Without hard data, I've relied on the idea that the Basal Metabolic Rate (BMR) in metabolism may be the trigger for a "conservation of energy" in the human body. That taking daily calories below the BMR triggers what many call "starvation mode" in the metabolism - that is, the body starts to use less energy as it prepares to survive what it perceives as a famine condition.
My recommendation has remained consistent over the years - one must try to determine their BMR and eat enough calories to meet that calorie requirement for basic function so the body will allow for loss of its stored energy - fat on the body. Interestingly, the BMR of most people is much higher than they realize and certainly higher than the often recommended calorie-restriction of 1200-1600 calories per day for weight loss.
For example, a 250-pound woman who is 5'5" tall and 35 years old has a BMR of approximately 1880-calories per day. If she restricts her daily calories each day to 1400 or 1600-calories, she will set her metabolism up to perceive a famine since she is not eating enough calories to meet her body's basic requirements to just function. Over the first few weeks she will lose weight, but she will also unwittingly be slowing her metabolism to perserve her stored energy - her fat stores - to survive the famine the body perceives with a calorie intake that fails to support basic function, which in turn causes the body to conserve energy, which slows or stops weight loss.
Recently hard data was published to support that such a metabolic slowdown exists and happens as one restricts calories.
In the Journal of Clinical Endocrinology and Metabolism, researchers published their findings in a study to investigate the effect of weight loss induced by caloric restriction on diurnal TSH concentrations and secretion in obese humans.
This was a "gold standard" study protocol - a rigorous clinical, prospective, crossover study that measured 24-hour plasma TSH concentrations at 10-min intervals along with the 24-hour TSH secretion rate. The results were fascinating!
- The 24-h TSH secretion rate was significantly higher in obese women than in normal weight controls
- Weight loss was accompanied by diminished TSH release (before weight loss, 43.4 ± 6.4 mU/liter·24 h; after weight loss, 34.4 ± 5.9 mU/liter·24 h; P = 0.02)
- Circulating free T3 levels decreased after weight loss from 4.3 ± 0.19 to 3.8 ± 0.14 pmol/liter (P = 0.04)
- Differences in 24-h TSH release correlated positively with the decline of circulating leptin (r2 = 0.62; P <>
Now I know some of you are scratching your head wondering - OK, what does that mean?
Well, the researchers stated it best in their conclusions: Elevated TSH secretion in obese women is significantly reduced by diet-induced weight loss. Among various physiological cues, leptin may be involved in this phenomenon. The decreases in TSH and free T3 may blunt energy expenditure in response to long-term calorie restriction, thereby frustrating weight loss attempts of obese individuals.
This data shows that there does exist a measurable slow down in the metabolism with calorie restriction. That slow down can significantly affect weight loss. That it is possible to restrict calories too much and affect the endocrine system in the body, making it adjust and slow the metabolic rate to conserve energy.
Now, researchers did not look at BMR in this study. What this study does confirm is the delicate balance the body has to survive - if you're not providing enough "fuel" the body will slow itself down and conserve energy so it can survive with less calories.
While many staunchly defend the "calories in calories out" perspective, those who have experienced weight loss stalls and stops during calorie restriction can now feel a sense of vindication and begin to understand how to lose weight effectively without damaging your metabolism!
Your first step is to eat enough calories to support your basal metabolic rate.
As you lose weight, your metabolism slowly does require less calories because it does not have the same "bulk" to support, so you must adjust your calories downward SLOWLY as you lose weight, not from the start. Give your body what it requires for calories to meet basic function so it does not try to conserve energy from the start.
In the above example, the calorie requirement each day should be no less than 1900-calories each day until 10-pounds are shed. Then re-calculate the BMR and adjust calories - at 240, the BMR is now approximately 1839-calories, so the requirement is adjusted down to 1850-calories a day for the next 10-pounds of weight loss. This is approximate...the key here is to make sure you're eating at or up to 10% above your BMR.
With each 10-pound loss, an adjustment of calories downward, of about 50-calories per day, helps the body maintain its basic function (blood flow, body temperature, heartbeat, etc.) and allow the loss of stored energy - fat - rather than starvation mode to conserve energy and slow or stop loss of stored energy. As one nears their goal weight, calorie intake is slightly adjusted up to the Active Metabolic Rate (AMR) to allow the body to have the calories it needs to support not only basic function, but also movement each day.
At goal, finding that balance where you no longer lose weight yet do not gain weight is not easy, but not impossible. It means taking the time to get it right and watching your weight to ensure you're not gaining again. If you start to gain, tweak your calorie intake slightly and watch. You will find the right calorie load to maintain your weight and keep your metabolism happy too!
This approach is highly individual - as it should be - and breaks the conventional wisdom that if you just restrict your calorie intake long enough you'll lose weight. Yes, you'll lose weight, but at what cost to your long-term health? To you long-term ability to keep the weight off? To you emotional well being as you struggle to maintain a calorie intake that is too low?