Friday, May 16, 2014

How accurate are gizmos for counting calories and predicting weight loss?

I'm going to keep checking on this as I go, month by month, but my impression is, they are wildly inaccurate. So far, I have been tracking my daily caloric intake on Mynetdiary (though I don't always include my exercise for the day, so if anything, they will be under counting how much of a caloric deficit I have). And I track my activity and enter my caloric intake on bodymedia fit for the gizmo, bodymedia fit core.

I don't have records for a full month yet as I was only sporadically counting calories at the end of the day in mid-April and didn't start wearing my gizmo full time until the end of April as I needed to larger armband, but they do figure in averages of what they "think" I would burn in a day (bodymedia), or exclude those days for figuring (mynetdiary).

What both come up with is that I'm burning "around" 1200-1250 extra calories a day more than I am actually consuming.

Here is the info from Mynetdiary which does weekly reports (as always click on the picture/chart to make larger):

I don't have any data for the first week, but let's "say" it's a 1200 average daily deficit, which is about right as I was on target every day - even the day I went to the baby shower and had a piece of cake, I ate within the normal calorie limit for the day. So, week one is 1200, week two is 1179, week three is 1285 and week four is 1255. that equals a four week average daily caloric deficit of 1230.

Now let's look at bodymedia fit's info for the last 21 days:

This information has all my sleep, step count and entered calories eaten for most of the 3 week period. I didn't wear the gizmo the first few days as I was waiting for the larger arm band. So, for the first 5 days, it's giving me less activity time than I usually do, so again, if anything, it's under counting caloric deficit for that time period.  So, according to this report, I am burning an extra 1239 calories a day.

One calculator says 1230, another says 1239. That's pretty close. So, let's take the average of those two and that makes it 1235. What does that then translate into a prediction of weight lost during that time? Well, 1243 times 28 days equals 34,580 calories. Divide that by 3500 (which is the number of calories in a pound) and that gives you losing 9.88 pounds in 28 days, or 10.5 pounds in a month (30 days).

Of course, it's not that simple. Even notice how a pound equals 3500 calories is a bit too precise? Well, it isn't 3500 calories precisely, of course and it's difficult to find the 'real' numbers. There are several things that talk about it, but what you learn is that the 3500 is an estimation.  Here is this from

1lb does not equal 3,500 calories

One of the most commonly held diet myths is “To lose one pound of fat you need to create a deficit of 3,500 calories”. This is wrong at every level. First of all, one pound does not equal 3,500 calories. You will see this formula in government literature, in just about every diet book, in private health booklets and all over the internet. The next time you see it, or hear it, ask where it comes from. You will not get an answer. (I asked the following seven UK organisations: the National Health Service (NHS); the National Institute for Clinical Excellence (NICE); the Department of Health; the National Obesity Forum; the Association for the Study of Obesity; the British Dietetic Association and Dieticians in Obesity Management and five of these have no idea where it even comes from. The two that tried to prove it failed by a factor of about ten.)
The first part of the calorie formula is the assertion that one pound of fat contains 3,500 calories. You will struggle to find anyone who can demonstrate the precise calculation behind this, so I’ll offer this as a suggestion:
1)    One pound equals 454 grams (decimal places aside, this is a fact);
2)    Fat has nine calories per gram (this is the universally accepted conversion, but it is an estimate and significantly rounded down from even the original estimate);
3)    Human fat tissue is approximately 87% lipid (this is a widely accepted conversion, but it is also an estimate).
Putting these together, we can derive the sum that 454 grams of body fat tissue has approximately the calorific energy of 395 grams of pure fat (454 grams x 87%), that is 3,555 calories (395 grams x 9).
3,555 is close enough to 3,500 you may think, until you see the absurdity of how precisely the formula is applied. According to those who believe this formula, this difference of 55 calories (in this case from the calculation being approximate) would make five to six pounds difference a year. The National Obesity Forum web site states “one less (sic) 50 calorie plain biscuit per day could help you lose 5lbs (2.3kg) in a year – and one extra biscuit means you could gain that in a year!” (Ref 2) No it won’t. I can’t even get an estimate of the formula to closer than 55 calories ‘out’. Even if the 3,555 were correct (and it isn’t), this would mean we all need a 55 calorie biscuit, no fewer, every day or we will be five pounds lighter in a year anyway. Every person who didn’t have that biscuit every day should have lost 141 pounds over the past 25 years.
With little effort I can find evidence in obesity journals that fat has anywhere between 8.7 and 9.5 calories per gram. The same (1911) obesity journal that says that human fat tissue can be 87% lipid also says that it may be 72% lipid.
Taking the extremes of these, we can establish a range whereby one pound of fat could contain anywhere between 2,843 and 3,752 calories. Given that it is currently held that one pound is 3,500 calories we could (according to this formula) inadvertently gain six stone every year at the low end of the calculation and lose almost two stone in the same year if one pound is 3,752 calories. Don’t worry about any of this – because the formula doesn’t hold at any other level either.
But there is even more. Heavier people lose more weight and more quickly. I think everyone has either witnessed it or experienced it. Weight loss slows as you go along. And why is that? Well, there's lots of reasons and I wont detail them all. 
  1. Your metabolism slows as you go along and your body adapts to this new caloric deficit
  2. Some people get more lax with their plan as they progress as they get more satisfied with how they look and/or feel. Or perhaps even find it more difficult to stick to the plan with sustained lower calorie diets.
  3. You burn less calories as you get thinner because it takes less calories to move a thinner body.
  4. And more...

In my last foray with weight loss I lost about 8-9 pounds per month the first few months and then it dropped little by little. By the end, I was happy to see a 3 pound loss for the month and I was working WAY harder to see that 3 pound loss too. The gizmos do change their formula a bit as you get thinner and a bit when you get older, but not enough usually as they don't figure in how long you've been at the diet - so they don't figure in any metabolic slow down you might have experienced - which ISN'T huge I want to say. You do not slow it down by more than 10% and it will usually recover when you've been maintaining for awhile.

So, back to what these gizmos predicted for me to lose and that was with under counting my activity level for both the mynetdiay calculator and the bodymedia fit calculator. According to them I should have lost 10.5 pounds in the last month. What I actually lost was exactly 9 pounds. Now, I think that is really, really good, but what if I had actually added in all my activity? It probably would say I should have lost 11.5 pounds for the month. That's a 20% inaccuracy!

But even all this that I have laid out is still inaccurate. I didn't lose just fat, but also lean mass and water. I didn't burn precisely 3500 calories to lose each pound and each of my meals were not those exact calories either. My fuji apple could have been anywhere between 90-150 calories, that piece of pork could have had more or less fat on it than an average piece and so on. These are ALLLLL estimates, but I wanted to get a baseline to show how it looks "now" and how it will compare to future months when everything slows down, yet the gizmos will keep telling me I "should" be losing at a much faster rate.

Even though I know the gizmos are highly inaccurate, I still find them fun and motivational and anything that keeps me on target for longer is good! I won't get discouraged when it tells me I should have lost 8 pounds and only lost 3 in a month when I get close to goal. I promise?

1 comment:

  1. I've been using a Fitbit for about 14 months and I've tracked my food on MyFitnessPal for all but 2 or 3 days of that. So I have a lot of data. There is inaccuracy, but I mentally adjust for it.

    1. I've done a few things to make the calorie burn in Fitbit a bit more accurate. If Fitbit gives me extra steps for riding in a car (it sometimes gives a few, other times it doesn't, I will "name" that time in the car if it is an extended trip and tell Fitbit I was riding in a car and it will drop the calories - it will drop them if I tell Fitbit I was driving but not by as much since driving does burn more calories than riding). Also, for true exercise (Fitness walking in a hilly neighborhood, using my rower, weight lifting), I use a heart rate monitor and send the data to Digifit which sends to Fitbit my calorie burn. That is usually more accurate than what Fitbit would have said for certain activities. All of that makes Fitbit more accurate.

    2. I recognize that all estimates of calorie burn are based upon estimates of your BMR. There are several formulas for BMR and I've seen them vary by as much as 100 calories per day. The one Fitbit uses will use your age, weight and gender. However, the Katch-McArdle formula is one that many people think is more accurate. It, however, uses your body fat percentage which most people don't know. When I use it (using the percentage from my scale which is probably not totally accurate either) I get a lower BMR than what a lot of calculators would say.

    3. I recognize that food intake calories may not be exact either. I am very careful on entering food. I enter it soon after I eat (sometimes before!) and I weigh a lot of my food. Sometimes though I eat out. And, I'm not thinking that restaurants are measuring things out that carefully. So I may record it according to what the restaurant says but that may not be what they served. Also, even packaged foods can be wrong. I was eating some popcorn that said that one serving was 3 cups (28g). I was measuring out 3 cups and counted. One day I couldn't find a cup measure so I weighed. I found out that 28g was way less than 3 cups! From now on I weigh stuff instead of measuring.

    4. And, of course, as you mention there are individual variations in metabolism with it slowing down with weight loss and maybe other things as well.

    5. Finally, I have found that weight loss often "lags" by a few days to as much as a week before it shows up. I've had several weeks where I know I had a good calorie deficit (even after adjusting for stuff as above) but the loss doesn't show up when I weigh in on Friday. Often, the loss shows up a day or two or a few later.

    Looking at my weight loss overall, I do loss somewhat less weight than what Fitbit would predict. But the thing is that it is pretty consistent that way if I allow for the fact that weight loss often shows up on the scale a few days latter than Fitbit would predict (some of that may be due to water retention at times). That is, if Fitbit says I have a calorie deficit of 6000 calories in a week I will usually lose more that week than if it says I have a calorie deficit of 3000 calories. Although, again, it may be off by a couple of days. Mentally, when looking at Fitbit if it says I have a calorie deficit for a day of 500 I figure that the real deficit is probably closer to 350 to 400.