Published by PMR of NY & CT
Eating fewer calories than your body uses is the basis of almost every weight loss plan, so it's fair to ask whether a low calorie diet actually works. The short answer is yes: when you consistently take in less energy than you burn, you lose weight. The longer answer is that how you cut calories matters as much as whether you do.
A moderate plan built on filling foods tends to hold up. Drastic cuts may bring fast early results, but they are harder to keep up, can cost you muscle and, at the extreme, need a doctor's oversight.
What counts as a low calorie diet
There's no single definition. In everyday use, a low calorie diet means eating noticeably less than you need to maintain your weight, through smaller portions and lighter food choices. How many calories that means depends on your size, age, sex and activity, so the right amount for you may be very different from someone else's.
A very low calorie diet is a different thing. The NIH describes it as a supervised plan of about 800 calories a day or fewer, often using meal replacement shakes or other formulas, for some people with obesity. It's a medical treatment, not a do-it-yourself diet.
What studies show about cutting calories
Studies are fairly consistent on a few points:
- Reduced-calorie diets lead to weight loss whether they cut fat, carbs or both. The pattern you can stick with matters more than the mix.
- Most weight loss happens in the first several months, then levels off.
- Regaining some weight over the following years is common, which is why long-term habits matter.
- Structured programs with regular support, such as group classes or visits with a dietitian, tend to do better than going it alone.
Even a modest loss can improve health for people who have excess weight, including blood pressure and blood sugar.
Very low calorie diets need medical supervision
Very low calorie plans can produce fast weight loss, and doctors sometimes use them for specific reasons, such as before certain surgeries. Because the body is under real strain, these programs include regular checkups and lab tests. Possible problems include:
- Gallstones, which become more likely with rapid weight loss
- Fatigue, constipation, nausea or diarrhea
- Muscle loss and thinning hair
- Blood sugar or blood pressure dropping too low in people taking medicine for diabetes or high blood pressure
These plans generally aren't appropriate during pregnancy or breastfeeding, for children and teens, or for people with a history of an eating disorder. Don't try one on your own.
Protecting muscle and joints while you diet
When you eat less, your body can lose muscle along with fat, especially if you are inactive or older. Muscle helps you stay strong, steady on your feet and able to keep up with daily life. Two habits help protect it:
- Eating protein at each meal
- Doing strength exercises at least twice a week
If arthritis or another painful joint makes exercise hard, that doesn’t rule it out. At Physical Medicine & Rehabilitation of NY & CT (PMR), our physical therapy clinicians can design a strength program around sore knees, hips or shoulders and progress it safely as you get stronger.
When to get medical help
Check with your doctor before you start a low calorie diet if you are pregnant or breastfeeding, live with heart or kidney disease, take medicine for diabetes or high blood pressure, or have had an eating disorder.
Get medical care right away for sudden, severe pain in the upper right belly, especially with fever, vomiting or yellowing of your skin or eyes, which can signal a gallbladder problem. Call 911 for fainting, chest pain or a racing or irregular heartbeat.
Frequently asked questions
Are meal replacement shakes a good idea for weight loss?
They can be. Replacing one or two meals a day with a shake or bar takes the guesswork out of portions, and research shows they can support weight loss as part of a structured plan. Look for a product with enough protein and little added sugar, and plan regular meals for the rest of the day. Replacing every meal with shakes, especially at very low calorie levels, calls for medical supervision.
Is intermittent fasting better than a low calorie diet?
For most people, the results are similar. Intermittent fasting limits when you eat, such as within an eight-hour window, which helps some people eat less overall. Studies comparing it with daily calorie cutting generally find comparable weight loss. Choose the approach that fits your schedule. If you take diabetes medicine, are pregnant or have struggled with disordered eating, talk with a doctor first.
Is it safe to cut calories after age 65?
It can be, with care. Older adults lose muscle and bone more easily, so a gentle approach that keeps protein intake up and includes strength and balance exercise is safer than a strict diet. Unplanned weight loss in an older adult can signal a health problem, so talk with your doctor before you start and about any loss you aren't trying for.
Stay strong while you lose weight
Cutting calories goes better when you keep your strength. If sore joints make exercise difficult, PMR can help you train safely, with physical therapy at our Danbury and Brewster locations. Request an appointment and our team will contact you to schedule. A registered dietitian or your doctor can help you choose the eating plan that fits.
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