It's Pretty Bad ^[1].

Think of it this way. There's 3500 cals in a pound of fat ^[2]. Which means, to burn that, in a basic sense, you should have your calories out be that many more than your calories in.

If you cut one can per day, that's 1120 cals per week. A significant amount of the cals you want to burn per week, to lose a pound.

[1] http://blisstree.com/feel/what-happens-to-your-body-if-you-drink-a-coke-right-now/
[2] http://www.weightlossforall.com/calories-per-pound.htm

It depends on how much you have in a day. I've heard arguments that fructose is particularly bad for you since it makes you less full. Sugar: The bitter truth ^[1] I'm not sure how accurate that talk is but it seems to make sense to me. People seem to drink soda all day and still have more even though they have effectively had more then a full days worth of calories

I've switched to iced green tea since it's still sugary, it does not have HFCS, they say grean tea is good for you, and I can make it at work. I've ended up having far less sugar since I make the iced tea a little light and I've saved a good deal of money since I just buy a big container of ice tea mix and stick it in my desk. As to how much good it did my health I have no idea.

[1] http://www.youtube.com/watch?v=dBnniua6-oM

Let me count that ways!

Regular Soda

Cavities - The soda has a low PH and makes the mouth acidic, and the sugar content promotes tooth decay when it comes into contact with bacteria in the mouth ^[1].

Diabetes - Over 130,000 cases of diabetes now linked to soda consumption, HFCS ^[2] (High Fructose Corn Syrup)

Obesity - Not only does soda contain a high concentration of sugar, and thus calories, but it doesn't satiate you (aka, you're stay hungry) and one study found that it actually INCREASES your appetite ^[3].

The sugar in soda pop not only provides a massive dose of calories, but triggers a vicious appetite cycle, said Ludwig, who wrote "Ending the Food Fight," about healthy eating for children.

"It's rapidly absorbed, which raises blood sugar and in effect causes the body to panic." The body releases insulin to break down the sugar, "but the body overcompensates, and blood sugar drops below the fasting level," lower than it was in the first place.

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Diet Soda

Artificial Sweeteners - Aspartame, Splenda, and Saccharin. These dangers of using artificial sweeteners could be handled its own question! Until we've created that question here, I'll link to The Hidden Dangers of Aspartame ^[4].

Obesity - Diet sodas have been shown to increase obesity as well. From WebMd ^[5]

For regular soft-drink drinkers, the risk of becoming overweight or obese was:

26% for up to 1/2 can each day
30.4% for 1/2 to one can each day
32.8% for 1 to 2 cans each day
47.2% for more than 2 cans each day.

For diet soft-drink drinkers, the risk of becoming overweight or obese was:

36.5% for up to 1/2 can each day
37.5% for 1/2 to one can each day
54.5% for 1 to 2 cans each day
57.1% for more than 2 cans each day.

For each can of diet soft drink consumed each day, a person's risk of obesity went up 41%.

Metabolic syndrome - Metabolic syndrome is a cluster of conditions that occur together, increasing your risk of heart disease, stroke and diabetes. ^[6]

This most likely coincides with a study which found there is a link between diet soda and the risks of a stroke or heart attack ^[7].

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Controversial

Causes Bone Loss - This was theorized out of the idea that too much phosphorous could deplete calcium and since many sodas use phosphoric acid, this could lead to bone loss. However this may not be the case anymore, see Soda may not hurt bones ^[8].

Indeed one could make the case that if you are drinking more soda, you are most likely drinking less of other things, like milk!

[1] http://www.dentalcomfortzone.com/template.php?aid=247
[2] http://www.naturalnews.com/028340_diabetes_sodas.html#ixzz1M4qWlCES
[3] http://articles.cnn.com/2007-09-18/health/kd.liquid.calories_1_nondiet-drinks-hfcs-soda/2?_s=PM%3aHEALTH
[4] http://maintainwealth.blogspot.com/2008/08/aspartames-hidden-dangers.html
[5] http://www.webmd.com/diet/news/20050613/drink-more-diet-soda-gain-more-weight
[6] http://getfitslowly.com/2008/02/12/the-dangers-of-diet-soda/
[7] http://www.aolhealth.com/2011/02/09/diet-soda-stroke-risk/
[8] http://preventdisease.com/news/articles/soda_may_not_hurt_bones.shtml

It may not have calories in it, but it has many chemicals that are toxic to your system. The secondary effects of those chemicals can in fact encourage weight gaining.

As an example, your body has certain reactions to something sweet. It is expecting something with sugar and responds in preparation for this. When you then don't give it that sugar you will start craving something sweet which will usually result in you eating more junk than you would have without the diet soda.

As a side note, many of the ingredients in diet sodas in the USA are banned from food in Europe due to evidence that they are toxic. Keep to water if you can, and once you get used to it you will wonder why you ever put those chemicals in your body.

How much soda do you drink is a big question here.

A 14fl oz bottle of coke has about 46g of sugar. A teaspoon is about 4grams. Do the math :)

Soda has no nutritional value, so regardless of your weight, yes you should cut it out of your diet completely. It's really no different than drinking liquid candy. The huge amount of sugar also taxes your pancreas because it has to pump out more insulin just to handle the blood sugar spike. Soda is unhealthy regardless of your weight.