Follow me on

Can Portion Sizes Really Drive Weight Gain?

out_to_lunchEven a casual visitor to North America will unlikely have missed the humongous portion sizes (enough to feed a small family) routinely served up at chain restaurants (and most other places).

Indeed, in any “folksy” discussion of what drives obesity, portion sizes are sure to be on the plate.

In addition, cutting portion sizes is one of the most common pieces of advise given to anyone trying to lose weight.

But how important are portion sizes in promoting weight gain?

This is where a new randomized controlled trial designed to test this hypothesis in free living individuals may provide some answers.

In this study, just published in OBESITY, Simone French and colleagues from the US and UK randomly assigned 233 volunteers (mean BMI ~30) to one of three lunch size groups (400 kcal, 800 kcal, and 1,600 kcal) or to a no-free lunch control group for 6 months.

The study was “disguised” as an intervention to test the “feasibility of providing box lunches to employees at a large metropolitan medical complex.

The free box-lunches were provided Monday to Friday. Fifteen different box lunch menus were created and menus were implemented on a 3-week repeating cycle. However, the conditions were on different cycles, so that participants assigned to different conditions could not directly compare their lunch box items on any given day. The same size box was used in all three conditions. Only water was served as a beverage.

As one may expect, people with the 800 and 1600 kcal lunches ate more that those with the 400 kcal lunches both at lunch and as total daily energy.

Interestingly however, while those on the 1600 kcal managed to gain about 2 pounds over the 6 months, there was no weight change in those eating fewer calories.

Incidentally, the individuals in the control group also gained about 2 pounds suggesting that the 1600 kcal was closer to what these folks were generally served as their “usual” meals.

Thus the authors rightly conclude that weekday exposure for 6 months to a 1,600 kcal lunch can cause significant increases in total energy intake and a weight gain.

On the other hand, the study also shows that simply limiting the size of lunch may not result in the significant weight loss that people may hope for.

I would interpret the overall findings as supporting the notion that our body’s homeostatic systems are much better in preventing weight loss than in defending against weight gain. Thus, while it takes a hefty 1600 kcal free lunch to drive weight gain, our bodies appear quite competent in regulating our body weights when served free lunches in the 400-800 kcal range.

On a side note, all box-lunch recipients increased their intake of fruits and vegetables compared to the control group – clearly showing (as noted in a recent previous post) that simply eating more fruits and veggies alone is not a viable recipe for weight loss (or even preventing weight gain).

So will efforts to reduce portion sizes (of weekday lunches) pay off in terms of preventing weight gain? Probably.

Will efforts to reduce portion sizes (of weekday lunches) help reduce obesity? Probably not.

Edmonton, AB

ResearchBlogging.orgFrench SA, Mitchell NR, Wolfson J, Harnack LJ, Jeffery RW, Gerlach AF, Blundell JE, & Pentel PR (2014). Portion size effects on weight gain in a free living setting. Obesity (Silver Spring, Md.) PMID: 24510841




  1. Your conclusions that reducing lunch portions probably won’t help seem hasty.

    First, there’s a lot of room between 800 and 1600 calories left “unstudied.”

    Second, that “hefty” 1600-calorie lunch isn’t so different from the lunch many adults eat: a deli-type sandwich, a large bag of chips, and a regular soda. In fact, the control group results support that observation.

    So if the study’s correct that this many additional calories indeed frustrate homeostatic regulation–and this is the portion size that’s most common–the results actually do suggest that working to reduce this to a lighter caloric load might well help.

    Post a Reply
    • @Marc – I agree that this is not of much use to dieters unless you read it as telling you that simply cutting portion size of one meal is probably not going to do much – most people will simply compensate at the other meals.

      @Leslie – Oh, I fully agree that 1600 kcal is hefty and probably closer to what some people eat than 400 kcal. Clearly, if your usual lunch is 1600 kcal, eating less will at least prevent weight gain.

      Post a Reply
  2. As described, this trial is of no help to a dieter.

    If it be possible to loose weight by reducing the calories of one meal 5 days a week, the problem of obesity would be very simple.

    The complexity of the problem makes this trial a perfect loss of time.

    Long term weight control, in my opinion, involves a management of a minimum of 25 nutritional and psychological considerations in order to loose weight. To manage the long term weight maintenance, add an extra 25 to 50.

    In order words, for a 300 pounds man, to loose 3 pounds a week at the begining of a weight program is very simple. (I stress it is not easy but it is simple)

    For the same person to loose half a pound a week, when the weight is down to 170, is probably easier than during the first 6 months, but it is almighty more complex.

    Post a Reply
  3. 2 pounds over 6 months? My weight may change just that much from one day to another depending on how many times I use the bathroom. Not to mention being on my period, then the fluctuation is even greater.

    For the record, my BMI is close to 30 and I have a 30-year weight loss/regain history, with long stable periods in-between. When I gain weight, the rate is closer to 2lbs per WEEK. 2lbs per 6 months do not alarm me much.

    Post a Reply
  4. For me at least there would also be a big Free(!) Lunch component to this.
    If I was expecting a big satisfying Free(!) Lunch at noon, then it would be easier to resist paying $3 for a Danish or muffin at 10AM. On the other hand — after having eaten an apple and plain bagel for lunch — the overpriced afternoon snack cart would look awful tempting.
    I was raised to appreciate bargains and to not waste food or other resources. Even in those years when I earned big dollars; most days I would bring a sandwich from home rather than spend $10 at the local restaurant.
    To me, being thrifty is deep-seated in my inner brain; as unconscious as my need to clear every scrap of food from my dinner plate.
    (I have noted that young workers seem to be quite opposite. Bringing lunch from home makes as much sense to them as stopping to pickup pennies from the sidewalk).

    Post a Reply
  5. I agree that this doesn’t tell us much about the utility of cutting portion sizes with the goal of weight loss. These participants didn’t know they were in an obesity study and certainly didn’t enter the study with the intention of losing weight. They also we’re not taught any other behavioral skills or experienced any other environmental changes that would reduce their likelihood of not compensating for the decreased calories at other times. In combination with other skills and changes, decreased caloric intake per meal by cutting portion sizes does result in weight loss, as demonstrated in obesity treatment trials. Perhaps as a public health approach though, small portion sizes prevent weight gain and then in combination with other interventions we can nudge people to weight loss.

    Post a Reply

Submit a Comment

Your email address will not be published. Required fields are marked *