Weight regulation and the obesity epidemic in itself is multifactorial in nature. There is a plethora of different factors at play when we start to consider why and how weight gain occurs, and subsequently persists long term for certain individuals and/or populations.
A widely researched topic of interest that provided us with valuable baseline information about weight regulation is known as human overfeeding studies. Essentially, these studies aim to figure out the ins and outs of weight gain and the adaptations that can happen to us as a result of chronic overfeeding.
While this topic seems pretty straight-forward (i.e. overeating for ‘x’ amount of time), there is a lot of data to consider with over 300 studies on the subject. Not only that, but these studies cover a very comprehensive range of variables and include various kinds of populations that vary in age, background, dieting history/disorder history, body composition/BMI status, exercise level, gender, and much more. Study variables differ greatly within these study designs as well, such as the types of diets prescribed and their macronutrient ratios (high fat, high carb, etc.), the kinds of foods eaten (candy vs. peanuts), and the methods in which subjects were eating (nibbling/grazing vs. gorging) to name a few.
So… there’s basically a LOT of variation within this topic.
With that said, it’s best we break it down and cover the main adaptations and concepts we’ve come to understand more about because of these studies:
- Body weight, body composition, adipose tissue vs. lean body mass
- Energy metabolism, behavioral traits, and human variability
Changes in Body Weight and Composition
It’s no surprise that a linear relationship exists between weight gain and the number of overfed calories. Between free-living and in-patient (metabolic ward) subjects in these studies, weight gain patterns were similar, albeit within a wide range of about 1.4 to 8.1 kg (or 3 to 18 lbs.) of weight gain.
There is a substantial amount of human variability in response to chronic overfeeding. This wide range in weight gain goes to show the first example variability that can be observed person to person. 3 pounds doesn’t seem like much, but 18 pounds is a very different story!
Factors to consider
It should be noted that the studies reviewed were ones that prescribed either a fixed number of overfeeding calories over their estimated maintenance intakes or a percentage of daily energy intake based on baseline RMR.
Some noteworthy variables that could play a part in this inconsistency include:
- The accuracy of baseline estimates as they relate to energy expenditure/weight maintenance.
- Macronutrient ratios of the overfed calories. Higher protein intakes show an impact on reducing variation in response to weight gain.
- Different levels energy expenditure among the subjects. Fidgeting and low-energy costing activities that take place constantly day to day. Non-exercise activity thermogenesis (NEAT) can add significant amounts of variability to one’s expenditure.
Fat Mass and Lean Mass
The moment we’ve been waiting for – what does our lean body mass gain look like if we drastically overfeed calories?
Well it doesn’t look too good as most of the weight gain that occurs from overfeeding is in adipose tissue relative to lean mass gains. Across the studies included in this review, lean body mass gains are likely to be less than the gains observed in body fat (about 30-40% from LBM and ~60% of the increase from fat mass).
Obviously, there’s quite a strong relationship between the number of calories overfed and the resulting weight gain. We knew this for the most part (calories in versus calories out). The more interesting part is the variances seen among the subjects; it’s reported that subjects who were considered the highest gainers gained three times as much weight as the lowest gainers.
It should be noted, however, that the majority of these study populations were considered to be overweight and obese and were not performing any extra physical activity or resistance training, so these outcomes should be interpreted considering those variables.
A Key Point
One major thing we can take away here is the idea that not every individual with respond the same under similar circumstances. This is especially so when body composition changes and weight management is concerned. We see this all the time with body fat setpoints, rates of weight loss/weight gain, and the many variations with caloric intakes/metabolic rates; it is simply something we cannot fully predict with so many factors at play.
There’s a much bigger picture to grasp regarding overfeeding. In the next part, we’ll dive more into changes in metabolic rates and energy expenditure, noteworthy behavioral traits that relate to overfeeding, and how we apply these points to ourselves and our clients.
Bray, G.A., Bouchard, C. The biology of human overfeeding: a systematic review. Obesity Reviews. 2020; 1-78.