How Much Does Sugar Contribute to Obesity?

Last week, the British Medical Journal published a review article titled "Dietary Sugars and Body Weight", concluding that "free sugars" and sugar-sweetened beverages contribute to weight gain.  But what are "free sugars", and why does the scientific literature suggest that the relationship between sugar intake and body weight isn't as straightforward as it may initially appear?

In a new review paper (meta-analysis), Lisa Te Morenga and colleagues review the studies evaluating the link between certain types of sugar intake and body weight in adults and children.  These studies include both observational studies and randomized controlled trials.  They conclude that the intake of "free sugars" and sugar-sweetened beverages are linked with higher body weight in both groups (1).

This conclusion appears sound and I have no quibbles with it.  But what are "free sugars"?  And why does this conclusion seem to be at odds with an older literature suggesting that people who eat more sugar tend to be leaner?

What are "free sugars"?

Here is the definition of "free sugars" they provide in table 1 of the paper, which is a standard definition used by the World Health Organization and the Food and Agriculture Organization:

All monosaccharides and disaccharides added to foods by the manufacturer, cook, or consumer; sugars naturally present in honey, syrups, and fruit juices.

So the term  refers to added sugars and fruit juices, but excludes the sugar that occurs naturally in fruit.  Importantly, it doesn't refer to total sugar intake, but rather to a major component of total sugar intake.

In plain language then, what the authors found is that added sugar and sweet beverage consumption are associated with a higher body weight in observational studies.  In controlled trials, these sugars increased body weight when calorie intake wasn't held constant, and had no effect on body weight when calorie intake was held constant.  For me, this conclusion is consistent both with the scientific literature I've read, and with common sense.

They do state in the paper that their result applies to specific types of sugar intake, rather than total sugar intake, but at certain points it sounds as if they're referring to total sugar intake.  For example, the title of the paper doesn't specify that the paper is specifically about added sugars and sweet beverages.  This could easily lead to misunderstandings about what they actually found.

The relationship between sugar intake and body weight is more complex than you may realize

When I first skimmed through the paper, I thought it was about total sugar intake, and I was surprised to see that they found an association between sugar intake and a higher body weight.  Why?  Because most of the observational studies that have examined the association between total sugar intake and body weight have found that people who eat more total sugar weigh less.  And the remaining studies found no association.  There is virtually no observational evidence that people who eat more total sugar weigh more than people who eat less, or gain more weight over time.

The hypothesis that sugar intake could be linked to weight gain is a pretty obvious one, and it's been around for a long time.  Consequently, many observational studies have evaluated it, beginning in the 1970s.  James Hill reviewed these studies back in 1995 (2), concluding:

Carbohydrates, particularly refined sugars, are still widely assumed to be fattening. However, there is now a substantial body of epidemiologic evidence refuting this view...  Almost all of the above studies support the contention that a high-carbohydrate, high-sugars diet is associated with lower body weight and that this association is by no means trivial.

Whether or not you agree with this hypothesis, it's still pretty interesting to note the marked difference between this conclusion and the result recently published by Te Morenga and colleagues, particularly since many of the same studies were available to be included in both review papers.

A major difference between the two review studies is that Hill was concerned with total sugar intake, while Te Morega was concerned with added sugars and sweet beverages specifically.  Today, few diet-health observational studies focus on total sugar intake as an outcome; usually the outcomes are focused on sugar-sweetened beverages, fruit intake, or other subsets of sugar intake.  Maybe people got tired of seeing that total sugar intake is associated with leanness?  Maybe the finding isn't novel enough anymore?  Or maybe it just makes the sugar-health story a little too complicated?  I find it a bit strange, personally.

These are observational studies we're talking about here, so we do have to be cautious about interpreting them.  Who knows how well self-reported sugar intake, and components of sugar intake, actually correspond to real intake.  And who knows to what extent differences in body weight are caused by differences in sugar intake, rather than other things that are associated with sugar intake.

Still, I think when we look at all of the data together, including the two papers I discussed in this post, a fairly logical and consistent picture emerges: added sugars and sweet beverages tend to be fattening because they lead us to eat too many calories, but whole fresh fruit isn't fattening and probably actually tends to be slimming.  When it comes to body weight, it's not so much the sugar itself, but the way in which it's packaged.

This is the same conclusion I reached in my 2012 post, Is Sugar Fattening?