The cited reason for the soda ban is to fight obesity. Which seems perfectly logical, since soda is a nutritionally bankrupt substance that certainly can't be helping people keep off the pounds.
But is there a scientific basis for what Bloomberg is proposing? The answer is yes and no.
|Cola with ice and lemon, by pic_p_ter|
Yes, there have been gazillions of scientific studies over the years on soda and obesity. And the results are basically what you would expect. One 2006 review of studies by the Harvard School of Public Health, for example, found that greater consumption of SSBs - particularly carbonated soda - was associated with weight gain and obesity. This was the case for both for kids and adults.
It's commonly believed that "empty calories" are the reason for the association between pop and obesity. Soda has a lot of sugar without the nutritional bulk (e.g. soluble fibre) that makes you feel full, so your body goes ahead and ignores the soda calories, making you want to eat more calories in other foods in order to feel satiated.
But on the other hand, there is no good scientific argument for why diet sodas would be exempt from the ban. The science shows quite clearly that a greater consumption of diet sodas, which contain Non-Nutritive Sweeteners, or NNSs, such as aspartame, are also associated with weight gain and obesity. This is the so-called sweetener paradox.
Now, the really intriguing question here is why soda is associated with packing on the pounds, no matter whether it contains many calories or few calories. Probably, soda contributes to the condition of obesity in some causal way. But calories can't be the entire story.
There are hints that the soda-obesity connection might actually have to do with the gut bacteria. These hints are gathered piecemeal from a variety of different studies, but the picture that is emerging has to do with how soda's ingredients affect the intestinal flora, and thus predispose a soda drinker to weight gain.
In regular soda, the culprit messing with the intestinal flora may be the sweetener fructose (often in the form of High Fructose Corn Syrup, or HFCS). An intriguing 2012 paper published by Swiss scientists in the journal Obesity Reviews suggested that fructose may affect gut bacteria in a way that alters the body's metabolic capacity. The sweet stuff that is so common in the Western diet may disrupt the normal relationship between humans and their microbes, perhaps by creating a gut with fewer bacterial species. Or maybe even by altering gene expression.
As for diet soda, it may be that Non-Nutritive Sweeteners (NNSs) are affecting the gut bacteria in some negative way. A recent review article by Pepino and Bourne at Washington University in St. Louis proposed several theories on how consuming NNSs might predispose a person to obesity. One idea was that NNSs may change the intestinal environment in a way that triggers inflammatory processes, once again disrupting normal metabolism.
While these proposals are preliminary, it seems within the realm of possibility that soda, both diet and regular, could affect the gut bacteria and therefore change metabolism in a way that makes it easy to put on weight. It's an intriguing avenue for further research.
The stakes are high, to say the least. A recent article in PLoS Medicine compared the soda industry to the tobacco industry: specifically, the fact that corporations (worryingly) try to boost sales of these harmful products through corporate social responsibility campaigns. In the case of the soda industry, the campaigns are often specifically designed to make young people buy more of it.
Malik VS, Schulze MB, & Hu FB (2006). Intake of sugar-sweetened beverages and weight gain: a systematic review. The American journal of clinical nutrition, 84 (2), 274-88 PMID: 16895873 Payne,, Chassard,, & Lacroix (2012). Gut microbial adaptation to dietary consumption of fructose, artificial sweeteners and sugar alcohols Obesity Reviews DOI: 10.1111/j.1467-789X.2012.01009.x Pepino, & Bourne (2011). Non-nutritive sweeteners, energy balance, and glucose homeostasis Current opinion in clinical nutrition and metabolic care DOI: 10.1097/MCO.0b013e3283468e7e
UPDATE, Summer 2012:
Read here about a hot-off-the-press study in Obesity Reviews that did link diet soda to changes in gut bacteria. Gut inflammation, resulting from artificial sweeteners, fructose, and sugar alcohols, is said to be the process affecting metabolism and satiety.
FURTHER UPDATE, September 2012:
Beware misleading reporting about this topic. Jonathan Eisen, prof at UC Davis, has awarded an "Overselling the Microbiome Award" to this article about the effect of sugary drinks on gut bacteria in MedicalDaily. It seems the article fell down on that old science tripwire: confusing correlation with causation. The study did find that soda was linked to changes in gut bacteria, and there's a process that could hypothetically account for this, but more research is needed before saying something as bold as, "Sugary drinks help bad microbes grow in the human gut."