Overfeeding studies can be categorized as studies that seek to investigate the consequences of chronic overfeeding, or consistently overeating calories, in humans and their biology. As we know from part 1, overfeeding studies may seem pretty straightforward, but they can actually teach us a lot about the unpredictability of humans and our bodies.
Alongside the obvious body composition and weight changes that come from overfeeding, changes in metabolic rate, total energy expenditure, and even disparities in eating behavior are also impacted and worth discussion. Consequently, there’s always something to take away in how we apply these points to ourselves and our clients.
More Fuel = More Burned?
As it relates to metabolic rate, metabolized fuel, and total energy expenditure, overfeeding spurs up some more questions.
Resting metabolic rate (RMR), for one, is shown to represent the largest portion of total daily energy expenditure in both sedentary and recreationally active people. Once overfeeding is thrown into the equation, however, no stark or reliable increases in RMR are observed when data is adjusted for body weight and fat free mass. These results are also independent of the carb or fat content within the overfed calories.
Thermic Effect of Food
Thermic effect of food (TEF), or diet-induced thermogenesis, is essentially the amount of energy we burn upon consuming foods. When drastically overeating calories, much of the present data shows that any increase in TEF is quite small and non-significant. It also remains unclear if this small increase is due to the rise in RMR. In addition to this, once exercise was added to an overfeeding protocol lasting 10 days, RMR was shown to have increased significantly.
Now considering individual macronutrients, protein intake may produce more variable results in RMR values. One study design had prescribed caloric surpluses, with protein contents ranging between 5%, 15%, and 25%. A significant increase in RMR was observed the following day in normal and high protein diets. Protein can also have an impact on TEF; protein is much more satiating as it takes longer for us to digest. In that, higher protein intakes are often associated with higher TEF responses.
Total Daily Energy Expenditure
As far as total energy expenditure is concerned, there are quite a few inconsistent results here as well. Some studies show increases in total EE, while others do not. Most of the evidence, however, suggests that total EE is increased although depending on the testing methods, it may not always be detected. When comparing carbohydrate and fat overfeeding, carb overfeeding was shown to increase total EE in subjects whereas fat overfeeding did not produce any changes.
Eating Behaviors Post-Overfeeding
Feelings and behaviors around and towards food can differ greatly person to person. This is dependent on a lot of factors like dieting history, experience with diets (restrictive backgrounds), body composition, current goals/weight status, surrounding environment (food/family), and so on.
Subjective feelings towards food or specific kinds of food (i.e. fast foods, carbohydrates, snacks/sweets, etc.) can change significantly after acute periods of overfeeding. This is influenced even more by the individual experiences and eating behaviors we all partake in as previously mentioned.
Hunger and the desire for foods, for example, are shown to change significantly after just 1 day of overfeeding or underfeeding. Upon using the three-factor eating questionnaire, which assesses disinhibition, restraint, and hunger, a greater dietary disinhibition was linked to adults who were more prone to obesity, compared to adults who were more resistant. This greater level of disinhibition, in turn, lead to a greater weight gain over time (shown in studies where follow-up periods were employed).
Another noteworthy mechanism in rebalancing intakes and recovering from bouts of overfeeding is called cognitive restraint. This can be defined as a sort of appetite control mechanism that we rely on to regulate our energy balance. Using cognitive restraint, however, seems to be quite inadequate once we take into account individual variability. Some subjects were observed to be “compensators,” or those who compensated with a significant drop in energy intake, while others were considered “noncompensators,” who failed to reduce their intakes to make up for the food surplus prior.
Almost all subjects who undertook overfeeding protocols were able to return, or closely return, back to their starting baseline weight over a timespan of weeks to months. Though, there is a significant amount of variability in response to overfeeding and the time period following it that gives us an understanding of just how many responses can be seen.
Subjects who are obese and non-obese prone, for one, proved to be a big factor influencing individual responses to overfeeding and subsequent appetite control mechanisms. This can easily be tied to the fact that obesity is a multifactorial issue – one unique to the individual, their environment, family, dieting history, health status, and much more. In addition to this, the differences in cognitive restraint, or any appetite control mechanism, show us that some people may need to employ more intentional cognitive restraint than others, rather than relying on “intuitive” responses.
These findings emphasize for us just how much variability exists with the human body, how we gain and lose weight, and how we control and regulate our own eating behaviors in response to particular scenarios. It’s not enough to simply prescribe a plan and predict every next step for the client because it’s nearly impossible to know how every individual will respond.
Taking into account the client’s dieting history, their mindset and attitudes towards dieting and food, and their individual circumstances are solid starting positions for understanding the steps you’ll need to take and the methods that will benefit the client most.
Bray, G.A., Bouchard, C. The biology of human overfeeding: a systematic review. Obesity Reviews. 2020; 1-78.