snapkin
Why track anything at all
The strongest evidence for logging is indirect. No trial has ever split people into a group that logs and a group that does not, with everything else kept the same.
Written and maintained by Mattias Geniar. Last updated .
- What the app does
- Asks for a photograph rather than a database search, because the biggest problem in this category is how few people are still logging after a month.
- How much we believe it
- People who log lose more weight. Every review finds that. Giving somebody an app does not make them lose weight. Those are two different claims.
What exists instead compares ways of logging, or compares whole programmes that happen to include logging. The best of it: Berry et al. (2021) pooled 12 randomised trials of digital self-monitoring and found −2.87 kg (95% CI −3.78, −1.96), with calorie intake down 182 kcal a day. Michie et al. (2009) looked across 122 studies and 44,747 people. Of the 26 behaviour-change techniques they examined, self-monitoring explained more of the difference between studies than any other. Harvey et al. (2019) found that people who lost 10% or more logged 2.7 times a day, against 1.7 times for those who lost less than 10%. The time spent logging made no difference at all.
And the usual warning from the same research: Burke et al. (2011) found the link every time, and then said plainly that “the level of evidence was weak because of methodologic limitations.”
Prediabetes is where the evidence is strongest, and it is not about the app
The Diabetes Prevention Program (3,234 people, followed for 2.8 years on average) is the landmark study. An intensive lifestyle programme, aiming for 7% weight loss and 150 minutes of activity a week, cut new cases of type 2 diabetes by 58% (95% CI 48–66) compared with placebo. Metformin, a drug, cut them by 31% (17–43). Average weight loss was 5.6 kg. The Finnish Diabetes Prevention Study (522 people) found the same 58% reduction on its own.
Writing down what you eat was part of both. In the American programme, Wing et al. (2004) reported that “dietary self-monitoring was positively related to meeting both weight loss and activity goals.” In the Finnish study, dietary advice was based on three-day food records.
On how much weight loss is enough: the usual “5 to 7%” figure is a programme target taken from what those two trials aimed for and reached. It did not come from a study designed to find the right amount. The real relationship is gradual, with no cut-off point. Hamman et al. (2006) found a 16% lower diabetes risk for every kilogram lost (HR 0.42, 95% CI 0.35–0.51 per 5 kg), and weight loss was the strongest predictor. Magkos et al. (2016) showed that 5% weight loss on its own improved insulin sensitivity in fat, liver and muscle.
The evidence against, and the two numbers we like least
Almost nobody keeps logging
Helander et al. (2014) followed 189,770 downloads of a free photo food-logging app.
2.58% used it actively did not
Apps in general do very little
Chew et al. (2022) put the pooled effect of apps at 2.18 kg at three months, falling to 1.63 kg at twelve, and called the effects “minimal in their current states.” Adding a human coach roughly doubles it (Chew et al., 2023). Snapkin has no human coach, and nothing here claims the effect of one.
Self-reported calories barely match real calories
Freedman et al. (2014) pooled five studies with 2,265 people in total. Each compared what people reported eating with doubly labelled water, the most accurate way to measure how much energy a person really uses. Under-reporting gets worse with body weight, by a further 5 to 7 percentage points at a BMI of 30 compared with 25. The correlation between reported and true calorie intake was r = 0.21 to 0.31.