Should you eat before you exercise or is it better to work out on an empty stomach? This is the question posed by researchers in several recent studies. And the answer to this age-old quandary, it seems, depends on your workout goals.

 

If fat loss is your main goal, exercising on an empty stomach – such as first thing in the morning – may encourage your body to burn slightly more fat for fuel. Researchers at Northumbria University asked 12 male volunteers to do a moderate-intensity treadmill workout at 10am either after an overnight fast or after eating breakfast (Gonzalez et al, 2013). They gave them a milk recovery drink 90 after their workout followed by a pasta lunch a little later to see whether there were any differences in their appetite or the amount of food consumed. They found that those who exercised on an empty stomach burned up to 20% more fat compared with breakfast eaters. What’s more, contrary to popular belief, they had a smaller appetite later in the day and did not consume more calories to compensate for their missed breakfast.

 

There’s the intriguing possibility that training on low carbohydrate diet followed by a high carbohydrate prior to competition (‘train low, compete high’) may benefit performance in competition (Hawley & Burke, 2010). The idea is that training with low muscle glycogen stores forces the muscles to adapt to using fat as a fuel instead of carbohydrate.

 

Indeed a recent study by Belgian researchers found that exercising in a fasted state not only burned more fat but also increased the capacity of the muscles to burn fat in preference to carbohydrate (Proeyen et al, 2011). Ten physically active males trained in the fasted state for 6 weeks, while ten consumed carbohydrates before and during training. The two groups increased their fitness (maximal aerobic capacity) equally but those who trained after fasting burned a higher % of calories from fat and increased the number of fat-burning enzymes in their muscles. In other words, exercising on an empty stomach caused metabolic adaptations on the muscles – it ‘trains’ the muscles to burn fat in preference to carbs.

 

However, despite increasing enzyme levels in the muscles, low carbohydrate diets have not been shown to enhance exercise performance. Another study by the Belgian researchers demonstrated found that when volunteers overate by 30%, they gained 0.7kg when they trained on an empty stomach (Proeyen et al, 2010). Those who trained after eating carbs gained 1.4kg over 6 weeks. But – and here’s the important bit – their VO2 max (maximal aerobic capacity i.e. aerobic fitness) increased significantly over the 6 weeks while the fasting group saw no change.

 

The bottom line is that if you’re exercising to lose or maintain weight, then exercising on an empty stomach may help you shed the pounds a little faster. But if you’re training to gain muscle or improve athletic performance then its best to eat carbs before your workout (Chryssanthopoulos et al., 2002; Neufer et al., 1987; Sherman et al., 1991; Wright et al., 1991).  In any case, if training on empty makes you feel lightheaded or hungry half way through your workout, then you should eat before you train.

 

Gonzalez JT, Veasey RC, Rumbold PL, Stevenson EJ (2013) Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. Br J Nutr. 2013 Jan 23:1-12. [Epub ahead of print

 

Van Proeyen K et al (2011),Beneficial metabolic adaptations due to endurance exercise training in the fasted state J Appl Physiol. 2011 January; 110(1): 236–245.

 

Van Proeyen, K (2010), Training in the fasted state improves glucose tolerance during fat-rich diet J Physiol. 2010 November 1; 588(Pt 21): 4289–4302.

 

Hawley J.A. & Burke, L.M. (2010) Carbohydrate availability and training adaptation: effects on cell metabolism. Exerc Sports Sci Rev, 38 pp 152 – 160.

 

Chryssanthopoulos, C. et al. (2002), ‘The effect of a high carbohydrate meal on endurance running capacity’, Int. J. Sport Nutr., vol. 12, pp. 157–71.

 

Neufer, P.D. et al. (1987), ‘Improvements in exercise performance: effects of carbohydrate feedings and diet’. J Appl. Physiol., vol. 62, pp. 983–988.

 

Sherman, W. M. et al. (1991), ‘Carbohydrate feedings 1 hour before exercise improve cycling performance’, Am. J. Clin. Nutr, vol 54, pp. 866–870.