Written by: Retha Booyens (Registered Dietician) When seeking to lose weight, every women’s magazine, lifestyle blogger and man on the street will be able to give you a whole list of relevant or irrelevant dietary advice. Or tell you to exercise more. It is a well-known fact that an increase in physical activity leads to weight loss. Currently, finding motivation to exercise is not a difficult task. Social media, ‘fitspiration’, crossfit-culture and popular fitness-icon’s like Kayla Itsines and Jillian Michaels are everywhere. The mathematical reason why exercise leads to weight loss is very basic, but easier said than done. More energy expended than energy intake leads to weight-loss.

  • Energy expenditure = Exercise
  • Energy intake = Eating

The unit of measurement of food energy is kilojoule (kJ) or kilocalorie (kcal).

  • 1 kJ = 4.2 kcal

Every person has a different energy requirement, based on their weight, height, gender, age, whether or not they want to lose weight, etc. Your energy requirement is the amount of energy you need to take in daily for all your body’s functions to happen correctly. A registered dietician can help you calculate your energy requirement.

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Studies have shown that when people start to increase their physical activity it is not generally accompanied by increases in energy intakes. But, when physical activity is reduced there is not generally an accompanying decrease in energy intakes.
But most of these studies were relatively short duration; more research of longer duration is needed to understand if energy intakes eventually adjust to the new levels of energy expenditure. The authors also found that there was often a short-term suppression of appetite during and immediately following high-intensity exercise but not after light or moderate intensity exercise.

Since the publication of the aforementioned review, a number of other experimental trials2 have been published on the relationship between exercise-induced increases in energy expenditure and intake.
Investigators divided 23 healthy, normal weight, men into categories of habitual non-exerciser, habitual moderate exerciser and habitual high-exerciser.  Subjects ingested either a 600kcal or 240kcal liquid meal and then were presented with a buffet one hour later.

 They found that the non-exercisers consumed the same amount, whether or not they consumed the higher or lower calorie liquid meal beforehand. However, subjects in the two exercise groups ate significantly less at the buffet following the higher calorie meal than following the lower calorie preload.

The authors concluded that the habitual exercisers were better able to compensate for their previous consumption of excess calories. Although it was not known if this was due to greater sensitivity in satiety signals, hunger signals or some other mechanism.

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In a cohort study3, 29 healthy, sedentary, normal weight, young men and women underwent a six-week moderate intensity exercise program. Both before and after the six weeks, participants also ingested a high energy (607 kcal) or low energy (246 kcal) milkshakes (referred to as preloads) one hour before a buffet style meal. After the lower-energy preload, the energy intake at the buffet was similar before and after the six weeks.  However, compared to before the exercise program, the energy intake at the buffet meal was lower after the higher-energy pre-load than before the program. The authors concluded this small effect was evidence of an improvement in appetite control following six weeks of exercise training. This might explain why numerous endeavours of sedentary people who start to exercise with the intention to lose weight end up failing. The ability to control energy intake with regards to energy expenditure comes with regular habitual exercising and not only exercising only. It has been theorized that regular exercise may aid in weight control via:

  • Increasing energy expenditure.
  • Improving the sensitivity of the appetite control system2.
  • Having difficulty in eating to match very high energy expenditures1.
  • Decreasing appetite through reductions in blood flow to the gastrointestinal tract with intense exercise4.
  • Blood flows to the muscles instead of to the GI-tract.
  • Decreasing appetite via enhanced insulin sensitivity2 or increased appetite-related hormones and substances (metabolites eg. polypeptide YY, glucagons-like peptide and pancreatic polypeptide3).

Take home points from this:

  • When you start a new exercise program with the intention to lose weight, you should practice mindful eating.
  • Keep a food journal to help keep track of your food choices.
  • Practice portion control (smaller plates, weighing your food portions, pre-pack meals in correct quantities)
  • Remove distractions; do not eat in front of the television or computer.
  • Sit around the table for meals; make every meal special.
  • Try not to eat on the run.
  • Enjoy meals with friends; conversation prevents over-eating.
  • If you stop exercising for any reason whatsoever (injury, change of the season, holiday, etc) try to decrease your energy intake to prevent weight gain.
  • When embarking on a lifestyle change, exercise can in the long-term lead to an improvement in appetite control.
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References:

  1. Blundell JE, King NA. 1999. Physical activity and regulation of food intake: current evidence. Med Sci Sports Exerc. 31:s573-583.
  2. Long SJ, Hart K, Morgan LM. 2002. The ability of habitual exercise to influence appetite and food intake in response to high- and low-energy preloads in man. Br J Nutr. 87:517-523.
  3. Martins C, Morgan LM, Bloom SR, Robertson MD. 2007. Effects of exercise on gut peptides, energy intake and appetite. J Endocrinol. 193:251-258.
  4. Blundell JE, Stubbs RJ, Hughes DA, Whybrow S, King NA. 2003. Cross talk between physical activity and appetite control: does physical activity stimulate appetite? Proc Nutr Soc. 62:651-661.

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