Not losing weight? It’s not uncommon for struggling dieters to blame the struggle to lose weight on a slow metabolism. But, what exactly does that mean?
The preponderance of the energy you burn every day falls under the category of resting metabolic rate. These are the baseline calories you burn when you’re not moving around or digesting your food. Your body needs energy for cellular, organ, and tissue function, including breathing, transporting oxygen, and pumping fluids throughout your body. These are basic functions you need to stay alive.
As long as you’re alive, there’s a baseline level of metabolic function that requires calories. For most people, this corresponds to 60% to 70% of total energy expenditure. Another component called activity thermogenesis are calories you burn during exercise ( between 20 and 25%) while the smallest component is the thermic effect of food. These are the extra calories your body needs to digest the food you eat. The thermic effect of food accounts for around 10% of total daily energy expenditure.
Since the bulk of your calorie burn takes place when you’re not eating or exercising, a slow metabolism is a slow resting metabolism. Of course, you can’t assume that you have a sluggish metabolism based only on the fact that you aren’t losing weight. Too many other factors can make it harder to shed weight and body fat.
What DOES impact resting metabolism? Genetics, how big you are, how much muscle mass you carry, all influence how rapidly you burn calories. Age is another important determinant – resting metabolism slows with age, partially due to loss of muscle tissue. In addition, men have a slightly higher metabolic rate than women, partly related to larger body size and muscle mass. Certain health problems, particularly an under-active thyroid gland, and even medications can slow your metabolic rate. These are all factors to consider if the weight isn’t budging. If you’re under-eating and over-exercising to lose weight, that can cause your metabolism to slow down as well, at least temporarily.
Is a Slow Metabolism Really to Blame?
For most people, it’s not a slow metabolism that accounts for slow weight loss. Often, it comes down to a mismatch between calorie intake and calorie expenditure. You may THINK you’re in a calorie deficit – but if you monitor your intake and calorie output more closely, you’re not. For example, most people underestimate how much they eat and overestimate how many calories they burn during a workout.
How bad are we at estimating calorie intake? In one study, researchers found that people visiting a fast food restaurant underestimated the calories they consumed by an average of 175 calories. A smaller group, around 25%, underestimated by a whopping 500 calories.
The fact that restaurants often double or triple standard portions distorts the perception of what a serving really looks like. Plus, we often don’t account for liquid calories – like coffee drinks loaded with sugar. Don’t forget, some Starbucks drinks have calorie counts that exceed 600 calories for a single drink.
We’re so influenced by the obesogenic environment and supersize portions that we’ve lost the ability to judge how many calories we’re actually eating. Yet another study carried out by researchers at Cornell found that overweight people underestimate calorie intake by as much as 40%. Even lean people underestimate the calorie content of meals, although not as much, around 20%.
Exercise Burns Calories, so Why Are You Not Losing Weight?
Do we misjudge calorie burn too? Unfortunately, we do. Not only do cardio machines overestimate the numbers of calories burned during exercise, both men and women are poor judges of calorie expenditure during a workout. In one study, individuals who exercised on a treadmill at 50% of their V02 max estimated their calorie expenditure to be 3-4 times what it actually was! If you plan your calorie intake around that, don’t expect to lose weight.
Unfortunately, there’s also a tendency to overcompensate for a workout by eating more simply because you exercised. Have you ever rewarded yourself after a workout with a brownie, cookie, or a frou-frou coffee drink? Chances are what you treated yourself to contained more calories than what you burned off. It’s a common problem.
Other Factors that Slow Weight Gain
How much do you sleep at night and is it quality sleep? Insufficient sleep can cause weight gain independent of the impact it has on your metabolism. When you’re sleep deprived, your appetite hormones, leptin, and ghrelin, are affected and it fires up your appetite. Longer term, too little sleep can raise your cortisol level and trigger a rise in belly fat. Long-term stress can do the same thing. It’s important to address these issues and take steps to correct them before assuming it’s a problem with your metabolism.
The Bottom Line
An inherently slow metabolism typically isn’t the reason people don’t lose weight. It’s more often a lack of awareness of calorie intake and output. Keep a record for a few weeks of what you’re eating, how much you’re sleeping, your stress level and how many calories you’re REALLY burning through exercise. When you write everything down, it helps get an objective picture of your diet and exercise energy expenditure. Make sure that you’re eating whole, unprocessed foods since packaged foods and foods high in sugar won’t fuel your body properly and trigger cravings that cause you to overeat.
Finally, if you objectively look at your diet and exercise routine and discover you’re doing everything right, check with your health care provider. Make sure you’re not taking a medication that hampers weight loss and that your thyroid function is normal. The most common cause of a slow metabolism in women is an autoimmune thyroid condition called Hashimoto’s thyroiditis, a condition that becomes more common after menopause. Hopefully, you’ll get a clean bill of health.
CNN. “You’re eating more calories than you think”
Cornell Chronicle. “It’s the size of the meal, not the size of the person, that determines how people underestimate calories, Cornell study finds”
J Sports Med Phys Fitness. (2010) 50(4):377-84.