If your goal is to lose weight, then you need to create a caloric deficit. This can be done by reducing eating less calories or by increasing caloric expenditure via exercise (ignoring the complexities of the body and nutrition such as reducing caloric intake by replacing added fats with nuts).
Of course, for weight loss to be successful, the intervention must be sustainable. This led Cameron et al to investigate how dieting alone or aerobic exercise alone differently affects appetite and appetite-related hormones, food hedonics and food reward, and olfaction.
Ten young, healthy male volunteers were recruited to undergo three 3-day interventions (with a 2-week washout between each) in which each participant first completed a control condition, followed by a diet or exercise conditions (in random order, so some started with diet and others started with exercise). Thus, this was a crossover study with the sequence of testing being either CON-DIET-EX or CON-EX-DIET (figure 1).
All testing was conducted before and after the 3-day interventions. During the control condition, food intake and physical activity levels were monitored so as to determine each participant’s weight-maintenance energy requirements. During both the DIET and EX conditions, a 25% caloric deficit was induced through a reduction in food intake or increase in exercise energy expenditure, respectively. This corresponded to roughly 710 kcal and 64 minutes per day of supervised treadmill exercise at 50% of VO2max (which was determined via pre-study testing).
To ensure that food intake was controlled for, the participants were sent home each day with a 62-item, pre-weighed food cooler. Thus, each participant could self-select their diet, but only from a calorically-controlled selection of food.
After completing each 3-day intervention, there were no significant differences in bodyweight or DXA-determined body fat mass, body fat percentage, or fat-free mass between groups. However, it is notable that bodyweight was about 1.5 kg lower after the DIET and EX periods compared to CON, primarily from a reduction in fat-free mass (which was marginally significant at p=0.06). This may very well just be water weight.
Appetite and hormones
On the morning of day 4 after each intervention, the participants came to the research lab fasted for appetite-related outcomes to be measured. A standardized breakfast (2 pieces of whole-wheat toast, 17 g peanut butter, 15 g raspberry jam, and 250 mL water for a total of 300 kcal) was consumed at 9:00 am, with appetite measures made before and every hour thereafter.
A standardized lunch (2 slices of cheese pizza at 780 kcal) was consumed at noon, followed by an all-you-can-eat dessert buffet from 12:30-1:00 pm where 5 snack food items and 4 fruit items were offered to participants. These items were individualized, being indicated as favorite foods by each participant during screening.
Both DIET and EX were associated with a significantly greater palatability of the standardized meals compared to CON. However, after eating breakfast, only DIET was associated with significantly greater desire to eat, hunger, and prospective food intake compared to CON, while EX was not significantly different from either group (it fell in the middle). Fullness showed the greatest values with EX then DIET then CON, but these differences were not statistically significant.
In line with the increased appetite ratings, the DIET group consumed significantly more calories during the buffet than the other groups. It also consumed significantly more food in terms of overall weight compared to CON, while EX once again fell in the middle and was not significantly different from either group. Interestingly, macronutrient composition and energy density of the buffet foods was not different between groups, suggesting that dieting leads to increased appetite and food intake without preference for a certain nutrient or type of food.
There were no significant differences between the groups for concentrations of ghrelin (hormone that makes you hungry) or leptin (satiety hormone).
Relative-reinforcing value of food (RRVF)
The reinforcing value of a stimulus refers to how much behavior the stimulus will support. This can be objectively observed as an increase or a decrease in the willingness to work for points to obtain a desirable food stimulus (compared with some alternative). Accordingly, the participants played a computer game between breakfast and lunch in which they had to match shapes in order to “earn” food points. These points could then be “spent” on their favorite fruit or snack item (determined via pre-study screening) that would be offered at the buffet for them (they didn’t know that everything would be there regardless).
Both DIET and EX resulted in significantly more snack points being earned compared to CON, indicating that the participants were willing to put forth more effort to eat their favorite foods during these conditions. Moreover, snack points were significantly greater in EX vs DIET.
Measuring the smell function of each participant was a relatively novel aspect of this study, based on the idea that smells play a central role in food hedonics and in determining food acceptability. The researchers hypothesized that both DIET and EX would lead to increased smell performance compared to CON because when your body thinks you are starving; why wouldn’t it increase your ability to smell?
This did indeed appear to be the case, as the odor threshold was significantly greater for DIET and marginally significantly greater (p=0.06) for EX when compared to CON. However, odor discrimination and odor identification were not different between groups, suggesting that smell was enhanced globally rather than specifically for certain smells.
Better to be more active than simply eat less
The exercise intervention used in this study was 1 hour of walking at a brisk pace – something that is realistic and achievable by most people. Similarly, the energy deficit corresponded to 500-700 kcal/day, which is common among weight loss interventions (theoretically ~1 pound per week). So if you had to choose between cutting calories or moving more, which would it be?
According to the results at hand, only cutting calories increases appetite and food intake despite exercise resulting in greater effort put forth to obtain favorite foods. However, both increase your ability to smell, possibly so as to make you more sensitive to delicious food that may come along. Therefore, if you are a young, healthy man looking to shed some weight, increasing physical activity may be the more sustainable option. Of course, we cannot readily infer that similar things would occur in the less healthy general population or in women.
We also cannot assume that more physical activity would benefit weight loss over the long-term. However, it is notable that a recent study showed increasing physical activity over eight weeks to not lead to increased food intake, thus resulting in a prolonged negative energy balance and weight loss. Moreover, physical activity is sustainable for life, unlike dieting that can only last until you starve to death. Furthermore, there are countless other health benefits to being more active.