TLDR: Fat burning and fat loss aren’t the same thing. Fat burning (fat oxidation) is about which fuel you use during a session. Fat loss is about your energy balance over days, weeks and months. Zone 2 isn’t special for fat loss. It can help if it means you’ll be more active overall and you enjoy it, but if weight loss is the goal, nutrition is the foundation. Choose your training based on your goals, the time you have and what you enjoy.
What is fat burning?
“Fat burning” and “fat loss” get used as if they mean the same thing. They don’t, and mixing them up leads a lot of people to train in a way that doesn’t match what they’re trying to achieve.
Fat burning, or fat oxidation, is the process of breaking down fat to produce energy. It happens all the time, at rest and during exercise, and it works in three steps:
- Mobilisation: fat stored in your fat tissue is released into the blood as free fatty acids.
- Transport: those fatty acids travel to working cells, such as muscle, and enter the mitochondria.
- Oxidation: inside the mitochondria they’re broken down to make ATP, your body’s energy currency.
How much fat you burn during exercise depends mostly on intensity. Fat burning rises as you go from very easy to moderate, then falls as the effort gets harder and carbohydrate takes over. The peak, often called “FATmax”, sits at roughly 59 to 64% of maximal oxygen uptake in trained people and 47 to 52% in the general population (Achten & Jeukendrup, 2004).
What is fat loss?
Fat loss is a reduction in the total amount of fat stored in your body. It happens when, over time, you use more energy than you take in. Your body makes up the gap partly by drawing on its own fat stores.
The key words are “over time”. Fat burning happens minute by minute during a session. Fat loss is the result of your energy balance across days, weeks and months, and it’s shaped by everything you eat and do, not just by what fuel you used on one session.
Fat burning zone vs calorie deficit: which matters more?
Here’s a simple illustration. Say someone jogs easily for 45 minutes, uses about 300 kcal, and 70% of that comes from fat. If, across the whole day, they still take in more energy than they use, they won’t lose body fat, however much fat they burned during the jog. The reverse is also true: you can burn mostly carbohydrate in a hard session and still lose fat if your overall energy balance is in a deficit.
The research backs this up. A meta-analysis of 13 trials comparing high-intensity interval training (HIIT) with steady moderate-intensity training found no difference in fat loss between them. Interval training took about 40% less time (Wewege et al., 2017). A larger review found that both reduced body fat percentage by a similar amount, with interval training producing a slightly bigger drop in absolute fat mass (Viana et al., 2019). Neither found an advantage for training in the “fat-burning zone”.
Should I add Zone 2 to lose weight?
Depending on which podcast you listen to, Zone 2 will build your mitochondria, melt your fat, fix your sleep, sort out your mortgage and get your neighbour to give back the strimmer they borrowed in 2019.
This is one of the questions I get asked most, and I’m going to give you the most non-committal scientist answer there is: it depends. Zone 2 isn’t special for fat loss. It’s a training stimulus, and a useful one, but it doesn’t unlock fat loss in a way other training can’t. Whether it’s a good idea for you comes down to three questions.
1. Will you stick to it?
Adherence matters more than the zone. If adding Zone 2 means you’re more active overall, week after week, it can be really useful. If it’s a plan you’ll drop after a fortnight, it won’t do much.
2. How much time do you have?
Zone 2 is a great way to build up training volume with less fatigue and a lower injury risk than lots of hard sessions. If you have the time, that’s a real strength. But if you’re short on time, more Zone 3 work and intervals will probably give you a better training stimulus for the hours you have. Both steady endurance training and intervals give big improvements in fitness, with intervals possibly slightly ahead (Milanović et al., 2015).
3. What are your performance goals?
If you’re training for a long event, easy volume has a clear place in your plan. If you want to know where your zones actually sit, my article on lactate threshold testing explains how they’re measured.
Expect your appetite to rise
As training volume goes up, appetite usually follows. Anyone who’s finished a long Sunday ride and gone back to the kitchen cupboard six times for “one last snack” will know this. That’s normal, and it’s your body matching intake to the extra work. It also helps explain why exercise on its own tends to produce less weight loss than people expect. When researchers looked at why, the main reasons were relatively small amounts of exercise and an increase in how much people ate (Thomas et al., 2012).
So where does that leave you?
If weight loss is the goal, look at your nutrition first. It’s the foundation for any fat loss, far more than the type of training you do. Choose your training based on your goals, how much time you have and what you enjoy. If that’s Zone 2, great. If it’s intervals, also great.
Common myths about fat burning and fat loss
Myth 1: Training in the “fat-burning zone” leads to more fat loss. A higher proportion of your energy comes from fat at lower intensities, but that doesn’t mean you’ll lose more fat. Fat loss comes down to energy balance over time, and studies comparing easy and hard training find similar results (Wewege et al., 2017).
Myth 2: Eating more fat makes you a “fat-burning machine”. A high-fat, low-carbohydrate diet does increase fat oxidation, but that doesn’t by itself lead to fat loss. You still need an energy deficit over time. For athletes, relying heavily on fat can also hurt performance at higher intensities, where carbohydrate is the better fuel.
Myth 3: Fasted training burns more body fat. Training fasted can increase fat burning during the session, but in a study where everyone was eating in a calorie deficit, fasted and fed training led to the same fat loss (Schoenfeld et al., 2014).
A note on getting lighter as an athlete
If you’re trying to lose weight while training, be careful not to cut back so far that you can’t fuel your sessions. Chronic underfuelling can harm your health and your performance. If you’re unsure, or if thoughts about food and weight are taking up a lot of headspace, it’s worth speaking to a registered dietitian or qualified sports nutritionist. My article on how to stop underfuelling is a good place to start.
The bottom line
Fat burning is a process that happens during exercise. Fat loss is an outcome of your energy balance over time. The “fat-burning zone” isn’t a shortcut to losing fat, and Zone 2 isn’t special for weight loss. It’s a useful way to build training volume if you have the time and enjoy it. If losing fat is the goal, start with your nutrition, then pick the training that fits your goals, your schedule and what you’ll actually stick to.
