Rumors about sugar substitutes never seem to die. Aspartame causes brain tumors in rats, Stevia is a miracle food suppressed by sugar lobbyists in Washington. Here, we separate fact from fiction and give you just the sweet truth.

Saccharin
What is it
Product of reaction between sulfur dioxide, chlorine, ammonia, and two biochemical acids. Found in toothpastes like Crest and Colgate, and many food products.
Calories
Officially 0
Flavor Profile
Minor aftertaste; 300 to 500 times sweeter than sugar
Side effects
None known. Over 30 human studies, over several decades, have found no significant side effects.
Notes:
Decades ago, one study linked saccharin to bladder cancer in rats (though never in humans). The FDA has since removed all warnings regarding saccharin because the evidence was deemed unacceptable, and in fact, today saccharin is considered to have the lowest risk profile of all available non-nutritive sweeteners, according to many leading authorities (e.g., Josh Leichtberg, MD here). In addition, saccharin is one of the oldest non-nutritive sweeteners on the market today, which means any true risks would have shown up by now. In our opinion, saccharin is the safest non-nutritive sweetener commercially available today.

Aspartame
Aspartame (NutraSweet and Equal)
What it is:
A combination of two amino acids; aspartic acid and phenylalanine. Found in Diet Coke, Diet Pepsi, and many other diet sodas, and foods.
Calories:
0
Flavor Profile:
Distinctly chemical; 180 times sweeter than sugar
Side effects:
If you have a rare genetic condition called "phenylketonuria", this product can kill you. Various reports have linked aspartame to a long list of health problems, including: headaches, migraine headaches, neurological abnormalities, seizures, cancer, anxiety, asthma and other allergic reactions, fatigue, dizziness, hair loss, impotence, nausea, tachycardia, and more.
Biochemically, aspartame has been shown to readily convert to formaldehyde once it has entered the body. Formaldehyde in turn has been shown to cause gradual but ultimately severe damage to the neurological and immune systems; it also causes permanent genetic damage at extremely low doses. After consumption of Nutrasweet, formaldehyde can be measured in the liver, kidneys, brain, and other tissues.
Notes:
This product is relatively new, which means its true effects on the human body are largely unknown. Given the risk profile, we recommend strict avoidance (no more than occasional very low amounts).

Sucralose
What is it:
Sugar molecules blended with chlorine; found in many foods and drinks. It is a synthetic chemical, NOT a natural product, as the advertising may imply.
Calories: 0
Flavor: Slightly chemical; 600 times sweeter than sugar
Side Effects: May cause diarrhea, bloating, abdominal distress. Studies have indicated that sucralose can cause shrunken thymus glands, as well as enlarged livers and kidneys.
Notes:
This product is very new on the market, which means its true effects on the human body are largely unknown. We recommend strict avoidance (no more than occasional very low amounts).

Sugar Alcohols
What it is:
Sugar molecules with added hydrogen, found in many "meal-replacement" and diet bars and mixes, and many other foods.
Calories:
Approximately ¾ the calories of sugar, but about 1/2 of those calories will either never be absorbed, or will be over a prolonged period of time (thus diminishing the "glycemic" effect).
Flavor:
None; same sweetness as sugar
Side effects:
Bloating, gas, diarrhea. Sugar alcohols are classified as polyols, a slowly absorbing carbohydrate. These are known to cause flatulence, bloating, diarrhea and cramping in many individuals, due to a common deficiency in the enzymes required to break these substances down. Sugar alcohols can be used in candy and baking in small amounts.
Notes:
Xylitol is a type of sugar alcohol that appears to have a safer risk profile than other sugar alcohols.

Stevia
What is it:
Dried leaves of the Stevia plant. Stevia is a plant that originated in the rainforests of Paraguay. It is up to 300 times sweeter than sugar, does not impact blood sugar and has zero calories. The leaves have been used for over 1,500 years by the Guarini Indians of Paraguay. Stevia has not yet been approved by the FDA for use as a sweetener in food products -- which simply means that manufacturers cannot claim that it is a sweetener, even though if is.
Calories:
0
Flavor:
150-400 times sweeter than sugar. Stevia tastes a bit fruity, and thus is perfect for some uses (lemonade, cooking), not so good for others (coffee).
Side Effects:
None known. Official clinical trials have not yet been conducted.
Notes:
Stevia has since become a very popular "sweetener" because it is essentially a natural herb, with a long history of safety in actual use. Since the FDA has not yet approved it as a sweetener - it remains classified today as a "dietary supplement".
We like this herb a lot. It's appears to be very safe, natural, and has zero calories. When the taste works in food, it is preferred above all other non-nutritive sweeteners (with saccharin a close second).