We’ve all seen those flashy headlines that promise effortless weight loss by simply adding a few key foods that are *guaranteed* to increase your metabolism. But is this true? Can what you eat actually alter your metabolic rate? To answer this question, we need to first broaden our understanding of what metabolism is and isn’t.
Metabolism and Energy Expenditure
‘Metabolism’ refers to a series of chemical reactions that break down what we eat into energy and other important compounds that are used to sustain life. In contrast, total energy expenditure is the amount of energy (calories) that we need to eat to maintain (1):
Basal energy expenditure (BMR) – the calories your body needs to perform basic, life-sustaining functions. Accounts for ~60% of total energy expenditure.
Thermic effect of food (TEF) – the calories your body spends digesting food. Accounts for ~10% of total energy expenditure.
Physical activity – this category can be further broken down into 1) non-exercise activity thermogenesis or NEAT – the calories you expend as part of daily living, and 2) exercise-related activity thermogenesis – the exercise you spend in structured activity. Adults expend, on average, between 0-10% of total energy expenditure on structured activity with the remainder being spent on NEAT.
To further clarify these two concepts, think of energy expenditure as the total number of calories you need to eat to sustain your current body weight and lifestyle, and metabolism as the biological pathways that determine how your calories will be spent.
Can Diet Increase Metabolism?
Like many nutrition questions, there isn’t a clear yes/no answer. One way to look at this is to see if what we eat can have a direct impact on speeding up/slowing down our energy expenditure (i.e. can what we eat alter the thermic effect of food?).
A recent meta-analysis (2) looked at 27 different trials to determine what effect different levels of energy intake and meal composition had on the thermic effect of food. The authors found that a higher-calorie breakfast resulted in a very slight increase in the thermic effect of food (each additional 96 calories increased the thermic effect of food by 1 calorie). The authors also found that meals with higher protein had a greater impact on the thermic effect of food compared to carbohydrates or fat. Earlier studies have also found this to be true and it’s reported that protein, carbohydrates, and fat are responsible for 20-35%, 5-15%, and 5-15% of total calories expended, respectively (3).
Other factors that may influence the thermic effect of food include the type of fat (i.e. medium-chain fats were found to have a higher thermic effect compared to long-chain fats) and consuming the calories of a meal at one time as opposed to multiple times throughout the day (2). Eating pace and palatability were not found to have any influence on the thermic effect of food (2).
The Bottom Line
At the end of the day, it’s important to recognize that your energy expenditure is largely out of your hands. However, what is in your control are your health behaviours. Eating an overall balanced diet that is slightly higher in protein (i.e. 25% of calories from protein), tracking your food intake in Cronometer, and incorporating regular strength-based activity is the best approach to achieving and maintaining a healthy body weight (and metabolism).
Collectively, these findings suggest that diet alone has a very small (and likely insignificant) impact on our total energy expenditure. However, what does influence our energy expenditure is fat-free mass (FFM). It’s well accepted that resistance training that results in muscle hypertrophy (growth in muscle size) and hyperplasia (increase in the number of muscle cells) leads to an increase in FFM and consequently, metabolism. On the other hand, significant weight loss and calorie restriction will inevitably lead to a lower metabolic rate because of 1) a lower calorie intake, 2) reduced body size, and 3) loss of muscle.
Quatela A, Callister R, Patterson A, MacDonald-Wicks L. The Energy Content and Composition of Meals Consumed after an Overnight Fast and Their Effects on Diet Induced Thermogenesis: A Systematic Review, Meta-Analyses and Meta-Regressions. Nutrients. 2016 Oct 25;8(11). pii: E670. Available from: https://www.ncbi.nlm.nih.gov/pubmed/27792142