Human beings did not evolve on three square meals a day. As hunter-gatherers we ate what we could find when we could find it. We used considerable energy to obtain food and as it wasn’t always accessible, we developed a dynamic metabolism that enables us to survive extended periods without eating. However, in today’s developed world with food never far from reach, we don’t often put our ancient metabolism to the test and we treat momentary hunger as something to be crushed, urgently.
With more than half the European adult population overweight or obese(1) the weight-loss industry is growing and expected to reach over $200 billion globally by 2019.(2) But as economists know all too well, where there’s a boom, there’s usually a bust not far behind, and dieting seems to follow the same pattern. Studies show that 95 percent of people who have lost weight through dieting end up putting all of the weight back on and more within 5 years.(3)
What is diet mode?
There has long been the belief that weight loss is simply a matter of calories in versus calories out... “Just eat less and move more!” However, as many dieters know, it’s not always that easy.
The theory behind diet mode, or starvation mode as it’s also referred, is that when you restrict calorie intake for an extended period of time, your body, unaware of your weight loss strategy, activates primal starvation mechanisms designed to keep you alive. It does this by manipulating a fine balance of hormones that impact the central nervous system in order to slow down energy expenditure (i.e. to reduce your metabolism).
What is metabolism?
Human metabolism is how the body stores and uses energy. It’s the process by which cells are able to respond to their environment. By way of biochemical reactions, the cells throughout our body can either break down molecules (releasing energy) or build up molecules (requiring energy). These processes of metabolism allow us to grow, reproduce, repair tissues and adapt to the world around us.
The liver plays a central role by acting as a sort of ‘metabolic processor’ converting nutrients into forms of energy that keep us going in between meals. Glucose is our body’s primary fuel source and is stored as glycogen in the liver and in muscle tissue. Excess energy is converted into fatty acids and stored in fat cells known as adipose tissue.
When your metabolism has “down-shifted” to diet mode, your rate of weight loss slackens, and the myth is that you can actually end up gaining weight while consuming fewer calories than you are burning. The debate on this topic is polarized and there doesn’t appear to be a scientific study that can prove the myth categorically. Some back it passionately while others argue that as long as an energy deficit is maintained (i.e. fewer calories going in than out) weight loss will continue. Many site the 1941 Minnesota starvation experiment where thirty-six conscientious objectors participated in a study to examine the effects of semi-starvation.(4) They were fed only half of their maintenance calories in a low-protein diet for six months and forced to walk twenty-two miles a week. By the end of the calorie-restricted period they had lost 25% of their body weight with a metabolic decline of nearly 40%.
Part of this metabolic drop can be explained by the fact that as a person loses weight, their reduced body mass requires less energy to maintain itself and unless calorie intake is reduced to compensate they will naturally see their weight loss slow down.
Calories In – Calories Out
The guidelines on daily energy requirements for the average-sized adult are 2,000 calories for women and 2,500 calories for men.(5) These values vary depending on age, activity levels and muscle-to-fat ratio, among other things. If we consider our individual daily calorie requirements as a budget, we could divide the energy between the following physiological needs (see table 1):
The importance of preserving muscle mass
Different tissues in the body have different energy requirements (see table 2). Skeletal muscle (i.e. muscle used for locomotion) makes up approximately 40% of an average person’s body weight and accounts for roughly 20-30% of the basal metabolic rate (BMR).(6) Muscle requires three times more energy to maintain itself than fat7 so increasing muscle mass with strength-based exercise increases ones BMR but by how much is often debated.
Gus Martin, a personal trainer and nutritional consultant based in London has performed studies on himself where he measured his BMR before and after a period of intensive weight training. “Over 8 weeks I put on 2kg of lean mass and low and behold my BMR had increased by 250 calories per day”, said Mr, Martin. This does correlate with a common anecdotal claim that one pound of muscle burns 50 calories per day, however this figure is not backed up by scientific studies and in that regard, one study concluded that a pound of muscle burns just 6 calories per day.(7) In any case, Mr. Martin claims “keeping muscle tissue active with weight-bearing exercise stimulates the body to hold onto lean muscle mass. When your body is in a calorie deficit it will burn muscle tissue to make up the energy gap unless the muscles are used and a sufficient amount of protein is consumed.”(8,9) However, this explains only part of the story.
Hormones: the driving force behind diet mode
‘Adaptive thermogenesis’ describes the metabolic adaptation that cannot be explained by merely a change in body mass. During extended periods of calorie deficit the body makes numerous metabolic adjustments in order to conserve energy.(10) These adjustments are regulated by a dynamic interplay between several hormones (namely insulin, leptin and the thyroid hormones)(11) that control the speed of our metabolism and are influenced by many factors, including our genes, the amount and type of food we eat, how much we exercise and our fat reserves to name a few.
Taking center stage in metabolism is the thyroid gland which produces hormones that stimulate our cells to work harder when energy needs increase, like when we’re cold or during pregnancy.(12) Aerobic exercise has been shown to stimulate the thyroid to produce more hormones that can increase metabolism.(13)
Leptin is a hormone secreted by fat cells that acts to suppress hunger. It plays a pivotal role in regulating energy intake and expenditure by driving primal mechanisms to maintain our fat stores. During starvation leptin levels fall, increasing hunger and influencing the thyroid to reduce our metabolism in order to conserve energy.(14)
Insulin is a hormone secreted by the pancreas following a meal to drive glucose out of the blood stream and into the cells to be used for energy. Muscle is the body’s primary tissue for glucose disposal, with four times the glycogen storage capacity of the liver.(15) The more sensitive our tissues are to insulin the less insulin is needed and exercise as well as increasing muscle mass are the best ways to increase insulin sensitivity.(16,17)
Dietary fat: Is it the key to maintaining metabolism?
Emerging research suggests that low-fat diets could be one reason why people experience a reduced metabolism when dieting. David Ludwig, Professor of Nutrition at the Harvard School of Public Health, describes insulin as “the ultimate fertilizer for fat cells”18 and explains that most calorie-restricted diets are low in fat and high in carbohydrates that increase insulin, driving glucose and fatty acids out of the blood. In this scenario, claims Professor Ludwig, the brain senses a lack of calories in the blood, triggering a hormone response like starvation that increases hunger, reduces metabolism and programs our adipose tissue to hoard fat. By increasing the ratio of fat-to-carbohydrates in the diet, the brain detects calories in the blood and fat cells are reprogrammed to stop hoarding, thereby reducing cravings, normalizing hormones and preventing the reduction in metabolism.(18,19) A controlled-feeding study in 2012 comparing three diets of equal calories with varying macronutrient ratios (i.e. protein, carbohydrates and fats) showed that a high-fat diet (60% of calories from fat) produced a higher basal metabolic rate (+325 kcal/d) than the low-fat diet (20% of calories from fat).(20)
Genetic varibility between individuals shows that some of us have built-in metabolisms that are better tuned to hold onto fat than to promote weight loss(21,22) which could explain why the subject of diet mode continues to divide opinion.
Our body fights back when faced with the prospect of starvation launching primal survival mechanisms that sabotage weight loss by reducing energy output and increasing hunger. Hormones are the great regulators of these metabolic adaptations and are a major factor in why dieting alone doesn’t work as a long-term weight loss strategy for most people.
We now know that the quality of calories going in has an effect on the quantity of calories going out (i.e. the rate of your metabolism) and including more healthy fats in the diet may be one way to negate the downward adjustment to metabolism when dieting.
Our muscles command a significant percentage of our daily energy needs and play a crucial role in glucose disposal and maintaining insulin sensitivity. Preserving muscle through exercise and protein consumption is vital to maintaining metabolism. When muscles aren’t used the body has a way of remodeling itself putting the energy contained in that tissue to better use elsewhere and proving that when it comes to muscle tissue and metabolism, there’s truth in the old saying “use it or lose it”.
- WHO – European food and nutrition action plan 2015-2020 (2014) http://www.euro.who.int/__data/assets/pdf_file/0008/253727/64wd14e_FoodNutAP_140426.pdf
- Markets and markets –Weight Loss and Weight Management Market... forecast to 2019. http://www.marketsandmarkets.com/Market-Reports/weight-loss-obesity-management-market-1152.html
- Bray GA & Champagne CM (2005). Beyond energy balance: there is more to obesity than kilocalories. J Acad Nutr Diet. 105:17-23.
- Keys A et al (1950) The Biology of Human Starvation, Vols. I–II. University of Minnesota Press, Minneapolis, MN.
- NHS Choices (2014) What should my daily intake of calories be? http://www.nhs.uk/chq/pages/1126.aspx?categoryid=51
- Zurlo F et al (1990). Skeletal muscle metabolism is a major determinant of resting energy expenditure. J Clin Invest. 86:1423-7.
- Wang Z et al (2011). Evaluation of specific metabolic rates of major organs and tissues: comparison between men and women. Am J Hum Biol. 23(3):333-8.
- Soenen S et al (2013). Normal protein intake is required for body weight loss and weight maintenance, and elevated protein intake for additional preservation of resting energy expenditure and fat free mass. J Nutr. 143(5):591-6.
- Longland TM et al (2016). Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss: a randomized trial. Am J Clin Nutr. 103:738-46.
- Heilbronn LK et al (2006). Effect of 6-Month Calorie Restriction on Biomarkers of Longevity, Metabolic Adaptation, and Oxidative Stress in Overweight Individuals: A Randomized Controlled Trial. JAMA. 295:1539-48.
- Camps SG et al (2013). Weight loss, weight maintenance, and adaptive thermogenesis. Am J Clin Nutr. 97(5):990-4.
- USNLM (2015) How the Thyroid Works? http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072572/
- Ciloglu F et al (2005). Exercise intensity and its effects on thyroid hormones. Neuroendocrinol Lett. 26(6):830-4.
- Rosenbaum M & Leibel RL (2010). Adaptive thermogenesis in humans. Int J Obesity. 34:47-55.
- DeFronzo et al (1981). The effect of insulin on the disposal of intravenous glucose. Results from indirect calorimetry and hepatic and femoral venous catheterization. Diabetes. 30:1000-7.
- Koopman R et al (2005). A single session of resistance exercise enhances insulin sensitivity for at least 24 h in healthy men. Eur J Appl Physiol. 94(1-2):180-7.
- Srikanthan P & Karlamangla AS (2011). Relative muscle mass is inversely associated with insulin resistance and prediabetes. Findings from the third national health and nutrition examination survey. J Clin Endocrinol Metab. 96:2898-2903.
- Hyman M (2015) The Fat Summit: interview with Professor David Ludwig. http://fatsummit.com
- Ludwig DS & Friedman MI (2014). Increasing adiposity: consequence or cause of overeating? JAMA. 311:2167-8.
- Ebbeling CB et al (2012). Effects of dietary composition on energy expenditure during weight-loss maintenance. JAMA. 307(24):2627-34.
- Dulloo AG (2012). Adaptive thermogenesis in human body weight regulation: more of a concept than a measurable entity? Obesity Reviews. 13(2):105-21.
- Hopkins M et al (2014). The adaptive metabolic response to exercise-induced weight loss influences both energy expenditure and energy intake. Eur J Clin Nutr. 68(5):581-6.
- Table 1. Gropper S, Smith J (2013) Body composition, energy expenditure, and energy balance. In: Williams P, Feldman E, eds. Advanced Nutrition and Human Metabolism. 6th edn.,Wadsworth, Cengage Learning., p288.
- Table 2. Gropper S, Smith J (2013) Body composition, energy expenditure, and energy balance. In: Williams P, Feldman E, eds. Advanced Nutrition and Human Metabolism. 6th edn.,Wadsworth, Cengage Learning., p289.