Eating less food than your body is used to can definitely lead to some changes as your body adjusts. When you reduce your calorie intake over a period of time, your body does go through some metabolic and physiological adaptations.
Does metabolism slow down when eating less?
One of the biggest concerns around eating less is that it will cause your metabolism to slow down. The metabolism refers to all the chemical processes in the body that convert food and drinks into energy. A “slower metabolism” means you are burning fewer calories at rest.
Research shows that metabolism does adapt when you reduce calories for weight loss. One study found that people who cut 500 calories per day had a 5-15% drop in resting metabolic rate over 6-8 weeks (1). Other studies show this metabolic slowdown can persist even after weight loss (2).
However, the effect tends to be temporary and smaller than expected. One review found that resting metabolism dropped by an average of 300 calories during weight loss (3). Another study found a 3% decrease in resting metabolism after 6 months of calorie restriction (4).
The drop in metabolism makes sense from an evolutionary perspective. During periods of famine, your body tries to conserve energy by slowing down non-vital processes. The extent to which metabolism changes also depends on factors like how much weight is lost and starting body composition.
Reasons for metabolic adaptation
There are a few reasons why resting metabolism slows down on a calorie deficit:
- Loss of muscle mass – Active muscle tissue burns more calories than fat at rest. Losing muscle during weight loss reduces resting energy expenditure.
- Neural and hormonal adaptations – Changes in appetite hormones like leptin and thyroid hormones affect metabolism.
- Reduced body size – Having a smaller body size and less tissue requires fewer calories for basic body functions.
How to minimize metabolic slowdown
The best way to counteract this metabolic adaptation is through exercise, especially strength training. Lifting weights and building muscle helps offset the loss of calories burned at rest. Other tips include:
- Losing weight slowly – A modest calorie deficit lessens the drop in metabolism.
- Eating adequate protein – Protein intake preserves muscle and stimulates metabolism.
- Taking diet breaks – Periodically reverting to maintenance calories can reset metabolism.
Do you get used to feeling hungry when eating less?
In addition to metabolic changes, eating less food can also increase feelings of hunger. This is because weight loss causes changes in appetite-regulating hormones.
Leptin is a hormone produced by fat cells. Lower leptin levels increase appetite and stimulate hunger. Reduced calorie intake leads to a significant drop in leptin levels (5).
Other hormones like ghrelin and peptide YY also adjust to signal hunger. Together, these hormonal adaptations intensify hunger cues in an attempt to get you to eat more food.
Fortunately, research indicates that appetite does seem to stabilize over time on a calorie deficit. One study in obese subjects found that hunger peaked after 3-5 weeks of calorie restriction but gradually subsided over the next 4 weeks of weight loss (6).
With consistency, most people do report getting used to the hunger. Here are some tips to help manage hunger when eating less:
- Eat plenty of protein and fiber – They help control appetite and make meals more filling.
- Drink water before meals – Being hydrated helps reduce hunger.
- Stay busy and active – This can take your mind off food cravings.
- Allow occasional higher calorie days – This provides a mental break from restriction.
Does your body temperature decrease?
There is some evidence that reducing calorie intake over a period of time can slightly lower core body temperature. Core body temperature refers to the temperature in the thoracic and abdominal cavities.
One study had participants consume a very low calorie diet providing 50% of estimated needs. After losing weight, they experienced a significant drop in resting core temperature of 0.24°C (7).
Another study had obese women follow a low calorie diet supplying 800 calories per day. After three months, resting core body temperature decreased by an average of 0.15°C (8).
The drop in temperature after weight loss seems to be mediated by a reduction in the body’s metabolic rate and processes. With less body fat for insulation, changes in circulation may also affect temperature.
However, the decrease in resting body temperature is modest. Core temperature is highly regulated and won’t drop enough to make you feel cold all the time. The hands and feet may feel colder, but major shivering is unlikely.
Do you lose muscle mass from eating less?
Cutting calories does unfortunately lead to some loss of lean muscle mass along with fat loss. However, the extent depends on several factors.
One study had overweight men and women cut 500 calories per day for 12 weeks. They lost an average of 5.3 pounds (2.4 kg) of lean mass and 33 pounds (15 kg) of fat (9).
Another study had obese men follow a very low calorie diet of 800 calories per day. After just 7 days, they had lost 5.1 pounds (2.3 kg) of lean mass but also 33.5 pounds (15.2 kg) of fat (10).
As these studies show, very low calorie intakes and rapid weight loss exacerbate muscle loss. Losing weight more slowly reduces lean mass loss. Exercise, especially lifting weights, can also help minimize muscle loss from eating less.
Do hormones like leptin change?
In addition to increasing hunger and appetite, calorie restriction also affects some of the hormones involved in weight regulation.
Leptin is an important hormone produced by adipose tissue that regulates energy balance. Lower leptin levels increase hunger and food intake.
Multiple studies show that leptin levels decrease rapidly with calorie restriction. One study found that only 1 week on a very low calorie diet decreased leptin levels by 63-65% (11). Another study showed an average 40% drop in leptin after 6 months of calorie restriction (12).
Another hormone affected by calorie restriction is peptide YY, which normally works to reduce appetite. Levels of peptide YY decrease with long-term calorie restriction (13).
These hormonal alterations in response to eating less all act to stimulate appetite and encourage eating. However, research shows the hunger-stimulating effects do seem to subside over time as weight stabilizes.
Does your cholesterol improve?
In addition to supporting weight loss, cutting calorie intake also has beneficial effects on heart disease risk factors like cholesterol levels.
LDL (bad) cholesterol levels tend to decrease significantly in response to calorie restriction. One study in 60 overweight women found that a low-calorie diet lowered LDL cholesterol by 15% more than a standard diet after 6 months (14).
Another study had obese subjects follow either a balanced diet reducing calories by 500 per day or a low fat diet reducing fat to less than 30 g per day. Both groups had a 10% reduction in LDL cholesterol (15).
Calorie restriction seems especially effective at lowering cholesterol when combined with exercise. One study found LDL decreased by 10% with calorie restriction but 20% when calorie restriction was combined with moderate exercise (16).
In addition to lowering LDL, some research shows that reducing calorie intake can also increase HDL (good) cholesterol. However, results have been mixed (17, 18).
Can it affect your fertility?
In men and women, substantial calorie restriction does seem to negatively affect reproductive hormone levels and fertility after prolonged periods.
In men, very low calorie intakes can lower testosterone levels. One study had obese men consume just 800 calories per day. After 8 weeks, their total testosterone levels decreased by 25% (19).
Multiple studies in lean and obese men have confirmed that calorie restriction causes a significant decline in testosterone, which can impair fertility (20, 21).
Women may also experience disruption to the menstrual cycle and reproductive hormone levels. One study had female rowers consuming 30% fewer calories than needed to maintain weight. After 6 months, 39% experienced menstrual disturbances (22).
Animal studies also report reduced fertility, ovulation issues, and changes in ovarian structures with chronic underfeeding (23, 24).
In both men and women, weight loss achieved through moderate calorie reduction does not seem to majorly impact fertility or reproductive function. But extreme calorie deficits could impair reproduction.
Does thyroid function decrease?
Some research shows that substantially cutting calories can reduce thyroid hormone levels, but results are mixed.
One study in overweight women found an 11% reduction in T3 thyroid hormone after 6 months on a calorie restricted diet compared to no change in the control group (25).
Another study in obese subjects found that a very low calorie diet providing about 50% of required calories significantly reduced T3 and T4 thyroid hormone levels (26).
However, other studies have found no change or only minor changes to thyroid hormones with moderate calorie restriction (27, 28). Weight loss achieved through regular aerobic exercise may help preserve healthy thyroid function compared to diet alone (29).
Overall, very severe calorie restriction is most likely to affect thyroid function, while more moderate calorie deficits have little to no impact on thyroid hormones.
Will you experience fatigue and feel cold?
It’s common to feel more sluggish and tired than usual when transitioning to eating less food. This temporary fatigue is partly due to lower energy intake from food.
However, metabolic adaptations and hormone changes also play a role in fatigue. As your metabolic rate decreases, you burn fewer calories and have less available energy.
Appetite-regulating hormones like leptin and ghrelin can also influence energy levels. Decreased leptin and increased ghrelin signal the body to conserve energy, which results in fatigue.
Additionally, calorie restriction reduces core body temperature slightly. You may feel chilled more easily with less insulation from body fat. Cold hands and feet are common.
Fortunately, this fatigue seems to dissipate over time. Taking it slowly when adjusting to fewer calories can help. Support your energy levels with regular physical activity, adequate protein and healthy fats.
Can eating less slow aging?
There is some intriguing research indicating that reducing overall calorie intake may help slow markers of biological aging and extend lifespan.
Animal research has found that restricting calories by 10–50% increased lifespan significantly in mice, rats, yeast, fish, flies, worms, and monkeys (30).
In humans, studies show links between reduced calorie intake and slower aging:
- Lower oxidative stress and DNA damage (31)
- Improved insulin sensitivity (32)
- Increased longevity (33)
Preliminary research suggests calorie restriction may work by triggering beneficial cellular repair processes and enhancing autophagy, the recycling of damaged molecules in cells (34).
However, current evidence in humans for the anti-aging effects of calorie restriction is limited. More research is needed on appropriate calorie levels and diet composition for longevity.
Does your immune system weaken?
There is concern that substantially cutting calories may weaken your immune system and increase illness risk. However, research indicates this depends on the degree of restriction.
One study had overweight subjects follow a diet providing 25% fewer calories than required. After three months, they had 30% fewer circulating immune cells and weakened immune responses (35).
However, other studies show that mild to moderate calorie restriction provides immune enhancing benefits:
- Reduced inflammation (36)
- Lower oxidative stress (37)
- Increased resistance to illness (38)
For optimal immunity, research suggests a modest calorie deficit of 10–25% of requirements. Severely restricting intake seems to impair immunity, while mild reductions may enhance it.
Conclusion
Eating less food forces your body to adapt metabolically and physiologically to a lower calorie environment. Your metabolism may slow slightly and appetite hormones adjust to stimulate hunger.
However, research shows your body can eventually adapt to feeling satisfied with fewer calories. Persistent fatigue, substantial muscle loss, and weakness are signs you may need to increase intake.
To minimize adverse effects, focus on consistent, modest calorie deficits combined with strength training and a nutritious, protein-rich diet.