The Pertinacity iPhone App asks you to eat less to lose weight. It's like other calorie counters, with two major differences:

  • (i) It's much easier to count your food with Pertinacity. You use a calorie proxy to count all food -- restaurant, packaged, and home-cooked -- the same way.

  • (ii) Your daily calorie limit is customized for you based on your recent eating habits which means you'll reduce your calorie intake little by little, at a comfortable pace.

The Calorie Proxy

The calorie proxy is this: You imagine squashing the food item you want to count. You make a fist, then estimate how many fists would be the same size as the squashed food item. We refer to this as a "fist-sized portion". You record the number of fist-sized portions your eat each day with the Pertinacity App.

Pertinacity's calorie proxy is as accurate as looking up food in a calorie table. You won't, on average, undercount (or overcount) calories with Pertinacity -- well, no more so than with a calorie table. People typically undercount calories whether they realize it or not no matter how they're reporting those calories. It's a fact of life that we accept and work around. Pertinacity's adaptive calorie limit compensates for this natural undercounting.

Like any weight loss plan or tool, Pertinacity only works if you use it, so care was taken to make it as easy for you to use as possible. While the calorie proxy is less precise than looking up food in a calorie table, the proxy also makes Pertinacity much easier to use. When people track their calories by looking up their food in a calorie table their answer is typically within 18% of what's expected. For Pertinacity's proxy that number is about 28%. Accepting this extra imprecision allows Pertinacity's interface to be dramatically simplified. This means you'll be more likely to use it, which means you'll be more likely to lose weight.

The Calorie Proxy Limit

Pertinacity always encourages you to eat slightly less than you are comfortable with. After your body and lifestyle adapt Pertinacity lowers the limit, again, to a level slightly lower than you are comfortable with. As time goes on you eat fewer and fewer calories and your weight decreases. This happens at a comfortable, customized pace.

Each day your goal is to keep your intake at or below the limit specified by Pertinacity. This limit is slightly less than the average of the past two weeks of daily calorie proxy values. For example, if you've been eating an average of 15.6 fist-sized portions over the past two weeks, Pertinacity will ask you to eat at most 15 fist-sized portions today.

That might seem like a small change, but that's ok, because smaller changes are easier to make.

After you've succeeded in eating at most 15 fist-sized portions per day consistently for two weeks, Pertinacity will change the limit to 14. It might take you longer than two weeks to stay consistently under the limit. If so, Pertinacity will wait until you're ready before lowering the limit.

The adaptive limiting method also solves the problem caused by underreporting. Consider this: Let's say I told a dieter that eating 1800 Calories/day would result in weight loss. The dieter then records 1800 Calories/day. We know from research that people tend to undereport calories by 20%, so a report of 1800 Calories/day could mean that the dieter actually ate 2250 Calories/day, which might be too high to result in weight loss.

One way to view this problem is to say that we're not comparing "apples to apples". The 1800 Calories/day limit comes from an unbiased estimate (a statisical model fit to a crosssection of the population) of the Calories/day a dieter needs. The dieter's report of 1800 Calories/day comes from the dieter and is biased toward underreporting. These two values of 1800 are like apples and oranges. They can't be meaningfully compared.

Pertinacity's adaptive limit works only with your reported numbers. When you look at the App on your phone you're comparing reported values to an average of reported values. You're making an "apples to apples" comparison.

Motivating Research

To lose weight you need to create an energy imbalance. This means you need to use more energy (Calories) than you consume to lose weight. To create an energy imbalance you could eat less or move more -- or both.

Generally speaking, solving one problem is easier than solving two, so we need to choose which problem to solve: eating less or exercising more. Pertinacity focuses on eating less because studies suggest that it's more powerful for weight loss than exercising more. Note that this isn't a statement that exercise isn't also effective for weight loss (just less so than eating less) or good for other aspects of your health.

Studies suggest that adherence to a weight loss plan improves results. In other words, for a weight loss plan to work you have to follow it. This might be unsurprising, but it is important to state because it prompts us to ask, "How do we improve adherence?"

We improve adherence by making a weight loss tool simpler to use. That is why Pertinacity uses a proxy rather than a calorie table. It is also why Pertinacity focuses on a only single weight loss method (eating less).

So, Does it Work?

Our data show that consuming fewer fist-sized portions today than a recent average is correlated to consuming fewer calories than a recent average. This implies that we can control calorie consumption by proxy.

No controlled experiments have yet been run directly on users of Pertinacity. However, Pertinacity was engineered to help you use what is possibly the most well-studied and understood weight loss "method": reducing calorie intake. Complete details of Pertinacity's research and development are available in the blog.

There's no new science in Pertinacity. There's old, boring science: eat less to weight less. The novelty of the App is, rather, in its approach to measurement and control, i.e., the calorie proxy and the calorie proxy limit.

I've had great success with Pertinacity. I've lost 13 pounds using it, and -- perhaps more importantly -- have kept it off. I use the App every day to track what I eat. In addition, I lost 5 pounds right before starting Pertinacity (using a regular calorie counter) for a total of 18 pounds since I decided to lose weight. My BMI has gone from from 25.1, "Overweight", to 22.6, "Normal".

I tell you this because I'm happy to be lighter and to let you know why I'm here writing this -- not because I place much stock in testimonials.

I do, however, think the methodology presented in the blog and summarized above should motivate you to give Pertinacity a try.

Try it. It's free. It's easy. Let me know how it goes.