Metabolic Adaptations – You’re trying to take off that last five pounds but your body is fighting you. You’ve dropped your calorie intake and are doing an hour of exercise daily and getting nowhere. Why, despite how hard you’re trying, is your body conspiring against you?
In reality, your body is protecting you. If there’s one thing your body needs, it’s a steady energy supply. If it perceives that you’re not taking enough calories and energy reserves are falling low, it starts to hoard that energy, using only enough to get by. Despite calorie restriction and the calories you burn through exercise, your body fights fat loss.
This economizing all relates to the concept of homeostasis – your body likes a certain degree of “sameness.” In fact, the word comes from the Greek word for “steady” and “same.” Just as your body tries to maintain your body temperature in a narrow range, it tries to do the same for your weight. Sadly, it’s not the least bit concerned about how you look in a swimsuit or that you’re struggling to drop your body fat percentage. Rude, isn’t it?
So, how does it get in the way of your goals? The way your body tries to maintain sameness with regard to your body weight is through metabolic adaptation. It does this in three primary ways. Let’s look at each one:
Metabolic Adaptations: Hormonal Adaptations
When you restrict calories too much, it places stress on your body. The degree of stress depends on how low you’ve dropped your calories and how much you’re exercising particularly aerobic exercise. This stress can impact the function of your thyroid gland, the “master gland” when it comes to your metabolism. Normally, your thyroid converts T4 to the active form of thyroid hormone T3. When your body is stressed by calorie restriction, it converts some of the T4 to reverseT3. Unfortunately, this form of thyroid hormone is inactive. So when you have an abundance of reverseT3, you have less active thyroid hormone to keep your metabolism primed.
Other hormones also change when you aggressively try to lose weight. If you diet for a prolonged period of time, your adrenal glands produce more cortisol. Although cortisol causes fat breakdown short term, long term it leads to the breakdown of muscle tissue as well and redistribution of fat. The results? You end up with more fat around your midsection. It also has the undesirable effect of suppressing your immune system.
In extreme cases of long-term calorie restriction and excessive exercise, you can experience a drop in sex hormones, like estrogen. This can lead to bone loss, a lack of menstrual periods, and infertility.
A real-life example of how your metabolism slows when you restrict calories or lose a significant amount of weight are the winners of the Biggest Loser show. A study found that the participants still had a resting metabolism that was 500 calories per day slower even 6 years after losing the weight. Despite regaining the pounds they lost, they still struggled with a slowdown in metabolism.
Metabolic Adaptations: Changes in Appetite
When you’re underfeeding yourself, your body tries to solve the problem by making you eat more. It does this by decreasing leptin, a satiety hormone produced by fat cells, and by increasing ghrelin, an appetite-stimulating hormone produced by cells that line your stomach. Once these two appetite hormones take on their respective roles, you experience hunger and cravings for foods high in fat and sugar.
How come those cravings are never for nutrient-dense, low-calorie foods like vegetables? Unfortunately, those foods don’t activate our brain’s dopamine response, the portion of the brain involved in reward. So, instead, we crave cookies, brownies, ice cream and other not-so-healthy items that lead to a dopamine surge. Plus, when ghrelin is screaming to you to eat, it’s hard to say no. That’s why dieting is partially a psychological battle.
Metabolic Adaptations: Nervous System Changes
Your body has another sneaky way of adapting to calorie restriction. You’ve probably heard of NEAT (non-exercise adaptive thermogenesis). NEAT refers to the movements you make when you’re not doing a structured workout. Do you tap your fingers on the table or shake your leg when you sit in a chair? Those are examples of NEAT and these activities can burn up to 300 additional calories per day.
When you restrict calories or when you expend lots of energy doing long periods of cardio, your body compensates by curtailing NEAT. In other words, you fidget less and become more efficient at doing activities so that you’re burning fewer calories, all in the name of energy conservation. This all happens outside of your awareness. The reduction in your activity level may be subtle, but over the course of a day, it adds up. Your sympathetic nervous system, the portion that hypes you up is toned down too to conserve energy.
Reducing Metabolic Adaptations to Fat Loss
Of course, you want to know what you can do to keep your body from hanging on to fat. Studies show that it’s best to approach weight loss in an incremental fashion by making a modest reduction in calorie intake. Some experts take it a step further. They recommend cutting calories below maintenance for a few days and then have a “refeeding” day where you increase your calories back to maintenance. In this way, your body senses that you’re not depriving it of energy for long periods of time. It’s particularly important to increase your carbohydrate intake every few days if you’re eating low carb. That’s because carbohydrates are closely tied to the proper functioning of your thyroid.
Studies also show that exercise reduces metabolic adaptations and the tendency to regain weight after you lose it. Rather than devoting most of your time to calorie-burning cardio, which could force your body to adapt and hold on to fat more, grab a pair of dumbbells or a barbell and build up more metabolically active muscle.
The Bottom Line
It’s not easy to lose weight when your body fights you. Avoid extreme calorie restriction and excessive cardio. If you restrict calories for a few days, follow it with a day where you eat enough calories for maintenance. Just as importantly, make sure you’re strength training.
J Int Soc Sports Nutr. 2014; 11: 7. Published online 2014 Feb 27. doi: 10.1186/1550-2783-11-7.
Am J Physiol Regul Integr Comp Physiol. 2009 Sep;297(3): R793-802. doi: 10.1152/ajpregu.00192.2009. Epub 2009 Jul 8.
Eurekalert.org. “Biggest Loser Study Reveals How Dieting Affects Long-Term Metabolism”
Best Pract Res Clin Endocrinol Metab. 2002 Dec;16(4):679-702.
Today’s Dietitian. Vol. 17 No. 7 P. 26. July 2015.
American Journal of Clinical Nutrition. Vol 53, 826-830. (1991)
Related Articles By Cathe: