The Physiology of Weight Management

We live in a culture in which body weight is often misunderstood. While it is true that obesity is associated with adverse health outcomes, the number on the scale is not necessarily indicative of overall health. Pressure to be thin can lead to eating disorders and low self-esteem, and weight bias in healthcare can have dangerous consequences. People are often told that if they just “eat less and exercise more” they’ll lose the extra weight. However, more research is showing that it’s not that simple. My goal is to give you an overview of the biological systems involved and illustrate why weight management is a lot more complicated than just swapping burgers for salad.

Two terms that come up repeatedly when discussing weight are BMI and BMR. BMI, which stands for body mass index, is a measure of weight in proportion to height. It is used to determine whether someone is classified as underweight (BMI of less than 18.5 kg/m²), normal weight (BMI of 18.5-24.9 kg/m²), overweight (BMI of 25-29.9 kg/m²), or obese (BMI of 30 or greater kg/m²). You can calculate your BMI by dividing your weight in kilograms by your height in meters squared, or by using a simple online calculator. It is important to note that BMI does NOT differentiate between fat and muscle, which could lead to a very muscular, healthy person being technically classified as overweight. BMR, which stands for basal metabolic rate, is a measure of how much energy per day a person expends doing nothing (i.e. lying in bed). The most accurate way to determine someone’s BMR is through a process called calorimetry, which involves measuring inspired and expired gas flows or heat. Since it is usually not possible to perform calorimetry in a standard hospital or outpatient setting, dietitians can use equations to estimate BMR that consider height, weight, and age. We then multiply the result by an activity factor to get an estimate of how many calories per day are needed.

Although it is important to promote body positivity and fight weight bias, it is also important to recognize that excess body weight can have a negative impact on health. Fat is more than just tissue; it is a metabolically active organ that, in excess, is associated with chronic systemic inflammation and an increased risk for chronic disease. A modest weight loss of even 5% in obese individuals can have significant positive metabolic effects, such as lowered triglycerides and blood glucose. For sustainable weight loss in healthy adults we generally recommend aiming to lose 1-2 pounds per week. Although standard advice to achieve this is to reduce intake by 500-1000 calories per day, it is important to keep in mind that not all calories are treated equally by the body. For example, protein takes more energy to digest than fat or carbohydrate.

Body weight is controlled by two systems: the hedonic system and the homeostatic system. The hedonic system is that of pleasure and reward; why we eat things that taste good. The homeostatic system is the complex interplay between organs and the brain through hormones and neural signals, which regulates how much energy is taken in and expended. A few key hormones to know are:

Insulin: A hormone produced by the pancreas that stimulates the uptake of glucose from the bloodstream into the cells so that it can be used for energy.

Leptin: A hormone produced by fat cells that helps regulate weight gain by decreasing appetite and increasing energy expenditure.

Ghrelin: A hormone produced in the gastrointestinal tract that increases appetite.

Cholecystokinin (CCK): A hormone produced in the gastrointestinal tract and secreted in response to dietary fat and protein, which leads to satiation (a feeling of fullness).

We now have a multi-billion dollar weight loss industry and people are more confused than ever about what and how to eat. For years, it was thought that low-fat diets were the way to go. Fat, at nine calories per gram, is more energy dense than either protein or carbohydrate, so it makes sense that eating less fat would be an easy way to cut calories. However, fat also promotes satiety (how long you can go between meals), so eating too little fat can backfire and cause you to eat more overall. As low-fat diets have fallen out of style, low-carbohydrate diets have quickly stepped up to take their place. Low-carb diets can produce a significant initial weight loss because of water loss associated with the depletion of glycogen stores, but weight loss will slow down after this. People tend to have the most success with long-term weight loss with low-carbohydrate diets if they decrease consumption of refined carbohydrates and increase the relative proportion of fruits, vegetables, and lean proteins. The low-fat and low-carb debate is further complicated by the concept of insulin sensitivity, which is how efficiently your tissues respond to insulin and take glucose from the bloodstream into the cells. Someone with high insulin sensitivity might do very well on a high-carb, low-fat diet, whereas someone with low insulin sensitivity would be better off on a low-carb, high-fat diet. This is just another example of how individualized nutrition can be.

Eating frequency has also been in the spotlight. Some claim that eating small frequent meals is the best way to keep hunger in check and metabolism running efficiently, while others swear by intermittent fasting. There are few randomized controlled trials (the gold standard of clinical research) that examine the relationship between eating frequency and weight loss. Eating small frequent meals may lead to weight loss if they keep hunger in check and prevent overeating but could backfire if an individual adds snacks without compensating by decreasing the size of his or her normal meals.

It is an old adage that breakfast is the most important meal of the day, but the research on the relationship between breakfast consumption and weight management is not conclusive. Eating breakfast may help you to keep blood sugar levels stable and make it easier to make healthier choices throughout the day. There has also been some research to suggest that eating more of your calories earlier in the day rather than late at night can have beneficial effects on your weight and metabolic health. However, more research is needed. Getting tired of hearing me say that yet? Welcome to nutrition science.

Physical activity is another important piece of the puzzle. The current recommendation from health organizations such as the American Heart Association and the Centers for Disease Control and Prevention (CDC) is that healthy adults get at least 150 minutes of moderate physical activity per week for overall health. While exercise has proven metabolic health effects, research has shown that it is very hard to lose weight with exercise alone. While you do expend more energy when you’re active, people often overestimate the number of calories that they’ve burned and overcompensate with food. Exercise can also stimulate the appetite and cause you to eat more overall. This isn’t to say that you shouldn’t exercise; exercise has proven to be beneficial for metabolic and psychological health. Building muscle can also help to boost your BMR because muscle tissue requires more energy to maintain than fat. Just don’t treat exercise as a free pass to eat whatever you want. Research has shown that the people who have the most success maintaining weight loss are those that make exercise a part of their everyday routine.

This article is only a brief foray into some of the complex mechanisms that govern weight management. It does not begin to cover all the processes involved, partly because many of them are still unknown, and partly because if it did, you would still be reading it tomorrow. The microbiome, genetics, and even exposure to different environmental chemicals throughout the lifespan may also play a part. I didn’t even touch on the social, political, and environmental factors that affect weight and health — food cultures and traditions, public health laws, food desserts, and the lack of unsafe spaces for outdoor physical activity in some places, to name a few. This is a particularly complex and controversial topic, and as always, I would love to hear from you. Please feel free to reach out with any questions or comments!

References

Anthanont, Pimjai, and Michael D. Jensen. “Does basal metabolic rate predict weight gain?.” The American Journal of Clinical Nutrition 104.4 (2016): 959-963.

Balthasar, Nina. “Genetic dissection of neuronal pathways controlling energy homeostasis.” Obesity 14.S8 (2006).

Blundell, John E. “Perspective on the central control of appetite.” Obesity 14.S7 (2006).

Gardner, Christopher D., et al. “Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women: the A TO Z Weight Loss Study: a randomized trial.” The Journal of the American Medical Association 297.9 (2007): 969-977.

Korner, Judith, and Louis J. Aronne. “The emerging science of body weight regulation and its impact on obesity treatment.” Journal of Clinical Investigation 111.5 (2003): 565.

Madjd, Ameneh, et al. “Beneficial effect of high energy intake at lunch rather than dinner on weight loss in healthy obese women in a weight-loss program: a randomized clinical trial.” The American Journal of Clinical Nutrition 104.4 (2016): 982-989.

Raynor, Hollie A., and Catherine M. Champagne. “Position of the Academy of Nutrition and Dietetics: Interventions for the Treatment of Overweight and Obesity in Adults.” Journal of the Academy of Nutrition and Dietetics 116.1 (2016): 129-147.

Seagle, Helen M., et al. “Position of the American Dietetic Association: weight management.” Journal of the American Dietetic Association 109.2 (2009): 330-346.