YES! And it's also a very addicting DRUG!
Take a look at the definitions. A toxin is a chemical produced by living organisms that causes harmful effects on the body at high enough concentration. Sugar easily meets that definition. It is plant derived and there are numerous studies that show a variety of harmful health effects from excess dietary sugar. Below are the definitions from Wikipedia.
A poisonous substance produced by living cells or organisms.
In the context of biology, poisons are substances that can cause damage, illness, or death to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism.
A drug, broadly speaking, is any substance that alters normal bodily function. Recreational drugs are chemical substances that affect the central nervous system, such as narcotics or hallucinogens. They may be used for perceived beneficial effects on perception, consciousness, personality, and behavior. Some recreational drugs can cause addiction and habituation.
OK, is sugar a drug? If you've ever had a feel-good high immediately after eating sweet "comfort" foods, it's a psycho-active drug. If you've ever had a "craving" for something sweet, it's an addicting psycho-active drug.
So, my point is that we need to start thinking of refined sugar as a toxin and drug. That makes sugar by far the most common and abundant toxin and drug in the modern food world!
The body needs small amounts of sugar to function properly. But the body can make all the sugar it needs. There is no minimum daily intake requirement for sugar or even for carbohydrates. There are many nutrients that in small amounts are necessary for good health, but in excess are detrimental. Zinc,copper, iron, iodine, and vitamins A and D are examples. Likewise, sugar in small amounts is necessary for health, but in excess causes problems.
When natural sugars are ingested unrefined, as in fruits or dairy, the amounts are generally low enough not to cause problems. However, when the sugar is refined and concentrated as a food additive, it is easy to get too much. Because of it's addicting and feel-good qualities, sugar is often added in large quantities to processed/manufactured foods to make them sell better. So, if you eat a lot of processed/manufactured or restaurant foods, it is very easy to get too much sugar. People eating a lot of these foods commonly get as much as 20 to 30 percent or more of their calories from sugars. What's worse is that most of that sugar is now in the form of high fructose corn syrup, which is even more harmful than table sugar. Most healthy primitive diets had only small amounts of sugar, mainly from raw dairy or from fruit when in season.
Too much dietary sugar certainly won't kill you right away, but it does have short-term harmful effects, like suppression of the immune system, even with relatively small doses. That leaves you more vulnerable to infectious diseases and cancer. One study showed that ingestion of 100 grams of sugar caused about a 40% reduction in white blood cell activity against pathogens and found that it took about five hours for immune function to return to normal.
Over many years, too much sugar can lead to metabolic syndrome diseases, including obesity, type 2 diabetes, heart disease, and cancer. These are the diseases of aging and that's exactly what too much sugar does - speeds up the aging process.
So, how much sugar should we allow in our diet? Probably no more than about 5% to 10% of total daily calories and the less, the better. That means if your normal dietary intake is 2,000 calories per day, you should not get more than 200 calories from sugar. Since sugar has 4 calories per gram, that means no more than about 50 grams (2 ounces) of sugar per day on a 2,000 calorie diet. And that's total sugar from all sources. Here's the amount of sugar you get from single servings of some common "comfort" foods, listed in grams, to the left of the food name (from NutritionData).
Grams of sugar:
96 cake (1/8 slice 9 inch white with coconut icing)
57 candy (4 oz Snickers)
56 milk shake (11 oz vanilla)
39 soft drink (12 oz cola)
39 yogurt (8 oz low fat strawberry Breyers)
37 coffee (12 oz caramel mocha Starbucks)
34 apple juice (12 oz Starbucks)
32 pie (1/8 slice 8 inch pecan)
30 ice cream (1 cup chocolate)
29 orange juice (12 oz McDonald's)
28 muffin (101 g blueberry Starbucks)
24 donut (5 inch)
21 brownie (56 g large chocolate)
18 granola (2/3 cup low fat fruit Nature Valley)
15 cereal (1 cup frosted flakes Kellogg)
07 hamburger (105 g McDonald's)
For comparison - grams of sugar:
19 apple (3 inch with skin)
15 peach (2 3/4 inch)
12 orange (2 7/8 inch navel)
12 grapes (15 red or green seedless)
07 strawberries (1 cup whole)
Breaking the Sugar Addiction
To break your sugar addiction, eliminate foods with added refined sugar and reduce intake of other refined carbohydrates. Increase intake of foods with good quality animal or dairy fat, preferably from pastured animals, and add probiotic foods. Also, try coconut oil. And be sure to eat a variety of foods to get good nutrition. If you have Excel or Open Office, try out my dietary nutrition calculator. Dropping sugar may seem difficult the first few weeks, but after a few months of abstinence from refined sugar and with intake of more good fat, probiotics, and nutrients, you should lose any cravings for sweets. If you still have difficulty, another tool to try is the Emotional Freedom Technique (EFT), a type of accupressure that employs tapping instead of the needles used in accupuncture. Once you have broken your addiction, you can still allow occasional sugary foods, but not very often or you run the risk of becoming addicted once again. You may also find that sugary foods that used to be "treats" no longer taste as good - they're too sweet!
If a food does not taste good without adding sugar to it, then either learn to like it without the sugar, or find other foods that you like that don't have added sugar. You may find as you lose your sugar addiction that many foods that formerly did not taste sweet enough now taste good. Sour and tart foods may become more appealing. You may even develop a new appreciation for all of the wonderful flavors in nature!
Update 2008 December 20
Fructose appears to be the worst dietary sugar for causing long-term health problems from excess consumption. Fructose is 50% of table sugar (sucrose) and typically 55% of high fructose corn syrup (HFCS). Read this interesting editorial from the American Journal of Clinical Nutrition: How safe is fructose for persons with or without diabetes?
Update 2014 January 5
A more recent discussion of health problems caused by too much dietary fructose:
Clinical Scientist Sets the Record Straight on Hazards of Sugar