Choose Simple if you want a beginner-friendly weight-loss app with an AI coach, meal logging and analysis, and lots of supportive hand-holding on your phone. Choose FastFlow if you own a Garmin or Apple Watch and want an app that reads your wearable live - heart rate, HRV, Body Battery, stress - and coaches you through six metabolic stages using your real-time data, with a calm, longevity-first tone aimed at people 40+.
These apps chase different goals. Simple is a weight-management coach built around behavior change: its AI assistant, food photo analysis, and gentle daily nudges make it one of the easiest on-ramps for a first-time faster, and its user base skews heavily toward women (roughly 73%). FastFlow is a wearable specialist for people who want to understand their physiology, not just lose weight - the only fasting app that turns a Garmin's live sensor stream into stage-by-stage insight over BLE. If your priority is guided weight loss with a coach in your pocket, Simple is excellent. If it's seeing what fasting does inside your body, that's FastFlow. Here's the honest head-to-head, including where Simple clearly beats us.
Comparison table
| FastFlow | Simple | |
|---|---|---|
| Live Garmin/Apple Watch sensor sync (BLE) | Yes - HR, HRV, Body Battery, stress during the fast | No - phone-only |
| AI coach | No | Yes - conversational AI guidance |
| Meal logging / food analysis | No | Yes - photo-based meal analysis |
| Metabolic stage coaching | 6 stages (Fed → Renewing) with live data | Timeline with coaching prompts |
| Primary framing | Effortless Longevity, physiology, 40+ | Weight loss, habit change |
| Beginner hand-holding | Light | Extensive |
| Platforms | iOS + Garmin/Apple Watch | iOS + Android |
| Price | Free + $4.99/mo or $39.99/yr; Garmin unlock $4.99 (3 free fasts) | Free + paid subscription |
Where Simple genuinely wins
Balanced beats hype, so here's the fair account. Simple's AI coach is a real strength - it answers questions conversationally, adjusts guidance, and gives beginners a sense of support that a data dashboard can't. Its meal analysis (photograph your food, get feedback) is a genuinely useful feature FastFlow doesn't offer. Its beginner onboarding is best-in-class - if you've never fasted and want your hand held, Simple is gentle and encouraging. It's explicitly built for weight loss, so if that's your primary goal, its whole design serves it. And it's cross-platform on iOS and Android. For a first-time faster focused on losing weight with lots of support, Simple is the stronger, more comfortable choice - and I'd say so plainly.
Where FastFlow wins
FastFlow does the one thing Simple can't: it reads your Garmin (or Apple Watch) in real time over BLE and shows live HR, HRV, Body Battery, and stress while you fast, mapping the data to six metabolic stages - Fed, Clearing, Shifting, Burning, Cleansing, Renewing. Instead of coaching prompts on a phone timeline, you see how your own physiology is responding as the metabolic switch toward fat and ketones unfolds (de Cabo & Mattson, 2019, N Engl J Med, PMID 31881139). For someone 40+ who cares more about metabolic health and aerobic longevity than a number on the scale, that shift in framing matters - FastFlow is anti-hustle and anti-obsession, built to work with the body rather than to drive weight down as fast as possible. No AI cheerleader, no food diary to maintain; just your watch, your data, and a calm read on your fast.
Which should you choose? By use case
- Your main goal is weight loss with support: Simple. Its coach and design are built for it.
- You want to understand your physiology and own a wearable: FastFlow. It reads your Garmin live.
- You're a complete beginner who wants hand-holding: Simple.
- You're 40+ and focused on metabolic health and longevity, not the scale: FastFlow.
- You want AI coaching and meal analysis: Simple.
- You're on Android: Simple. FastFlow is iOS-only today.
- You want both: Simple for coaching and food feedback, FastFlow for the live wearable read - they don't conflict.
Still comparing? See how the whole field stacks up in best fasting apps that sync with Garmin, or read FastFlow vs Zero.
Frequently asked questions
Does Simple sync with a Garmin watch?
Not in real time. Simple is a phone-first fasting and weight-loss app. It may read some data through Apple Health after the fact, which can include Garmin-sourced steps or weight, but it does not receive your Garmin's live sensor stream - HR, HRV, Body Battery, stress - during a fast. There's no live BLE connection to the watch. If you want your wearable's real-time physiology reflected while you fast, Simple isn't designed for that; FastFlow is the app that does it. For phone-based coaching and meal analysis, Simple is strong; for live wearable data, it isn't the tool.
Is Simple better for weight loss than FastFlow?
For weight loss specifically, yes - Simple is built for it. Its AI coach, meal photo analysis, and behavior-change design all point at helping you eat less and lose weight, and its onboarding supports that goal. FastFlow is framed around metabolic health and longevity rather than the scale; it can support weight goals indirectly through fasting, but it won't log your meals or coach your diet. If dropping pounds with guidance is your priority, Simple is the better fit. If understanding your fasting physiology is the priority, FastFlow. They optimize for different outcomes.
Which app is better for someone over 40?
It depends on your goal. If you're 40+ and focused on metabolic health, aerobic base, and aging well - working with your body rather than forcing weight down - FastFlow's calm, wearable-driven, anti-hustle approach fits that mindset, especially if you own a Garmin. If you're 40+ and your main aim is weight loss with a supportive coach, Simple's hand-holding may serve you better. Neither is "the 40+ app" outright; FastFlow simply leans toward longevity and physiology, while Simple leans toward guided weight management. Match the tool to what you actually want to change.
Does FastFlow have an AI coach or meal tracking?
No, and that's deliberate. FastFlow doesn't include an AI chat coach or a food diary - it focuses on reading your Garmin or Apple Watch live and coaching you through metabolic stages with your real physiological data. Simple owns the AI-coach and meal-analysis space; FastFlow owns the live-wearable space. The design choice is intentional: FastFlow is meant to be low-maintenance and anti-obsession, not another app asking you to log every meal. If you want conversational coaching and food tracking, Simple is the right pick; if you want quiet, data-backed fasting insight, FastFlow is.
How do the prices compare?
Both are freemium. FastFlow's core app is free, with premium at $4.99/mo or $39.99/yr and a one-time $4.99 Garmin companion unlock (3 free trial fasts). Simple is free with a paid subscription for its AI coach and advanced features. Costs are broadly similar, though Simple's value is concentrated in its ongoing coaching subscription, while FastFlow's Garmin link is a small one-time fee. The philosophical difference: FastFlow favors tools you set once and mostly ignore, whereas Simple is designed for daily engagement with its coach - which is a feature if you want it and a cost if you don't.
Can I use both apps together?
Yes. They don't conflict, and their strengths are complementary. You could run Simple for AI coaching, meal analysis, and beginner support on your phone, while using FastFlow for the live Garmin or Apple Watch read and metabolic-stage insight. That's redundant for pure timing but reasonable if you value both guided weight-loss coaching and wearable physiology. Most people eventually settle on one based on their primary goal, but there's no technical barrier to using both during a transition or while you figure out which approach fits your life.
This article is for general education and is not medical advice. Fasting is not appropriate for everyone. Consult a qualified healthcare professional before starting any fasting protocol, especially if you take medication or have a medical condition.