Why Refined Carbohydrates are Bad
High-glycemic refined and processed carbohydrates are bad because they make us fat and increase the risk of diabetes and heart disease, and possibly even dementia. To be sure, not all carbohydrates are the same. Some are worst than others.
Sugar is a carbohydrate. If you read food packaging labels, you will notice that sometimes they give total carbohydrate amount and then break out the sugar amount as a sub-category of that.
Learn to watch this sugar amount, because sugar is a bad carbohydrate. Sugar is part glucose and part fructose. Fructose is one of the worst carbohydrates. By now, you probably have heard of the fact that high-fructose corn syrup is bad for you. In his book, The UltraMind Solution, Dr. Mark Hyman tells us to throw everything out that contain high-fructose corn syrup. Refined and processed carbohydrates such as in baked goods, pastry, cookies, and highly processed foods that come in boxes are not good.
On the other end of the spectrum, vegetables and fruits are complex carbohydrates and are good carbohydrates. They come right out of nature and has not be processed. Try to get organic ones whenever possible so that there is no pesticides.
The fiber in the fruit and vegetable will alleviate to some extent the problems of carbohydrates by slowing down the rise in blood sugar. That is why it is better to eat the orange rather than drink the orange juice. They both have sugar, but the orange juice has lost its fiber. And some orange juice has added sugar put in by the manufacturers. The fruits and vegetables contains a lots of vitamins and antioxidants that are essential for good health.
The carbohydrates from some vegetables (such as potatoes and corn) are worst than others. Carbohydrates from white bread and white rice is not good either. Better are breads with whole grains. Better are whole grain rice such as brown rice, basmati rice, and wild rice. Or use quinoa instead of rice.
Carbohydrate Glycemic Index
How do we tell if a carbohydrates is good or bad? The Glycemic Index (GI index) will tell you. The higher glycemic index food, the worst they are. The lower the index the better. The index is a scale from 0 to 100 that tells you roughly how fast the carbohydrate turns into sugar in the body. Pure sugar is given the reference value of 100. Roughly speaking, the more digestible the food, the higher the glycemic index. That is why mashed potatoes is worst and has higher GI than boiled potatoes.
You can find out the Glycemic index of foods from the GI database at glycemicindex.com. GI values of 70 or above is consider high glycemic index foods. GI values of 55 or below is considered low GI foods. And anything in between is moderate. In general, fruits tend to have a higher glycemic index than vegetables since they have more sugar and are sweeter.
The reason why high GI carbs are bad is because they causes a spike in the blood glucose level. When glucose increases in the blood, the pancreas has to pump out more insulin which is a hormone that tries to gets the glucose into the cells for energy. Having chronic high levels of blood glucose and insulin levels will cause the cells to become insulin resistant, increasing the risk of diabetes, metabolic syndrome, and heart disease.
GlycemicIndex.com says ...
"Recent studies from Harvard School of Public Health indicate that the risks of diseases such as type 2 diabetes and coronary heart disease are strongly related to the GI of the overall diet. In 1999, the World Health Organisation (WHO) and Food and Agriculture Organisation (FAO) recommended that people in industrialised countries base their diets on low-GI foods in order to prevent the most common diseases of affluence, such as coronary heart disease, diabetes and obesity."
Low glycemic index food improves diabetes management, improves blood cholesterol levels, and reduce the risk of heart disease. The glycemic index applies only to carbohydrates because consumption of meat and fish and fat does not change blood glucose level in as significant of a way as carbohydrates do.
"Why We Get Fat"
Carbohydrates Make Us Fat
High glycemic index carbohydrates make us fat. Contrary to the belief that we get fat because we are eating too much or exercising too little, many experts are saying that we get fat because we are eating too much high glycemic carbohydrates. It is not the number of calories that matter, but the type of calories.
Carbohydrates causes high insulin levels in the blood. Insulin is that hormone that causes body to accumulate fat.
Gary Taubes say in his book "Why We Get Fat":
"we do not get fat because we overeat; we get fat because the carbohydrates in our diet make us fat. The science tells us that obesity is ultimately the result of a hormonal imbalance, not a caloric one -- specifically, the stimulation of insulin secretion caused by eating easily digestible, carbohydrate-rich foods: refined carbohydrates, including flour and cereal grains, starchy vegetables such as potatoes, and sugar, like sucrose (table sugar) and high-fuctose corn syrup." (page 10)
In his earlier book, Good Calories, Bad Calories, he says the same thing but in a more technical manner and with scientific explanations that are heavily researched and referenced.
Sugar is one of the worst carbohydrates to eat. It has a high GI index of 100. You can learn more about how sugar makes us fat in my other article linked here.
Carbohydrates increase risk of diabetes
Type 2 diabetes (or adult onset diabetes) by definition is a chronic state of high blood sugar. Since it is high GI carbohydrates that raises blood sugar the most, eating lots of high GI carbs increases the risk of diabetes and the other symptoms associated with it such as insulin resistance and metabolic syndrome.
Article on Health.com cites a diabetes nutrition specialist Linda Sartor saying that ...
"The goal ... is to take in enough carbohydrates to nourish ourselves, but never so much that it causes high blood sugars," 
It also helps to space out meals so that you eat more frequently but less each time so that you do not get a large spike in blood sugar.
Exercise helps mitigate the effect of high blood sugar because exercise improve insulin sensitivity and improves the movement of glucose from the blood into the cells.
The problem with Insulin
A person who is tested fine for blood glucose level may still have chronically high insulin levels. The ration of glucose to insulin level should be 10:1. When food is eaten, insulin level rises put push the glucose into the cells. And then insulin levels drop again. This is what should happen.
But when there is a high carbohydrate load that provide so much glucose in the blood that glucose is not able to be shuttled into the cells (either because the cells have become resistant to the effects of insulin, or because there is just too much glucose), you may get a chronically high insulin levels. Chronically high insulin results in obesity and other metabolic disorders.
Carbohydrates Increase Risk of Heart Disease
ScienceDaily articles cites Dr. Shechter saying ...
"Foods like cornflakes, white bread, french fries, and sweetened soda all put undue stress on our arteries. We've explained for the first time how high glycemic carbs can affect the progression of heart disease." 
Eating high GI carbohydrates causes high glucose levels in the blood. This triggers the pancreas to pump out insulin. Insulin triggers the liver to take glucose in the blood and turn it into triglyceride fats. High triglycerides is associated with risk of heart disease and are an even better predictor of heart disease than cholesterol.
In short, carbohydrates increases the level of triglycerides.
It is true that decreasing the amount of saturated fat will decrease the LDL cholesterol (the bad cholesterol). But if you have high triglycerides, then you have to decrease carbohydrates in an attempt to bring the triglycerides levels down.
Also, carbohydrates breaks down into sugar. And sugar can cause arterial damages.
The Acid Alkaline Balance Diet notes a study of where 85% of pigs fed high levels of sugar developed heart disease, whereas pigs which had 10% of the sugar replaced by coconut oil and beef tallow did not. [page 28]
"Good Calories, Bad Calories"
Alzheimer's May Be a Type 3 Diabetes
What is bad for the heart is also bad for the brain. Afterall, the brain is full of arteries and capillaries that bring oxygen from the blood cells to the neurons.
So if eating high GI carbohydrates is bad for the heart, it is bad for the brain as well. Some people are calling Alzheimer's Disease a "Type 3 Diabetes".
Diabetics have an twice the risk of getting Alzheimer's than non-diabetics.
In Good Calories, Bad Calories, Gary Taubes writes ...
"So whatever dietary factors or lifestyle factors lead to Type 2 diabetes will always increase the likelihood of manifesting dementia." (page 207)
Alzheimer's is characterized by amyloid plaques in the brain that contains advanced glycation end-products (AGEs). AGEs are haphazardly linked sugar and proteins that are the result of a glycation process. Diabetics have excess AGEs found in various organs and tissues.
The insulin-degrading enzyme (IDE) is supposed to degrade and clear insulin and amyloid proteins from the brain. However, when there is excess insulin, the IDE focuses on clearing the insulin and thereby leaving more of the amyloid around which then clumps into plaques in the brain -- that's the theory at least.
AGEs and recative oxygen species
High blood glucose level (which can result from too much high-glycemic carbohydrates) causes a glycation process that results in greater number of advanced glycation end-products (AGEs). This is when sugar molecule stick haphazardly to protein molecules, which in turn clump together with other similar molecules. This process is known as cross-linking which has the end result of making your arteries, joints, and skin less flexible. Although this process happens all the time, but when sugar level is low, the sugar molecule disengages and no major clumping occurs.
Another process that happens all the time is oxidative stress. This is a natural by-product of metabolism, such as the burning of glucose. Reactive oxygen species is produced. These are reactive free radical molecules that oxidizes other molecules in its path. Anti-oxidants help neutralize these free radicals and is why antioxidants are known to be beneficial.
Low Carb Diets
Because of the problems with carbohydrates. Many authorities believe in the low-carb diet. Some of which includes Dr. Bernstein (author of Diabetes Solution), Gary Taubes (author of Good Calories, Bad Calories), and Dr. Andreas Eenfeldt of Sweden, and many others.
However, many other experts argue that one should not cut out carbs completely. These experts includes Dr. Dean Ornish, Dr. Neal Barnard, Dr. T. Colin Campbell, Dr. Caldwell B. Esselstyn. You still need nutrients from fruits and vegetables to maintain a healthy diet even though they are complex carbohydrates.
Note that not all studies indicate a conclusive result. There often may very well be conflicting studies that reach different conclusions.
Based on my readings, I take a moderate approach. I try to avoid sugar. I limit refined and processed carbohydrates such as bread, pasta, white rice, and pastries. But I eat a lot of vegetables and fruits.
Article was updated February 2012 and is only opinion at the time of writing.
-  How High Carbohydrate Foods Can Raise Risk For Heart Problems
Doctors have known for decades that too much carbohydrate-laden foods like white bread and corn flakes can be detrimental to cardiac health. In a landmark study, new research now shows exactly how these high-carb foods increase the risk for heart pro
-  Big Fat Lies with Gary Taubes on Google Video
Gary Taubes, an award-winning writer and author of "Good Calories, Bad Calories" says that it is carbohydrates that drive insulin levels which drive fat deposits.
-  Good Calories, Bad Calories