Is your workout ruining your diet?
The relationship between appetite and exercise is complex. Following these few simple tips can help with appetite control, experts say.
How hungry are you after exercise? Post-workout hunger pangs have long been blamed for stalling weight loss – but how and why some people feel ravenous when they finish a workout while others can barely stomach the thought of food has remained a mystery.
Only now are researchers uncovering the biological reasons why exercise blunts or spikes our appetite, helping or hindering us to lose weight. And it partly depends on the type of activity you do and how much sweaty effort you put in completing it.
The relationship between appetite and exercise is complex. Anyone who has embarked on a new workout regimen will vouch that pounds rarely drop as quickly as hoped, and it is known that everything from sleep patterns, body weight, fitness and genetics influence our individual urges to eat after exercise. Levels of hunger hormones, such as ghrelin (which increases appetite) and peptide YY (which reduces it), released after activity ebb and flow, making you feel more or less hungry.
An international team of scientists, led by Jonathan Long, an assistant professor of pathology at Stamford University, describe in their recently published paper a molecule called Lac-Phe. A hybrid of the compounds lactate and phenylalanine it’s released in higher amounts during intense exercise and seems to stifle appetite.
A study showed that when mice bred to not produce Lac-Phe were made to run intensely several times a week, they gorged on high-fat kibble each time they stopped, gaining 25 per cent more weight than regular mice.
“The team had previously shown that mice injected with Lac-Phe weren’t as hungry and ate up to 30 per cent less food,” says Gareth Wallis, associate professor of exercise metabolism and nutrition at the University of Birmingham, who was involved in the research.
For the latest study, they turned their attention to people, because they suspected the molecule might hold the key to understanding how exercise could aid weight loss. Eight young men were asked to exercise three times: a gentle bike ride, an indoor bike session with sprints, or weight training. The bike sprints were found to induce the most dramatic spike in blood levels of Lac-Phe, then resistance training and the slow cycle.
“What it showed us is that the Lac-Phe molecule goes up during vigorous exercise,” Mr Wallis says. “And since we know from animal studies that more Lac-Phe leads to fewer calories consumed, a natural conclusion would be to assume that it is involved in the suppression of hunger.”
For many people there will be a sense of familiarity about the findings. A long, slow jog of 80 minutes or more could easily see me raiding the fridge when I get back whereas a faster or hilly run can result in me not wanting to face food for several hours.
This, Mr Wallis says, is probably an example of Lac-Phe in action. “Generally the harder we work, more of the molecule is produced,” he says. “Although not everybody’s response to every type of exercise will be the same.” Swimming, for example, appears to be an anomaly.
Having conducted decades of appetite research at Loughborough University, exercise physiologists there were puzzled as to why swimmers, more so than any other exercisers, claim to be starving after a session in the pool or outdoors. As a student I shared a house with an international swimmer who would devour an entire loaf of white bread and lemon curd after her intense morning training. Even a few lengths in the local lido make me want to chew my own arm. What is it about exercise in water that makes us extra hungry?
To find out, the Loughborough team recruited 32 healthy recreational swimmers and cyclists and recorded their food intake after 60 minutes of swimming, 60 minutes of cycling – both performed as interval sessions with a repeated eight minutes of effort and two minutes of rest – or sitting, with each activity performed on a separate day, and with at least a four-day intermission between.
Half an hour after they finished each bout of exercise they were invited to a pasta buffet, where they could eat as much as they wanted until “comfortably full”. After swimming, they reported feeling hungrier and ate an additional 132 calories afterwards compared with sitting – the equivalent of a 25g packet of crisps or two digestive biscuits.
Cyclists consumed 94 extra calories, but it was not considered enough of a difference to be statistically significant to the effects of rest. “Temperature plays a role in appetite control,” Mr Wallis says. “Hunger after a swim is likely linked to the cool temperature of water and the extra energy used by the body to generate heat – we want to eat after a swim because food simulates warmth for our bodies.”
None of this means that exercise, even if it does make you hungry, isn’t worthwhile. “We know that exercise alone isn’t the best weight-loss tool and that it should be done in conjunction with healthy eating if you want to lose pounds,” Mr Wallis says. “But we also know that exercise does help to prevent you from gaining weight and it has an enormous number of beneficial metabolic effects.”
Embracing the new science of appetite control will help.
1. Consider gentle exercise before breakfast
Moderate activity on an empty stomach first thing in the morning has been shown to have two benefits by researchers at the University of Bath. Not only are you burning extra calories by working out, but your overall calorie consumption for the rest of the day is likely to be slightly lower too. It could be a “useful strategy to induce a short-term energy deficit”, the Bath team said.
2. If you do a hard workout, try to eat something two to three hours before you start
“What you consume in advance is very important in determining hunger afterwards,” Mr Wallis says. “This is partly because you will have burnt through your body’s stored fuel and the signals are to stock up reserves.” Consuming some carbs two to three hours beforehand is important because they are an easily accessible energy source during high-intensity exercise – reserves of which can be stored in the muscles for when you need them. “Also try to include protein at meals as it has a satiating effect to fill you up for longer,” Mr Wallis says. “And definitely after intense exercise, protein will help with the muscle repair process as well as stem appetite.”
3. Prone to post-workout bingeing? Exercise harder
According to Mr Long, the key outcome from the new data is that “intensity matters” if you want to feel less hungry when you finish. Whatever your fitness level, it pays to pick up the intensity or pace. That means adding hills or short bursts of sprinting or lung-busting effort to your walk, run or cycle, lifting weights or doing a regular HIIT or circuit class. “The evidence is that it will help to suppress hunger at least in the short term but possibly for longer,” Mr Wallis says.
4. Limit temptation
When unfit, overweight people first start exercising, they don’t necessarily feel hungrier afterwards. Tanya Halliday, an assistant professor of health and kinesiology at the University of Utah, asked 24 sedentary men and women to perform a brisk walk on a treadmill or to lift light weights for 45 minutes in the morning.
Levels of hunger-suppressing hormones were slightly elevated after each session, but that did not mean the exercisers ate any less at a lunch buffet compared with non-exercisers. All participants consumed about 950 calories of lasagne, salad, bread rolls and fruit pancakes, suggesting that “other factors” – namely the smell and sight of the food – enticed them to eat, Ms Halliday reported in Medicine & Science in Sports Exercise journal. The message? Have some healthy food on standby for when you finish.
– The Times
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