Understanding the Human Metabolism

This article aims to explore the human metabolism in more detail and how it’s adaptive nature can provide obstacles for weight loss. Having a grasp of how the human metabolism operates will provide a clearer understanding of why plateaus occur and how to push through them.

So firstly, what is the ‘metabolism’? 

You can view the metabolism as a kind of internal ‘furnace’. It represents all the internal chemical reactions taking place within the cells of the body. There are 3 principle components of human metabolism: -

  • Basal Metabolic Rate (BMR) - Energy expended when a person is supine (lying down) at rest, in the morning, after sleep. BMR represents the rate at which the body expends energy to sustain basic life processes. In sedentary populations, BMR accounts for around 60% of total daily energy expenditure.
  • Thermic Effect of Food (TEF) - Relates to the increased energy expenditure associated with the digestion, absorption and storage of food and accounts for between 10-25% of total daily energy expenditure.
  • Activity Thermogenesis - Can be split into Non-Exercise Activity Thermogenesis (NEAT) and exercise-related activity. NEAT refers to any energy expenditure of daily living that is not formal activity, such as walking to work, taking stairs, fidgeting, shopping etc.

So what is ‘Adaptive Thermogenesis’? 

The best definition I have seen is by Hall (2009) who defines it as “an adaptation of metabolic rate that opposes weight change”. One of the earliest demonstrations of the existence of an adaptive decrease in thermogenesis in response to energy restriction was the Minnesota Study of 1950. Not only does human metabolism decrease in response to an acute (short term) reduction in energy intake, but the Biosphere 2 Study also demonstrated that there is a decreased energy expenditure during the maintenance of a reduced body weight over the longer term (2 years in the case of this study) (Weyer, et al, 2000). The adaptive nature of the human metabolism is why the mythical 3,500kcal weekly energy deficit has been debunked (Hall and Chow, 2013). One of the most pervasive weight loss ‘rules’ is that an energy deficit of 3,500kcal per week (500kcal per day) is required to lose 1lb (0.45kg) of body weight. However, this is a gross simplification, and doesn’t take into account the dynamic nature of human metabolism.

One of the most variable aspects of metabolism is activity thermogenesis. For example, Rudolph et al (1995), found that after a 10 or 20% loss in body weight, there was a decline in total energy expenditure, resulting from decreases in both resting and non-resting energy expenditure. Levine (2005) also found that with studies that underfed subjects, NEAT and physical activity both decrease.

“This creates an intriguing scenario whereby NEAT might act to counterbalance shifts in energy balance” (Levine, 2005)

Why does Weight Loss Often Plateau? 

There is a clear body of evidence indicating the adaptive nature of the human metabolism. This creates difficulties for people wanting to lose body weight and is often the reason why people hit weight loss plateaus despite seemingly being in an energy deficit. Recommendations we like to use are periods of ‘diet breaks’ or periods of 1-2 weeks returning to a clients’ baseline calories between periods of dietary restriction. In general, 12-16 weeks of sustained negative energy balance is as long as we go before giving the client a period of time of increased calories. This has the effect of not only providing the client with a psychological break from dieting, but it also gives the body a chance to increase it’s metabolism. This is because adaptive thermogenesis occurs both ways; decreased metabolic rate with decreasing energy intake and conversely increasing metabolic rate with increasing energy intake. Returning a client to energy balance will increase TEF as more food is being consumed. Returning a client to energy balance will also increase thyroid hormone output (Weinsier et al, 2000), decrease appetite, increase activity, and make fat mobilisation easier (McDonald, 2005). Essentially, the goal of a ‘diet break’ or period at maintenance calories is to correct the adaptive reduction in metabolic rate & allow the diet to proceed more effectively once calories are reduced again. The study by Wing & Jeffrey (2012) found that prescribing occasional breaks to a weight loss program had no negative impact on overall weight loss.

Non-linear dieting is another approach to dieting that counter-acts this decreasing metabolic rate. Oftentimes diets will prescribe a moderate caloric deficit every single day (usually around 10-20%). As seen above, a linear approach such as this will only work for so long before a plateau is reached and weight loss stalls. Non-linear diets will involve calorie fluctuations across the week. For example, 2 days of the week could see an aggressive caloric deficit (40%) and these days could coincide with days a person does not train. 3 days could involve a more moderate deficit (10%-15%) and 2 days could involve eating at maintenance calories or even in a slight surplus. So for someone with a maintenance calorie intake of 3,000 calories the week may look something like this: -

Monday: 15% Deficit (2,550kcal)
Tuesday: 40% Deficit (1,800kcal) 
Wednesday: Maintenance (3,000kcal) 
Thursday: 15% Deficit (2,550kcal) 
Friday: 40% Deficit (1,800kcal) 
Saturday: Maintenance (3,000kcal) 
Sunday: 10% Deficit (2,700kcal) 
Total Weekly Deficit: 3,600kcal

So as you can see from the example above, the average calorie deficit across the 7 days is 514kcal, which is an average of a 17% deficit from a baseline of 3,000 calories. However, because of the higher calorie days, there is far less metabolic slow down, allowing fat loss to occur without hitting a plateau. I also find this kind of dieting far easier for people to tolerate and adhere to in the long term. There are only 2 days with very low calories and there is the psychological break of knowing that there are 2 higher calorie days. This approach is usually far better tolerated than trying to maintain the same moderate calorie deficit day-in, day-out.
Here is another approach that works well with people who are social at weekends: -

Monday:  25% Deficit (2,250kcal)
Tuesday: 25% Deficit (2,250kcal)
Wednesday: 25% Deficit (2,250kcal)
Thursday: 25% Deficit (2,250kcal)
Friday: 25% Deficit (2,250kcal)
Saturday: Maintenance (3,000kcal)
Sunday: Maintenance (3,000kcal)
Total Weekly Deficit: 3,750kcal

The approach above calls for larger deficits from Monday-Friday, which then allows more freedom at weekends, which is typically when people eat out more often to socialise etc.

It must be emphasised here that as weight loss occurs, a persons’s Resting Energy Expenditure (REE) will naturally decline, so this will naturally alter what someone’s maintenance calorie intake will be. Therefore it is important to periodically re-calculate someone’s calorie needs to ensure the deficit there are in is sufficient to elicit the appropriate rate of weight loss.
It is not the scope of this article to go into how large calorie deficits should be. However, as a general rule, the more body fat you have to lose, the greater the calorie deficit you are able to utilise without running the risk of losing lean muscle mass. The leaner an individual becomes, the more conservative you have to be with caloric deficits, as there will be less body fat available as a fuel source and hence the risk of losing muscle mass increases. 

At this point the role of resistance training in the process of weight loss and improved body composition should be emphasised. Engaging in resistance training throughout the process of dieting is strongly advised. Resistance training elicits a range of morphological and neurological adaptations that contribute to changes in muscle function with respect to size, strength and power. Not only can these adaptations support improved athletic performance, but can also improve health-related musculoskeletal function (Egan & Zierath, 2013). Resistance training, through enhancement of physical activity-related energy expenditure, elevation of post-exercise basal metabolism, and preservation of lean mass can compensate, at least partly, for the decrease in energy expenditure in response to low energy intake (Major et al, 2007). Performing resistance training will assist in the increase in muscle tissue, which will, in turn, help to increase a person’s BMR.

Take Home Points: -

  • Human Metabolism is adaptive and increases or decreases based on energy intake and a person’s lean muscle mass
  • The adaptive nature of the metabolism often means purely linear approaches to dieting hit a plateau at some point
  • ‘Diet Breaks’ and other non-linear approaches can be utilised to prevent plateaus from occurring
  • Undertaking resistance training whilst dieting is of the upmost importance to preserve lean muscle mass, elevate metabolism post-workout, improve insulin sensitivity as well as help protect against a raft of health problems such as Type II Diabetes, obesity, hypertension, heart disease etc.
  • When setting up a diet the most important factor is whether someone can stick to it or not. There are numerous ways of structuring diets to suit the individual which is something we strive to do here at Physique Wise. 

References: -

Hall, K, “Predicting Metabolic Adaptation, Body Weight Change and Energy Intake in Humans”, American Journal of Physiology, 2009.
Hall, K, “What is the Required Energy Deficit Per Unit Weight Loss”, International Journal of Obesity, 2008.
Rudolph, L et al, “Changes in Energy Expenditure Resulting from Altered Body Weight”, New England Journal of Medicine, 1995.
Levine, J, “Nonexercise Activity Thermogenesis (NEAT): Environment and Biology”, American Journal of Physiology, 2005.
Weyer, C et al, “Energy Metabolism after 2 Y of Energy Restriction: The Biosphere 2 Experiment”, American Journal of Clinical Nutrition, 2000.
Major, G et al, “Clinical Significance of Adaptive Thermogenesis:, International Journal of Obesity, 2007.
Gropper, S & Smith, J, “Advanced Nutrition and Human Metabolism”, 2012.
Hall, K & Chow, C, “Why is the 3500kcal per Pound Weight Loss Rule Wrong?”, International Journal of Obesity, 2013.
Weinsier, R et al, “Do Adaptive Changes in Metabolic Rate Favour Weight Regain in Weight Reduced Individuals? An Examination of the Set-Point Theory”, The American Journal of Clinical Nutrition, 2000.
Egan, B & Zierath, J, “Exercise Metabolism and the Molecular Regulation of Skeletal Muscle Adaptation”, Cell Metabolism, 2013.
McDonald, L, “A Guide to Flexible Dieting”, 2005.