Could ‘diet’ drinks actually promote weight gain?

Diet (artificially-sweetened) drinks are sold on the basis that, compared to sugary varieties, they are better for our health and, in particular, our weight. However, there is at least some evidence that artificially sweetened drinks do not offer the health boons they promise.

This week, for instance, saw the publication of a study which assessed the relationship between the intake of both diet and regular (sugary) soft drinks and risk of the condition metabolic syndrome (characterised by factors such as excess weight around the middle, raised blood sugar and triglyceride levels and raised blood pressure) [1]. The analysis involved data obtained from nearly 9000 American men and women.

What the study showed is that compared to individuals drinking one or less soft drink a day, those drinking one or more drink of sugary soft drink a day were 81 per cent more likely to suffer from metabolic syndrome. What might come as a surprise to some is that when looking at diet soft drink consumption, risk of metabolic syndrome was also up ” this time by 80 per cent (almost the same as the sugary soft drink consumers).

An ‘epidemiological’ study of this nature cannot be used to prove that the consumption of diet drinks (or sugar soft drinks for that matter) actually cause metabolic syndrome. It is of course possible that relatively unhealthy individuals also tend to consume more in the way of soft drinks, diet or otherwise. Against this, though, is the fact that the researchers attempted to account for other factors that might influence the results including activity levels and intake of other foods.

Most individuals find it relatively easy to rationalise how it could be that glugging down sugary soft drinks might promote weight gain and metabolic syndrome. However, it’s harder to imagine how this might be for diet drinks. There has been, of course, no shortage of parties keen to diffuse any idea that diet drinks might promote weight gain. For instance, Susan K. Neely, president and chief executive officer of the American Beverage Association, is quoted as saying: The association found between diet soda and metabolic syndrome is particularly implausible. Diet soda is a beverage with zero calories, and it is 99 percent water. And Dean Ornish, clinical professor of medicine at the University of California, San Francisco and, ahem, chairman of the PepsiCo Health & Wellness Advisory Board is quoted as saying: “There is no plausible physiological mechanism to explain this and causes me to question the accuracy of the methodologies used in this study.”

But is it really so ‘implausible’ that diet drinks might promote weight gain and metabolic syndrome? There is some evidence, for instance, which shows that consuming artificial sweetener aspartame ingestion may actually lead to increased food intake [2,3]. Also, this week’s study points out, there is also some thought that the highly sweet nature of diet or regular soft drinks may lead to a greater preference for sweetened foods [4].

And here’s another thing: could it not be that artificial sweeteners can act to disrupt the normal metabolic function of the body, and therefore promote weight gain? For instance, one breakdown product of aspartame (Nutrasweet/Equal/Canderel) is formaldehyde, which is used to preserve dead bodies. Another of aspartame’s breakdown products is methanol ” another known poison. Could it be that these and perhaps other breakdown products of artificial sweeteners might ‘jam’ the metabolism a little?

The only real way to know whether artificially sweetened drinks are superior to regular sugared varieties in terms of weight would be to conduct what are known as double-blind placebo controlled studies. In such research, individuals are assigned to consume the substance being tested (e.g. aspartame) or something else (e.g. sugar). Neither the individuals in the study nor the investigators are allowed to know what they are having. After some time, the ‘code’ is broken, the scientists find out who was taking what, and the results of the two groups are compared.

Curiously, to date, not one single double-blind placebo-controlled study designed to assess the effects of artificial sweeteners on weight has been published.


1. Dhingra R, et al. Soft Drink Consumption and Risk of Developing Cardiometabolic Risk Factors and the Metabolic Syndrome in Middle-Aged Adults in the Community Circulation 2007: [Epub print 23 July 2007]

2. Lavin JH, et al. The Effect of Sucrose- and Aspartame-Sweetened Drinks on Energy Intake, Hunger and Food Choice of Female, Moderately Restrained Eaters Int J Obes. 1997 21:37-42

3. Tordoff MG, et al. Oral stimulation with aspartame increases hunger. Physiol Behav 1990 47:555″559

4. Davidson TL, et al. A Pavlovian approach to the problem of obesity. Int J Obes Relat Metab Disord. 2004;28:933″935.

6 Responses to Could ‘diet’ drinks actually promote weight gain?

  1. Claire Weeks 25 July 2007 at 10:08 am #

    OOps you left a key word out of one sentence – What might come as a surprise to some is that when looking at DIET soft drink consumption, risk of metabolic syndrome was also up ” this time by 80 per cent (almost the same as the sugary soft drink consumers).

    A plausible theory is that the body expects to receive a surge of sugar into the bloodstream when a sweet tast is experienced in the mouth and therefore prepares a boost of insulin to cope with it. The low-cal ‘diet’ drinks of course do not send the expected sugar, so anything else consumed at the same time is much more likely to be turned into fat by the excess insulin. And of course insulin surges are associated with metabolic syndrome. This is of course a simplistic sysnopsis but one well worth testing.

  2. ethyl d 25 July 2007 at 4:30 pm #

    Based on my own experience, I would guess that the reason why both drinkers of regular and diet sodas showed evidence of metabolic syndrome is that both groups are eating a high-carb and junk food diet in general; the drinkers of diet sodas just rationalize their choice by thinking the diet drink is saving them a few calories. This is what I was doing a few years ago when I switched from regular sodas to diet ones. I knew I needed to lose weight, but had not yet learned about low-carb eating, so was eating the “expert” recommended low fat-high carb diet and getting fatter and fatter. When I ate fast food, I rationalized ordering a diet soda by saying to myself that at least I was saving 150-200 calories by drinking the diet drink, despite my “value” meal with an enormous bun and enough french fries for several people. Now that I’ve given up fast food and eat low-carb real food, I still choose diet soda with Splenda, and am forty pounds lighter and feel so much better than I once would ever have thought possible. It’s not the diet sodas–although I accept that we might well be better off without them–it’s the rest of the poor food choices people make along with whatever they drink.

  3. Dr John Briffa 26 July 2007 at 12:02 pm #

    Thanks Claire – omission corrected!

  4. James G 27 July 2007 at 9:34 am #

    As ethyl d pointed out, doesn’t this really point to a correlation, rather than a causation?

    The way I read these results, people who drink soft drinks, whether diet or non-diet soft drinks, are more likely to show metabolic syndrome than people who don’t, with no significant difference between the two. Or did I miss something here?

    It seems that if you are just as likely in either circumstance, then there must be other factors affecting metabolic syndrome, which should be controlled for. Such as a high-carb and junk food diet, as ethyl d pointed out.

  5. Dr John Briffa 27 July 2007 at 9:42 am #

    Yes, the study showed correlation and cannot be used to ‘prove’ causation. The point is, though, that the concept that diet drinks may actually cause weight gain/metabolic syndrome is not as ‘implausible’ as some would have us believe.

  6. John Spottiswoode 31 July 2007 at 8:18 am #

    This is something that I have suspected for a long time. Could it be that the sugar substitutes fool the body into thinking that it has plenty of sugar and puts to fat what it can, robbing the person of energy whilst directly causing an increase in fatness?

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