Eating 700 calories a day for a month will almost certainly produce weight loss on the scale, but much of the early drop is water and stored carbohydrate rather than body fat, and the side effects extend far beyond hunger. Your metabolism slows, your hormones shift to conserve energy, you lose muscle alongside fat, and your body enters a state of physiological stress that can affect everything from your hair to your gallbladder. The number on the scale tells a misleadingly simple story compared to what is actually happening inside.
The First Week Feels Dramatic but Is Mostly Water
If you suddenly cut from a typical intake down to 700 calories, you can expect to see a surprisingly large drop in body weight within the first few days. Most of that is not fat. Your body stores carbohydrate as glycogen in the liver and muscles, and each gram of glycogen is bound to roughly three to four parts water along with potassium.1PubMed. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition When you eat far fewer calories than your body needs, glycogen gets burned first and all that water goes with it. Someone might lose four or five pounds in a week and believe it is fat, when in reality most of it would come back immediately if they returned to normal eating.
This glycogen effect explains a pattern that trips people up repeatedly: quick apparent success in the first week, followed by slower progress in weeks two through four when actual fat loss is doing the heavy lifting. It also explains why stopping an extreme diet leads to a sudden “rebound” in weight that feels like you regained everything overnight. You did not. You refilled your glycogen stores and the water came back with them.
Your Metabolism Fights Back Almost Immediately
Within the first week of severe calorie restriction, your body begins spending less energy than you would predict based simply on having a smaller body. Researchers call this adaptive thermogenesis. A study measuring 24-hour energy expenditure in overweight subjects found that after just one week of calorie restriction, daily calorie burn dropped by about 178 calories more than body composition changes alone would explain, though the range varied widely from person to person.2PubMed Central. Early Adaptive Thermogenesis Is a Determinant of Weight Loss after Six Weeks of Caloric Restriction in Overweight Subjects That is your body actively dialing down its energy use to resist the calorie deficit you have created.
This metabolic slowdown is not small and it is not uniform. People who lose the most fat tend to experience the most pronounced drop. Research on prolonged calorie-restricted weight loss found that participants who lost the greatest amount of fat mass had an adaptive thermogenic effect of roughly 79 calories per day beyond what their changed body size predicted, compared to only about 10 calories per day in those who lost the least fat.3The FASEB Journal. Contribution of Changes in Body Composition and Adaptive Thermogenesis to the Decline in Resting Metabolic Rate During Prolonged Calorie‐Restricted Weight Loss In plain terms, the harder you push, the harder your body pushes back. A month of 700 calories per day is exactly the kind of aggressive deficit that tends to trigger a strong metabolic response.
A study specifically tracking women on a very-low-calorie ketogenic diet for one month confirmed that total energy expenditure dropped significantly by the end of the four weeks, driven by losses in lean tissue and declining thyroid hormone levels.4The Journal of Clinical Endocrinology & Metabolism. Effects of 1-Month Very-Low-Calorie Ketogenic Diet on 24-Hour Energy Metabolism and Body Composition in Women With Obesity The net result is that your 700-calorie diet creates a smaller calorie deficit as the weeks go on than it did on day one. Weight loss stalls or slows, even though you are still eating the same tiny amount.
You Lose Muscle, Not Just Fat
One of the most consequential things that happens at 700 calories a day is the loss of lean tissue, including skeletal muscle. When your body cannot get enough energy from food, it does not exclusively raid fat stores. It also breaks down muscle protein to convert amino acids into glucose for the brain and other tissues. How much muscle you lose depends on your starting weight, the severity of the deficit, the protein content of whatever you are eating, and whether you are exercising.
In people who are overweight or obese, fat-free mass typically accounts for about 20 to 30 percent of total weight lost during dieting. In people closer to normal weight, that fraction can climb above 35 percent.5Advances in Nutrition. Preserving Healthy Muscle during Weight Loss At 700 calories a day, you are almost certainly eating too little protein to fully protect your muscles, and the deeper the calorie deficit, the greater the risk to lean mass.6PubMed Central. The impact and utility of very low-calorie diets: the role of exercise and protein in preserving skeletal muscle mass Losing muscle is a problem that compounds: less muscle means a lower resting metabolic rate, which means your body needs even fewer calories, which makes maintaining any weight loss harder in the long run.
This is why researchers and clinicians who supervise very-low-calorie diets emphasize high protein intake and resistance exercise during the diet. Without those protections, a month at 700 calories can leave you lighter on the scale but with a less favorable ratio of fat to muscle than when you started.
Hormonal Upheaval
Severe calorie restriction sets off a cascade of hormonal changes, most of which are your body’s alarm signals that it is not getting enough fuel.
Thyroid Hormones Decline
Your thyroid gland regulates metabolic rate, and it responds quickly to starvation-level eating. In patients consuming 400 calories per day, active thyroid hormone (T3) dropped by as much as 66 percent, while the inactive form (reverse T3) rose by about 27 percent.7PubMed. Effects of a very low calorie diet on weight, thyroid hormones and mood At 700 calories you are eating somewhat more, but you are still in a range where significant thyroid suppression occurs. Lower T3 means you burn fewer calories at rest, feel colder, and may notice sluggish digestion, all symptoms people commonly report on extreme diets.
Hunger and Stress Hormones Shift
Calorie restriction drives up ghrelin, the hormone that signals hunger, while suppressing leptin, the hormone that signals fullness, and lowering fasting insulin.8PubMed Central. Satiety Associated with Calorie Restriction and Time-Restricted Feeding: Peripheral Hormones The result is that you are not just willpower-deficient when you feel ravenous on 700 calories. Your body has chemically turned up the hunger dial and turned down the satisfaction dial.
Cortisol, the primary stress hormone, also rises during calorie restriction. One study found that restricting calories significantly increased total cortisol output.9PubMed Central. Low Calorie Dieting Increases Cortisol A meta-analysis looking across multiple studies found that cortisol tends to spike most during the early phase of restriction and may drift back toward baseline after several weeks, though the effect is strongest with the most severe calorie cuts.10PubMed. Systematic review and meta-analysis reveals acutely elevated plasma cortisol following fasting but not less severe calorie restriction Chronically elevated cortisol promotes fat storage around the midsection, disrupts sleep, and can impair immune function, which somewhat undercuts the purpose of the diet.
Menstrual Disruption in Women
For women, eating 700 calories a day for a month puts you at real risk of menstrual irregularity or loss. Weight loss-related amenorrhea can occur when someone loses more than about 10 to 15 percent of their body weight in a short period. The underlying mechanism is that low energy availability disrupts the pulsatile release of gonadotropin-releasing hormone from the brain, which in turn suppresses the hormones that drive ovulation and estrogen production.11PubMed Central. The effects of weight loss-related amenorrhea on women’s health and the therapeutic approaches: a narrative review Even if full amenorrhea does not develop within one month, luteal phase defects and anovulation can begin earlier in the process. These changes are not just a fertility concern; suppressed estrogen affects bone density, cardiovascular health, and mood.
Hair Loss, Feeling Cold, and Digestive Problems
Beyond the hormonal shifts, a month at 700 calories can produce a set of visible and uncomfortable physical symptoms.
Hair shedding is one of the most distressing. Rapid weight loss triggers a type of hair loss called telogen effluvium, where a large number of hair follicles shift prematurely into their resting phase and then shed two to three months later. A retrospective study found that patients began losing hair an average of about one month after significant weight loss, though the good news is that shedding typically resolves on its own within about five months without treatment.12PubMed Central. Telogen Effluvium Associated With Weight Loss: A Single Center Retrospective Study The trigger is not just calorie restriction itself but the broader metabolic stress, nutritional deficiencies, and hormonal disruption that accompany it.13Brazilian Journal of Hair Health. Telogen Effluvium and Metabolic Stress in Modern Weight Loss Interventions: A Narrative Clinical Review
Feeling cold all the time is another hallmark. Calorie restriction lowers core body temperature as part of the body’s energy-conservation strategy. Research on long-term calorie restriction found that people eating significantly less than their energy needs had lower 24-hour, daytime, and nighttime core body temperatures compared to people eating normally or exercising to a similar body weight.14PubMed Central. Long-term calorie restriction, but not endurance exercise, lowers core body temperature in humans The study’s authors noted this appeared to be a direct effect of the calorie restriction rather than just a consequence of being thinner. Gonadal hormones also play a role in this thermoregulatory shift.15PubMed Central. Gonadal hormones influence core body temperature during calorie restriction
Constipation is common as well. When you eat very little food, there is simply less bulk moving through your digestive tract. Reduced intake of fiber, fats, and starches all contribute. Low-fiber intake on a very-low-calorie diet is essentially unavoidable unless you are deliberately supplementing, and the decreased thyroid function described earlier further slows gut motility.
Gallstone Risk Goes Up
One of the more serious medical risks of eating 700 calories a day for a month is gallstone formation. When you lose weight rapidly, the liver secretes extra cholesterol into bile, and the gallbladder does not empty as frequently because you are eating so little fat. That combination creates conditions for cholesterol crystals to form into gallstones. A large matched cohort study comparing people on very-low-calorie diets to those on less extreme low-calorie diets found that the very-low-calorie group had roughly 3.4 times the rate of gallstones requiring hospital care.16PubMed Central. Risk of symptomatic gallstones and cholecystectomy after a very-low-calorie diet or low-calorie diet in a commercial weight loss program: 1-year matched cohort study About 61 percent of gallstone events in the study led to surgical removal of the gallbladder. The risk was attenuated somewhat when researchers adjusted for the amount of weight lost, but it remained significantly higher in the very-low-calorie group.
This is not a theoretical risk that only applies to people eating 400 calories. At 700 calories per day, you are still in the zone where rapid weight loss can shift bile composition. If you have a history of gallbladder problems or are at higher risk due to age, sex, or genetics, this deserves serious consideration.
What Happens to Your Gut
Your gut microbiome responds to dramatic calorie restriction in ways researchers are still working to fully understand. A study tracking people through a very-low-calorie diet phase found that the microbial community shifted significantly, with certain bacteria like Akkermansia, a mucin-degrading species, blooming during the restriction period. Analysis of the bacteria’s gene activity suggested that when dietary fiber drops to very low levels, gut bacteria capable of feeding on the mucus lining of the intestine expand to fill the gap.17Cell Host & Microbe. Does caloric restriction prime the microbiome for pathogenic bacteria? The microbiome returned roughly to baseline after participants moved to a less severe diet, but the short-term shift raises questions about gut barrier integrity during the restriction period. When bacteria eat your intestinal mucus instead of dietary fiber, the protective barrier between gut contents and your bloodstream thins.
The Weight Comes Back Easily
Perhaps the cruelest irony of eating 700 calories for a month is what happens when you stop. The metabolic adaptations described earlier, including the lowered resting energy expenditure, the suppressed thyroid hormones, and the elevated hunger signals, do not snap back to normal the moment you return to a regular diet. Research on weight regain identifies a vicious cycle: obesity, followed by weight loss, followed by regain, in part because the decrease in metabolic rate and the increase in hunger persist after the diet ends.18PubMed Central. Mechanisms of Weight Regain following Weight Loss The body, having registered a period of severe food scarcity, essentially keeps its defenses up for months afterward.
Meanwhile, the classic Minnesota Starvation Experiment, which remains the most comprehensive study of severe calorie restriction and refeeding in humans, documented that participants’ appetite and preoccupation with food persisted long after normal eating resumed, and that weight regain initially favored fat over lean tissue.19PubMed. Physiology of weight regain: Lessons from the classic Minnesota Starvation Experiment on human body composition regulation When you regain weight after losing muscle, you end up with a higher body-fat percentage than you started with, even at the same scale weight. This outcome, sometimes called “fat overshoot,” is a well-documented consequence of severe restriction followed by ad libitum eating.
The implication is straightforward: a month of 700 calories may produce a number on the scale that feels like progress, but if the lost weight returns as a higher ratio of fat to muscle, you have made your body composition worse, not better.
When Doctors Actually Prescribe Very-Low-Calorie Diets
All of the above does not mean very-low-calorie diets are never used. They are a legitimate medical tool, but with significant guardrails. University-based weight-loss programs have used prescribed diets in the 500 to 800 calorie range alongside exercise recommendations and behavioral counseling, with retrospective analysis of over 2,000 patients showing effective weight loss across groups with normal blood sugar, prediabetes, and type 2 diabetes.20Nature. Clinical efficacy of a medically supervised outpatient high-protein, low-calorie diet program is equivalent in prediabetic, diabetic and normoglycemic obese patients The key differences between these supervised programs and doing it on your own are medical monitoring, protein-fortified meal replacements designed to minimize muscle loss, scheduled bloodwork to catch gallstone formation or electrolyte imbalances early, and a structured transition plan to prevent the metabolic rebound that makes weight regain so likely.
Without that infrastructure, eating 700 calories a day for a month is essentially running the same physiological experiment on yourself without the safety net. You face the same gallstone risk, the same muscle loss, the same hormonal disruption, but without anyone checking your labs or adjusting course if something goes wrong.
Immune Function and Micronutrient Gaps
At 700 calories per day, it is nearly impossible to meet your body’s micronutrient needs through food alone. Even well-designed meal plans struggle to deliver adequate iron, calcium, zinc, B vitamins, and fat-soluble vitamins at that calorie level. The immune system is particularly sensitive to nutritional shortfalls. Malnutrition compromises immune defenses and increases susceptibility to infection, though the exact mechanisms connecting undernutrition to impaired immune cell function remain an active area of research.21PubMed Central. Effects of Malnutrition on the Immune System and Infection and the Role of Nutritional Strategies Regarding Improvements in Children’s Health Status: A Literature Review Over a single month you are unlikely to develop a frank deficiency disease, but if you are already marginally low in iron or vitamin D, as many people are, a month of extreme restriction can push you below the threshold where your body functions well.
This is especially relevant during cold and flu season or for anyone with an existing health condition. Combining the cortisol increase from dietary stress with micronutrient gaps creates a one-two punch against your immune system that a simple calorie count does not capture.
Sleep, Mood, and Cognitive Effects
People on very-low-calorie diets frequently report difficulty concentrating, irritability, and disrupted sleep. The mood effects are partially explained by the hormonal changes already described: elevated cortisol, low thyroid hormones, and high ghrelin are all associated with anxiety, low mood, and obsessive thoughts about food. The Minnesota Starvation Experiment famously documented that participants became consumed by food-related thoughts, lost interest in other activities, and experienced significant psychological distress during the restriction phase.
Sleep effects are somewhat mixed in the research. A study using a moderate calorie deficit of 500 to 700 calories per day from baseline found no significant change in sleep duration but did find an improvement in sleep quality.22MDPI. Dieting Behavior Characterized by Caloric Restriction and Relation to Sleep: A Brief Contemporary Review That study, however, involved a moderate deficit in overweight adults rather than the extreme restriction of an absolute 700-calorie intake. Anecdotally and in clinical observation, more severe restriction tends to produce restless sleep and early waking, likely because low blood sugar and high cortisol at night interfere with normal sleep architecture.
The cognitive effects are worth taking seriously if you rely on mental sharpness for work or school. Your brain runs primarily on glucose, and while it can partly switch to ketone bodies during prolonged restriction, the transition period and the overall energy shortfall typically produce a noticeable fog that most people find incompatible with peak performance.
Who Is Most at Risk
Not everyone faces identical danger at 700 calories a day, but certain groups face disproportionate risk. Adolescents and young adults who are still growing can suffer lasting effects on bone mineral density and hormonal development. Women of reproductive age face menstrual disruption at rates that make this a near-certainty rather than a remote possibility during aggressive restriction. People with a history of eating disorders are at risk of relapse, since the psychological effects of severe restriction can reactivate disordered patterns. Anyone on medication for diabetes or blood pressure needs to be particularly cautious, because calorie restriction can cause blood sugar or blood pressure to drop dangerously low when combined with drugs designed to lower those values.
Older adults are another vulnerable group. They tend to have less muscle mass to spare and a harder time rebuilding it, so the lean tissue lost during a month of extreme dieting may never fully return. The combination of muscle loss and bone-mineral depletion from low calcium and vitamin D intake can meaningfully increase fall and fracture risk.