How can a high level of triglycerides affect my health if my cholesterol is normal and my LDL and HDL are both low?
I presume that you meant that LDL and HDL were both normal. This is more important than the actual triglyceride number (I will actually get to your answer after I get the rest of this off my chest). LDL cholesterol is what we call the "bad" portion of cholesterol that gets modified to become a major constituent of atherosclerosis leading to heart attacks, peripheral vascular disease and strokes. HDL cholesterol is actually the "good" portion of cholesterol and protects us from the "bad" portion. If HDL cholesterol is actually low, it can only be raised by discontinuation of smoking and/or an increase in daily exercise patterns. Doctors and laboratories calculate a risk index based on the LDL and target this number as the goal of our treatment. If you have risk factors for early stroke, heart attack or death, we aim at an LDL cholesterol of less than 100 (no matter what the HDL cholesterol is); if you have had a heart attack, stroke or bypass surgery we aim at an LDL cholesterol of 70.
Lowering LDL can be done by weight loss (if you are overweight), changes in the diet (the "Mediterranean style" diet is good for this), exercise and medications. Raising the HDL (the "good cholesterol") can only be accomplished by exercise and discontinuation of smoking (plus some medications that are not terribly effective). Now you know why everyone is after you about a good clean life without cigarettes, with lots of exercise, while watching your diet. As far as the HDL raising medications are concerned, I am not impressed with the data so far. While there is some encouraging data for the fibrate class of drugs, for niacin and for fish oils, the only really good data demonstrates that eating a healthy amount of fish (not fried, though) may make you live longer. Somehow, I find eating fish far more appealing than paying an extraordinary amount for the capsules containing oil squeezed from the parts of the fish that I wouldn't ever eat.
Triglyceride level hasn't really been proven to be a risk factor for heart attack or stroke by itself. Triglycerides give us a peek at your metabolism, and how effectively you are storing and using your "metabolic fuel". High triglycerides occur in metabolic states such as insulin resistance syndrome and may reflect what we used to call "pre-diabetes". As with diabetes, there is a genetic predisposition (fancy words for "it runs in my family"). There are certain families that have extremely high triglycerides. Think of the triglycerides as a fuel residue. If they get too high, they "junk things up" (please pardon a very non scientific analogy). For this problem we do have some medications that help. Once again, though, keeping your weight in the NHBLI (National Heart Lung and Blood Institute) recommended range, amazingly keeps this issue in good control without medications for most people. High triglycerides are not as usual in countries with smaller refrigerators.