What should we eat before exercise? How about nothing?

A few months ago I was chatting with a gentleman who was having difficulty losing weight despite doing ‘all the right things’. Almost every morning he would start the day with a vigorous and quite lengthy exercise session. He would ‘fuel’ this session with a bowl of cornflakes. I suggested he pull back a bit on the cornflakes, on the basis that it would likely cause his blood sugar and insulin levels to skyrocket, which in turn would impair his body’s ability to mobilise fat for burning during the exercise session.

Not one for doing things by halves, he declared he would just stop eating the cornflakes and do the exercise on an empty stomach. Hey presto, the last I heard he was thrilled with the results.

This conversation came back to me today when I was reading a study which explored variety of nutritional approaches to enhancing fat-burning during exercise [1]. Most attention in this paper is paid to the nature of carbohydrate consumed prior to exercise. Specifically, the authors put forward the idea that carbohydrates of relatively high glycaemic index (generally disruptive to blood sugar) will lead to high levels of insulin that inhibit what is known as lipolysis. Lipolysis is a word used to describe the process through which fat is mobilised from cells (usually fat cells) to appear in the bloodstream as ‘fatty acids’. These fatty acids can make their way to the muscle cells to be burned during exercise, though this is unlikely to happen to the same degree if they end up ‘trapped’ in the fat cells as a result of the influence of insulin.

The authors concede that the evidence regarding the influence of the glycaemic index of food eaten prior to exercise on fat-burning is not utterly consistent. However, taken as a whole, there is more than enough evidence to think that there are benefits to be had from avoiding spikes in sugar and insulin prior to exercise. To quote from the paper:

“… studies have demonstrated up to 2-fold increases in the amount of whole-body fat oxidation during treadmill running. This occurred during exercise intensities ranging from 50 to 70% VO2max in both males and females, differing in activity levels and differences even occur during the first 15 min of exercise. Furthermore, similar findings are seen with carbohydrate intakes ranging from 1 to 2·5g/kg BM [body mass] and when the pre-exercise meal is ingested from 30min, up to [12 hours] before exercise.”

Another interesting note from this paper concerned the impact of the sugar fructose on fat-burning. Although fructose is a low glycaemic index food, the authors point out that it inhibits fat burning during and after exercise to a greater extent than glucose. There is some thought that one of fructose’s effects is to increase levels of lactate – which is known to inhibit lipolysis.

I’m not a big believer in aerobic exercise for weight loss, but I do think that it’s perhaps worth bearing in mind that what one eats prior to exercise can influence fat burning and any weight loss results that might be achieved. Also, for those engaged in endurance exercise, enhanced fat-burning can reduce reliance on carbohydrate as a fuel, and reduce the risk of ‘glycogen depletion’ and ‘hitting the wall’ (also known as “bonking”). Cornflakes, with their high glycaemic index and relatively fructose-rich nature make a pretty poor choice of pre-exercise food.

So, what should people eat prior to exercise? Maybe nothing.

I know we’re often told we should have something inside our stomach during exercise to ‘fuel’ our efforts, but I’m not sure this is always a good idea. If someone exercises in the morning and can do this on an empty stomach, so much the better I think. Insulin levels are likely to be low and this might help fat-burning. It might, essentially, force the body to dig into its fat reserves for the energy it needs during the exercise. As long as there is not undue weakness or lightheadedness, I see no issue with exercising on an empty stomach.

This may not be the best approach for those seeking to crack their personal best for the marathon. But for common-or-garden exercise, I think it’s the way forward for most.


1. Gonzalez JT, et al. New perspectives on nutritional interventions to augment lipid utilisation during exercise. British Journal Nutrition epub 5 December 2011.

16 Responses to What should we eat before exercise? How about nothing?

  1. Jake 6 December 2011 at 3:05 am #

    Two additional benefits of fasted exercise.

    1. Stimulates the production of growth hormone which aids in fat burning and muscle building.

    2. Stimulates autophagy in the liver which helps the liver operate at high efficiency.

  2. rod tucker 6 December 2011 at 6:54 pm #

    I’ve been exercising on an empty stomach for about 3 years now. My program involves once weekly sprints, HIIT training on a stationary bike, one session of cardio on a cross trainer and two sessions of resistance training. Oh and I play squash once a week. I eat a high fat-protein low carb diet and feel fine with plenty of energy during my fasted exercise. So yeah, fasted exercise seems Ok to me

  3. kem 7 December 2011 at 8:54 am #

    It’s a no brainer, really. Once you get “fat adapted”, you can forget about running out of fuel.

  4. Jeremy 8 December 2011 at 1:42 pm #

    One additional thing to add is that cornflakes were invented by a seven day Adventist. They were used to lower libido in men. So cornflakes are not the best breakfast to help lose weight due to the effects on your hormonal profile. If male clients are struggling to get the message you can always tell them that!

  5. Craig 9 December 2011 at 6:22 pm #

    “As long as there is not undue weakness or lightheadedness, I see no issue with exercising on an empty stomach.”

    I’m glad you added this. I’ve been low carbing for 12-months. Before I go to the gym to do interval training, circuits, weights, whatever… I normally have a protein shake with whole milk and a shot of expresso. This keeps me going from when I leave work until when I leave the gym and by the time I get home I am ready for my evening meal.

    It means I don’t have too much liquid in my stomach when I’m jumping around, jogging, etc.

    I made the mistake a couple of weeks ago of exercising without my usual protein shake. After my workout, I felt dizzy, disoriented and (almost, I’d say) confused.

    I made it home, ate something, and within 10 minutes it all went away.

    I’m not diabetic and I had eaten earlier in the day. Suffice to say, it was a lesson for me NEVER to exercise on an empty stomach.

  6. Jean 9 December 2011 at 7:26 pm #

    I cannot exercise without eating, I have type 2 diabetes and if I do not eat regularly my blood sugar levels fall too low, and I feel dizzy and ligh headed. Any suggestions as to what constitutes healthy food for me to eat before exercise? At the moment I have organic oats made up as porridge, with lactose free milk and sugar free apricot conserve to sweeten (the conserve has fruit, and pectin to set and no other ingredients but then apricots contain fructose don’t they?

  7. Will 9 December 2011 at 7:37 pm #

    I agree with what I think is the gist of the article, although I am confused about one thing. In the extract from the paper it seems to be saying that fat oxidation was similarly increased regardless of pre-exercise carbohydrate, which is the opposit of the point I thought Dr Briffa was making.
    I find the point about fructose interesting, thank you for that. However at 1.5g/100g fructose, I would not call cornflakes a high fructose food. The dominant sugar is sucrose, as it is in most breakfast cereals without fruit.

  8. Marly Harris 9 December 2011 at 7:49 pm #

    Anecdotal evidence, of course, but I workout six-days-a-week (weight resistance)and I’ve tried both methods = eating before exercising and exercising before eating. For me, definitely, not eating before the gym is the right way.

    I’ve lost 133 pounds (29 more to go)and at an advanced age (78), my body is improving in strength and tone and appearance. And do I enjoy that first meal after my workout (fatty meat/fish).

    I’ve been reading Dr. Jack Kruse’s material and he advocates eating within a half hour of arising and not exercising in the morning. Well, it obviously worked for him but my method is working fabulously for me.

  9. majkinetor 12 December 2011 at 8:12 pm #

    “The dominant sugar is sucrose, as it is in most breakfast cereals without fruit.”
    Sucrose is 50% fructose.

    Exercise on lowcarb requires adaptation (around 2 months or so)


    Examining the results of these two ketogenic diet performance studies together indicates that both groups experienced a lag in performance across the first week or two of carbohydrate restriction, after which both peak aerobic power and sub-maximal (60–70% of VO2max) endurance performance were fully restored.

  10. Richard 13 December 2011 at 7:36 pm #

    Hi Dr B, one of your fans and my personal training clients introduced me to your blog and i love it!

    Im a sports scientist.

    This post however is correct in many ways does not take into account – EPOC – excess post exercise oxygen consumption. What are your thoughts on the effects of this?

    The studies mentioned only look at fat oxidation during exercise. I have been taught that the amount of fat oxidised during regular/cardio based exercise is small when compared with the amount of fat oxidised during and post H.I.I.T. and I have observed a 5 times increase in metabolism for 2-3 hours post exercise(buring 300+ calories per hour at rest)

    I think for weight loss its good in the short term or done in cycles(carb cycling/intermittent fasting) but as you say for performance its no good.

    Keep up the great work

    Richard Clarke

  11. katey mcendoo 14 December 2011 at 1:08 am #

    I am positive that exercising on an empty stomach is a really silly idea, I have bad experiences of it myself, you end up under performing, bonking out and end up shovelling loads of rubbish food into your system to make yourself feel better afterwards. A high protein 400 min calorie snack with some carbs a minimum of an hour before you go out will see you right for a good couple of hours hard mountain biking.

  12. Matt 15 December 2011 at 9:06 pm #

    From the points raised, I would say that maybe if you were just trying to lose weight then exercising on an empty stomach may be beneficial. But there are other factors to consider, namely the intensity the individual is working at. If they are say working at only 55% of max then this approach could be ok, but if you are training at a high intensity then this would be a disadvantage training on an empty stomach, and certainly people need to these cautious in doing so, especially as performance will be limited and other undesirable effects such as feeling light headed may be experienced.

    But again it comes down to the individual and what works best for them, but if we are going to advise individuals that the best way to shed fat is to exercise on an empty stomach, they should discuss this with a health professional first to make sure their are no contraindications in doing so, especially if they are diabetic or on certain medications.

    But I guess if their are those training on an empty stomach, then post-workout nutrition is obviously going to be even more important.

  13. jake3_14 16 December 2011 at 1:08 am #

    I’m not a sports scientist, but I’ve read that the body metabolizes fat more slowly than it metabolizes glucose. So, if you eat nothing before a workout, aren’t you limiting the intensity of your workouts?

  14. Jeremy 26 December 2011 at 11:03 pm #

    By the way, exercising on an empty stomach is a great idea. I did it for a year whilst running and after becoming fat adapted I felt great during my workouts. I did not hit the wall as many people do because my metabolic pathway to utilise fat was so well tuned. People depend on carbohydrate too much and hit the wall as they put it. In terms of limiting exercising intensity, many crossfit athletes myself included, undertake severely anaerobic workouts, yet eat a very low carbohydrate and higher fat and protein diet.

    It really depends on the person, I am ok with it. Try it for a month, if you hate it, don’t do it. But it does help to becoming well adapted to utilising fat as a main energy source.


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