do not add sugar to the blood stream. This is a lot less true than is widely supposed.
Here are five general sources of energy for the body:
Alcohol (which this post will not deal with), fats, proteins/amino acids, complex carbohydrates, and simple carbohydrates.
I don't actually know how the body gets energy from alcohol so I'm gonna leave it alone for now. Complex carbohydrates and some simple carbohdrates require some breakdown before turning into glucose (some simple carbohydrates are glucose to begin with) but I think that's pretty straight forward.
So this post is about how the body turns proteins and fats into glucose. The process of turning something that's not a carb into sugar is called gluconeogenesis. Gluconeogenesis takes place in the liver and occurs more when the body is under starvation conditions or is eating a diet low in carbohydrates. Gluconeogenesis is driven by glucagon. Glucagon production in type 1 diabetics is often abnormally high (because in the normal pancreas, insulin kinda tells the alpha cells to slow production of glucagon), but may also be normal or low (such as in diabetes where the entire pancreas or islet cells have been damaged or are missing).
When the body breaks down fat for energy, it separates the fat into two parts to start with. One part is a fatty acid, (whether this can be turned into glucose in humans is debatable), and the other part is glycerol.
Glycerol is considered a complex carbohdyrate if you eat glycerol as is. It takes some work for your body to convert it into glucose, but it happens.
The things your nutrition label calls proteins may be amino acids actually together in a chain (in which case they are proteins) or they may be plain amino acids. If you eat them as proteins, your body nonetheless breaks them down into their component amino acids. Your body can then use the amino acids to build the proteins it needs. If there are extra amino acids left over, the body may store them as fat or turn them into glucose. Most (I have seen this number at 12, 13, and 14) of the amino acids are typically turned into glucose; another four or five can be turned into glucose if needed. Two amino acids never turn into glucose in human beings (those two are leucine and lysine and they are called the ketogenic amino acids because they are ketogenic).
So, the body gets from food to glucose at different speeds (and at different percentages of the food) depending on what is consumed. Typically, when people take insulin in a basal-bolus pattern, they take bolus insulin based only off of the carbohydrates they eat, because the carbohydrates are absorbed most quickly and because it's easier to just count carbs. Some people also count proteins and inject for a smaller portion of the protein (I do not do this because I have trouble figuring out just how much to inject for the proteins and because counting just carbohydrates seems to work reasonably well for me).
Proteins and fats and even some of the carbohydrates are converted to sugar slower and are often covered by basal insulin. In people whose bodies make some insulin, often that insulin can prevent rises in blood sugar when the sugar is going into the blood stream at slower rates.
In the DCCT, the large scale trial often used as evidence that more intense blood sguar control prevents, delays, and reverses complications in type 1 diabetics, higher fat diets were not associated with lower insulin needs or lower A1cs. Other studies have had had other results, but it is safe to say that if you take, for instance, 70% of your insulin as bolus and 30% as basal while on a high carbohydrate diet, if you do not make insulin, going to a low carb diet will not drop your insulin needs to your current basal needs.
Here is the DCCT authors' analysis.
And just for fun, the glucose song: