You'll hear some people who strongly believe the body can go into "starvation mode" and others on the complete opposite end of the spectrum that don't believe it's a mode at all.
But we know that people can starve, so starvation is a real thing that the body can endure, thus there are physiological responses to that state.
But what about standard dieters, do they undergo a physiological response to dieting similar to that of starvation?
How the body responds to a calorie deficit:
The body has a beautiful ability to adapt to just about everything and can either be acutely adaptive to it's environment like jumping out of the way of a moving car, or it can adapt to environmental stimuli long-term, like a diet. When we diet, we are likely placing ourselves in a calorie deficit (provided it's being done correctly and monitored well).
(Tangent time) I say "likely" because if you don't know how much you are consuming on average then how can you possibly know if you're in a calorie deficit?
For example, say this is what your week of calorie intake looks like:
Monday- 1350 calories
Tuesday- 1600 calories
Wednesday- 1550 calories
Thursday- 1650 calories
Friday- 1800 calories
Saturday- 2600 calories
Sunday- 1300 calories
That's an average of 1693 calories. But you aren't consistently eating that amount daily so your body has adapted to this inconsistent cycle of caloric intake. It would be smart to eat roughly 1700 calories consistently to see how your body responds to that and then place yourself in a small caloric deficit to theoretically elicit weight loss.
We see a caloric deficit as a way to lose body fat, but our body sees a calorie deficit as a state of famine and food unavailability. We are prehistoric beings living in a very modern world which is why our bodies do not realize that we are trying to lose weight for aesthetic/strength/wellness purposes. This is why it's important not to slash calories quickly because that can be a shock to our bodies but rather, cut calories slowly overtime to allow the body a safer transition to fat loss.
When our bodies sense a caloric deficit, i.e. less energy intake for metabolic processes, it will pull energy from storage (see my previous blog post explaining the main metabolic pathways), first through glycogenolysis (depletion of glycogen stores), then gluconeogenesis (energy from fat, pyruvate, amino acids), then through lipolysis (breakdown of fat). Once the body sees that there are very limited energy options for glucose conversion, the body will switch over to ketone production and use that for it's main fuel source (ketosis). All of these pathways occur at the initial signs of poor food availability and decreased oral intake from the typical oral intake that the body has adapted to, i.e. dieting and starvation.
Important to note: our bodies don't work in a step-wise process like this, generally these pathways are all working simultaneously at different rates during a calorie deficit depending on the physiological and psychological state one is in but one thing our bodies hate to do, is pull energy from muscle; meaning breaking down muscle into amino acids to use for energy. This is very costly, typically occurs in acute traumatic events (think heart attack, respiratory distress, intensive care patients, etc.) and generally results in starvation and malnutrition if carried out for a long period of time. However!! Muscle break down can occur to a small degree during long periods of dieting which is why it's smart to diet slow to mitigate muscle loss.
The clinical criteria for starvation:
As a clinical dietitian, one of my responsibilities is to monitor the risk for and diagnose malnutrition. There are three categories of malnutrition we assess, 1) in the context of acute illness or injury, 2) in the context of chronic illness or injury (indicated by a condition lasting 3 months or longer) and 3) in the context of social or environmental circumstances marked by starvation. Moderate starvation is characterized by consuming less than 75% of your caloric needs for 3 months or more (definitely seen/elicited with dieting), mild muscle and fat loss, and a certain percent weight loss over time. Severe starvation is characterized by consuming less than 50% of your caloric needs for one month or more, severe muscle and fat loss, and a certain percent weight loss over time.
To determine mild or severe muscle and fat loss, dietitians are educated on performing a nutrition focused physical exam (NFPE) to assess certain areas of the body for muscle and fat wasting that is outside of what the norm would be for a healthy individual. We also look at weight history to determine how much weight has been lost over a given amount of time (for example, 7.5% weight loss in 3 months is clinically significant) in order to use that a criterion for a malnutrition diagnosis.
You've started the diet, now what happens?
Since you aren't giving your body the typical energy intake it's been used to in order to perform all it's physiological processes, it's now pulling energy from storage which can and most likely will elicit weight loss. As a dieter, this is your goal!
Your body is so smart though and it's main goal is survival so it's biggest fear is being in a state where it can't protect you (aww love you body). So, your body will work hard to adapt to the caloric deficit you put it in, meaning it will downregulate it's metabolic capacity so it doesn't require as much energy to function (i.e. your body now needs less calories/energy to perform that it did before you started dieting). For example, utilizing the average caloric intake calculated above at 1700, say now you're consuming 1500 calories and have been for the last two months. You've lost about 8 pounds (1lb. per week) but noticed you aren't losing weight at the same rate anymore. This is what dieters know to be a 'plateau' in the their progress. Science calls this metabolic adaptation or down-regulation and it's a beautiful protective mechanism that keeps you alive! If our bodies could not adapt, you would continue losing weight until you were next to nothing and dead.
So while we need to be grateful for this, it makes weight loss harder, meaning you have to drop calories more from 1500 to probably 1300 which is LOW and generally not sustainable long-term. When you eat too low of calories for too long the body will continue to work to adapt to this lower energy intake while trying to stimulate hunger by ramping up ghrelin and depressing leptin in order to get you to eat so you can gain the weight back (a large reason why people have a hard time maintaining weight loss). "Attempts to sustain weight loss invoke adaptive responses involving the coordinate actions of metabolic, neuroendocrine, autonomic, and behavioral changes that “oppose” the maintenance of a reduced bodyweight."
The body also 'shuts off' or 'shuts down' certain bodily functions that the body deems to be expendable processes for the sake of survival. In women, this can be loss of a menstrual cycle and/or loss of libido because having sex and bearing a child are extremely energy expensive processes that the body can't afford right now. Women can also feel cold all the time, have thinning hair, impaired cognition and feeling fatigued quicker. Men can experience low libido, fatigued quicker than usual, impaired cognition and feeling cold all the time as well. A lot of these symptoms are indicative of impaired thyroid function, gut health and endocrine disruption.
This table below outlines the effects of sustained weight loss of only 10% of your starting weight:
So, to recap this scenario: you started your diet realizing that your average energy requirements were 1700 calories so you cut calories to start your diet to 1500 calories. You've lost 8lb. on 1500 calories per day for 2 months. You've now dropped your calories to 1300 because you weren't seeing the same progress. You've been dieting on 1300 calories now for another month and have lost about 5 more pounds (a total of 13lbs.). You're happy because you've dropped a pant size and you're liking the way you look. BUT, you're tired a lot, gym sessions are more exhausting and sometimes you aren't even completing the workout, you're relationships are strained because you'd rather be home on the couch and sticking to your diet is getting tougher so being around food is more mentally fatiguing, you're cold and don't feel like being intimate with your S.O. and your productivity at work is taking a hit because you aren't as focused and clear-headed as usual so you're just doing the bare minimum.
You're kinda fed up with the diet at this point. Your body has now adapted to function on 1300 calories, you've hit another plateau which means you need to cut calories even more to see weight loss progress. This isn't advisable given the fact that 1300 calories is so low already and you're body is turning off various processes to conserve energy. This point finally brings me to an answer to the originally question...
DIETING IS BASICALLY A FORM OF CONTROLLED STARVATION, 1) I say 'controlled' because it's intentional rather than due to social determinants impacting food availability and nutritional status and 2) because the body goes through the same metabolic processes during dieting as it does during starvation (not nearly as fast but the same); your body is undergoing alterations and adaptations to mitigate further weight loss in order to keep you safe.
Would I call this 'starvation mode'? No, I'd call it metabolic adaptation.
I wouldn't say 'starvation mode' is inherently wrong though either, but just not the best term to use. True starvation would fall into those clinical categories I explained above or complete cessation or oral intake for an extended amount of time with quick weight loss, reduced strength and a host of other impaired physiological processes.
It's important to note that your metabolism is not damaged when you're dieting, it's adapted, and your body will adapt and your metabolism will regulate back up when you stop dieting and start eating more. This is often accompanied by weight gain if done too quickly, but sometimes weight gain can be minimal if done efficiently through a reverse diet which is something I help clients with when they are done dieting so they can continue to sustain some/most of their weight loss in the cut phase!
If you believe you need further help, guidance and accountability from a coach so you can have someone else monitor all of this for you, then please apply to work with me here! I'd love to help you achieve your goals if you're truly ready for a change!