Losing weight

What is the fat burning zone? Should I run more slowly to lose fat?

You may have heard the claim that you should run more slowly to burn more fat. My local gym has signs suggesting that if you want to burn fat, you should exercise in a “fat burning” heart rate zone.

This idea is based on a misunderstanding of the science. It is true that when you exercise at a lower intensity, your body is able to draw more of the energy directly from fat. But it is not correct to conclude that exercising at this lower intensity will result in your having lower overall body fat.

There are two reasons why the claim about a “fat burning zone” is wrong.

  • First, it does not matter where the fuel comes from while you are exercising; in the end (ie later in the day) your body will replenish and rebalance your energy stores. So a calorie surplus will always end up increasing your body fat; and a calorie deficit will always end up reducing your body fat. The source of fuel while you are exercising is irrelevant.
  • Second, what matters is the total amount of calories you burn; not the proportion that come fat. If you exercise at lower intensity, a higher proportion may come from body fat, but this may be a smaller absolute number of calories.

In other words, the fat burning zone is a myth caused by misunderstanding the science.

So how can I lose weight?

To lose fat, you need to eat fewer calories than you burn. The extra calories you burn will be taken from your body fat.

Suppose your daily calorie requirement is 2800 kcal per day; and suppose that you consume 2710 kcal per day. Then you are consuming 90 kcal a day fewer than you are burning. Because fat is about 9 calories per gramme, you will lose about 10 grammes of body fat a day.

You can reduce weight either by reducing your calorie intake (eg by eating smaller portions) or by increasing your daily calorie requirement (eg by exercising more) or both.

How quickly should I lose weight?

If you do decide you want to reduce your body fat, you should not attempt to lose weight too rapidly. In particular, you should not aim to lose more than ½ kg a week; nor should you try to consume less than 85% of your daily equilibrium calorie needs. This is to avoid illness, and because if you eat less than this, your body goes into starvation mode, your metabolism slows down, and your rate of weight loss decreases.

The best approach to estimating how fast you should be losing weight is:

  • decide how much running you realistically intend to do per week;
  • estimate your equilibrium daily calorie requirement at that level of exercise, given your current bodyweight;
  • work out how many calories a day you need to eat to lose weight without consuming less than 85% of your daily needs or losing more than 500g a week.

You can do this calculation here.

See also

14 Responses to Losing weight

  • Great discussion on the weight loss and the term Fat burning zone is not a myth but needs some more advise on it.

  • While we’re talking about misunderstanding science, we should talk about whether or not calories have anything to do with fat storage in animals. A growing body of evidence suggests that the old formula “weight gain = calories in – calories out” is misleading at best, and possibly completely wrong. It is not widely known that calories in food are measured by burning food in a calorimeter-not exactly an accurate model of human metabolism.

  • Alex:

    There is a fat burning zone, however it is misunderstood. If your goal is weight loss then it is faster to train at a higher intensity as it burns more calories per unit time, however training at 50-65% of max heart rate does improve the efficiency of fat metabolism and burns fat as opposed to carbohydrates.

    • Frank:

      In my opinion, the misunderstanding comes from looking only at how much you burn over a given time. It is correct that you burn more (for the same amount of time) when you work harder, but you cannot do it for as long. The thing people call the fat burning zone is the zone where you burn the most calories taking into account how long you are able to keep it up. Training for 3 hours at 70% will burn much more caloriers than training for 1 hour at 85%. It is a combintion of how much you burn per hour and for how long you are able to keep it up. The “fat burning zone” is the zone where this combination is optimal.

    • Frank:

      As he states in the article, it is irrelevant where the calories come from when you train. The body only has one way of storing excess calories, and that is as fat. Even if you can choose to burn fat, it is irrelevant that you do it, because whatever calorie defict you have will end up as less fat on your body. There is no fat burning zone.

      • It is not necessarily irrelevant since some people would like to burn the fat and the excess calories both, which I presume would increase the rate of fat burning.

  • Master Trev:

    Interesting article, and I agree with most of it.
    However, the statement that eating less than 85% of your caloric needs will make you ‘ill’ is somewhat of an old wives tale and not based on science (ironically, this is the same rationale you use to discredit other theories) and is, well, somewhat old-fashioned.
    Surely, one that is eating less than their needs will get tired quicker, and will not be able to exercise as usual, and may feel a general ‘sluggishness’, but this is not indicative of any ‘illness’ per se. Western medicine seems to be really into EAT EAT EAT for ENERGY because if you dont EAT EVERY 4 HOURS then YOU’LL GET SICK AND DIE! heh
    Unless you have a body like a twig, you likely have plenty of energy stored up to last for several days, even weeks, of reduced caloric intake. Sure, you’ll get hungry, and this is just your body’s natural response.

    • Frank:

      I totally agree. I am able to burn up to 15.000 kalories in one week from training, which almost equals burning everything I eat. That does not make me ill or tired. Burning only an excess of 15% of what I eat is nothing. That is something like 30 minutes on a bicycle. I typically ride 2-3 hours every day.

  • Alan Farrance:

    If you run slowly most of the energy comes from burning fat and less from glycogen. If you run fast most of your energy comes from glycogen and less from burning fat. The question is how the glycogen replaced. If, as usually recommended, you eat some sort of carbohydrate afterwards then the glycogen is replaced from that carbohydrate not from your fat store. You would likely lose more fat from a 2 hour jog than a 1 hour fast run even though the total energy used might be the same. But you are very unlikely to lose as much fat from a 1 hour jog as you will in a 1 hour fast run.

  • Tim:

    I can see what Mr James is saying, in that the fat-burning zone is actually a useful zone to train in for endurance athletes who are going to need to get a lot of the energy for their event from fat. However, it is completely useless for anyone else, and gyms are definitely still perpetuating the myth that this is the best zone to train in for weight loss.

    On the other hand, I don’t see the “starvation mode” myth as being any better. The idea that my body will go into some kind of energy-saving mode because I failed to eat 85% of my calorie needs for one day is ridiculous and not borne out by any evidence. I do agree that gradual weight loss is far healthier than rapid loss, and more likely to be long-lasting, but for very different reasons. Rapid weight loss does not encourage healthy habits that can be maintained long term, and makes it very difficult to maintain muscle mass. The 1lb-per-week target is pretty sensible, but why the calories have to be spread so evenly over 7 days is not explained, and for good reason: there is no explanation.

  • Su-Chong Lim:

    I agree with Tim. I am an experienced runner and had got away with training exclusively at Zones 3-4 or higher. I tended to bonk out at marathon events so I avoided them, but did well at half marathons. However I graduated to triathlon competing, and heading into my first Half Ironman this year decided to get metabolic/respiratory gas testing, being nervous about my ability to hold out the 6 hour exertion duration. My testing physiologist was somewhat pessimistic when I described my training intensity, and sure enough, at Zone 1, I was only able to mount a fat-burn ratio of 11%, and 9% at zone 2! at zones 5 I was burning 0%. I embarked on a training regime exclusively in Zone 1 (it was awful — it just killed me!) and a high fat low calorie diet and in 11 weeks was able to document 62% fat burning in Zone 1. I realise how poorly I had understood this aspect of exercise physiology and my understanding of this mechanism really improved.

    • chaz:

      Su-Chong, thank you. that was one of the most useful things I have ever read on the need for Z1 and Z2 training. I have been ignoring those zones, but your statistical details give me the nudge I needed to spend more time in them

  • kay:

    I ran/walked up until 3 years ago. I am 67 and wondered if it was feasible to start running again. If so, could you recommend a way to start.

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