Unclear causal relations between sugar, obesity, and dental decay…
A recent article published in PLoS One (a fairly good open access journal) has a closer look at the potential relation between “Obesity and Dental Decay” with a focus on the role of common sugar. Looking at more than 8000 around 11 year old kids, the authors surprisingly found that higher obesity correlates with better dental conditions. Based on their results the authors question that high sugar consumption is the main cause for obesity (see 1s figure). The study seems to suggest that other factors might be essential to cause obesity (see 2nd figure).
Many sugars (most prominently sucrose, our conventional table sugar) have some very nice properties. First of all, they taste good and are able to add a desirable sweetness to many different products. They also are commonly used to preserve food, for example in jam or sugar confectionery. On the other side, sugar nowadays seems to be far away from is glorious past as sought after luxury goods1. Today, sugar is not only a cheap mass product, it also has an ambiguous reputation and high sugar intake is often associated with widespread health problems linked to changed diets and current eating habits. At this point I should say that if I say ‘sugar’ I generally mean the most typical sugars which are monosaccharides and disaccharides (this includes glucose, fructose, and sucrose).
It is widely accepted that sugar has negative effects on your teeth and promotes what scientist would call ‘dental decay’ (Touger-Decker, 2003; Hayden, 2013). In addition sugar has many calories although it is not considered to be particularly nutritious, so some people speak of ‘empty calories’. It is hence rather intuitive to suspect that high sugar consumption could lead to increased weight, in other words to cause obesity. Studies indeed indicate that eating a lot of sugar increases to risk for obesity (Te Morenga, 2012). OK, so sugar is bad for your teeth and it makes you gain (too much) weight. Now you would naturally expect that obese people are people that eat a lot of sugar and thus must in average have worse teeth!
A study published this October in PLoS One by Goodson and others, however, reports that dental conditions were better for obese people when compared to normal or underweight people. The researchers looked at more than 8000 children of around 11 years old and found that the percentage of decayed or filled teeth went down the higher the body mass index (BMI). While nearly 16% of the underweight children had decayed or filled teeth, only less than 8% of the obese kids were showing similar dental conditions.
There are two major aspects to look at when it comes to judging a typical scientific study or experiment. First: Is the shown data or the reported observation solid? Imagine you ask two obese and two underweight people what their favorite color is. The two obese say blue, the underweight both say green. This data is not worth anything, mostly because asking only two people is not representative. In addition, it is very questionable if the question itself actually makes sense unless you have a good reason to assume a causal link between favorite color and obesity. In the presented paper by Goodson and others, more than 8000 children were tested. Further, looking for potential relations between obesity and dental decay makes sense, in particular due to a suspected link via sugar consumption. That means, yes, the observations in the study at first sight seem to be entirely OK.
Second: Does the presented data allow the conclusions made by the scientists?
In my everyday science experience I am regularly surprised that this second part is quite often a very weak point of many scientific publications. In many cases, authors tend towards too spectacular or broad conclusions that are often only indicated but NOT shown by the data presented. The same is maybe true to some extend in the paper we are talking about here.
What the authors DO show is that it cannot be that sugar is the only major cause for dental decay AND the only major cause for obesity2. Because in that case, obese people would show more tooth decay3 . Since the influence of sugar on dental decay is relatively well documented and studied, the authors doubt that sugar is a main driving force for obesity. Sounds good, right? Sugar doesn’t make you fat, or what?
I think that the authors make a too strong conclusion when they say that their finding “contradicts the obesity-sugar and the obesity-dental decay relationship hypotheses”. Their data does not allow ruling out that high sugar intake will increase the obesity risk, as it suggested by many other studies (including the metastudy by Te Morenga and colleagues).
The study by Goodson et al.(2013) indicates that other factors than sugar consumption are key causes for dental decay.
But the new data indeed indicates that sugar consumption might not be the most dominant factor leading to obesity4. In that case it becomes possible that obesity itself, or factors other than sugar intake that lead to obesity, correlate with dental protection that counteracts the negative effects of sugar on teeth. This would be in agreement with the observations made by Goodson et al. (see the second diagram I made).
Or what if obese people simply like to brush their teeth more often?
That might sound ridiculous, but it shows that human biology and behavior are incredibly complex, and simple explanations are seldom found.
- Goodson JM, Tavares M, Wang X, Niederman R, Cugini M, et al. (2013) “Obesity and Dental Decay: Inference on the Role of Dietary Sugar” PLoS ONE 8(10): e74461. doi:10.1371/journal.pone.0074461
- Lisa Te Morenga, Simonette Mallard, Jim Mann (2012) “Dietary sugars and body weight: systematic review and meta-analyses of randomised controlled trials and cohort studies” BMJ 2013;346:e7492
- Riva Touger-Decker and Cor van Loveren (2003) “Sugars and dental caries” American Journal of Clinical Nutrition; 78(suppl):881S–92S
- Hayden C, Bowler JO, Chambers S, Freeman R, Humphris G, Richards D, Cecil JE (2013) “Obesity and dental caries in children: a systematic review and meta-analysis.” Community Dentistry and Oral Epidemiology 2013; 41: 289–308.
- Harold McGee, “On Food and Cooking: the science and lore of the kitchen”, 2nd edition, 2004, ISBN 0-684-80001-2.
- Ferris Jabr (2013) “Is Sugar Really Toxic? Sifting through the Evidence“ Scientific American Blog
- W.D. Edwards, “The science of sugar confectionery”, RSC, 2000
- That is way back when sugar had not yet become an extremely cheap mass product. See also Harold McGee’s book on “On Science and Cooking”. [↩]
- Hence sugar can still be ‘evil’, but at least not the only ultimate evil that some claim it is. [↩]
- Speaking in terms of formal logic it means that the two simple premises “IF x eats a lot of sugar, x becomes obese” and “IF x eats a lot of sugar, x’s teeth will decay more” cannot both be valid in light of the reported observation on dental decay. [↩]
- So either sugar does not lead to obesity, or more realistically sugar intake is only a relatively mild cause for obesity when compared to other factors. That would still be in agreement with many studies, as for example the meta study by Te Morenga et al. (2012) which finds a clear but moderate effect of sugar consumption on obesity. [↩]