Weight Loss Nutrition

Low Calorie Diets: Dangers and Considerations

National Academy of Sports Medicine
National Academy of Sports Medicine
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By Dominique Adair, MS, RD

The prevalence of adults with overweight or obesity represents an alarming public health challenge. Recent figures from the CDC show the majority of American society has become 'obesogenic,' characterized by environments that promote increased food intake, non-healthful foods, and low levels of physical inactivity (1, 2).

While numerous complex factorial mechanisms have been investigated, experts agree that weight loss occurs when energy intake is less than energy expenditure. This imbalance in the energy equation can be achieved by manipulating either side – meaning a decrease in intake or an increase in expenditure (or both) will provide the imbalance that will result in weight loss (3).

According to a recent study examining Behavioral Risk Factor Surveillance Survey (BRFSS) data, about 86% of the men and 92% of women attempting weight loss report eating less energy (energy restriction) or eating less fat. The proportion of people who reported using energy restriction alone doubled between 1996 and 2003, and low-fat dieting dropped by one-third (4).

Weight loss programs based on Kcalorie restriction are divided into two main categories: Low Calorie Diets (LCDs) and Very Low Kcalorie Diets (VLCDs). The primary difference is one of energy value, with VLCDs providing <800 Kcalories/d. LCDs diverge so profoundly in energy provision that there is no consensus as to how many Kcalories are provided.

VLCDs (i.e., Optifast and Medifast) have been used since the 1970s to induce rapid weight loss. They are often a treatment of choice for people who are more than 30% overweight with comorbidities, and require the monitoring of a health professional. LCDs include most popular weight loss programs (i.e., Weight Watchers, Nutrisystem, Jenny Craig, etc.)

The Dangers of Low Calorie Diets

Given these dieting trends, it is important for fitness professionals to consider if there are risks to following a Kcal restricted diet. Some of the proposed risks include:

  • Suboptimal nutrient provision.
  • Long-term metabolism depression.
  • A behavioral “backlash” from sustaining extended food deprivation.
  • Associated health risks.

Inadequate Nutrients

Many popular weight loss diets use 1,200 Kcal as the suggested Kcal minimum. This number derives from a scientific estimate of the lowest number of Kcalories required to deliver adequate nutrients. However, as fitness professionals are aware, everyone is different, so while some people will lose weight easily on 1,800 Kcal, others will need to decrease their intake to 1,200 to produce weight loss.

The primary risk in taking in less than 1200 Kcal is nutrient deficiency. Theoretically, the delivery system for all nutrients (macro- and micro-) is food. When we decrease food intake below a certain amount, we are also decreasing all the nutrients that foods contain, risking deficiency. It is therefore extremely important that clients following Kcal restricted diets choose nutrient dense foods.

Try combining caloric restriction with a healthy diet rich in micronutrients like the Mediterranean Diet

Metabolic Depression

Despite an abundance of popular advice about food and metabolism, reduced energy needs with weight loss are not solely attributable to metabolism. By losing weight, the body actually decreases its energy needs (the “cost” of moving around a smaller body is lower). Therefore, some decrease in energy requirement is an unfortunate “side effect” of weight loss, regardless of how the weight is lost. Scientists call this term “energy gap” and it has been quantified at approximately 8 Kcal/pound– In other words, if someone loses 50 pounds, they will need 350-400 less kcal to maintain their smaller body (5).

Behavioral Backlash

Regaining weight is common following restrictive weight loss programs and most evidence suggests that the vast majority of people who lose weight regain it during the subsequent months or years (6). However, there is little data to support that the rate of regain is positively related to the degree of restriction, in fact studies show that both VLCDs and LCDs are associated with weight gain over time (7).

Health Risks

If the diet does not provide adequate nutrients to support good health, these deficiencies can also be considered secondary health risks. In terms of direct risk to health, the most common serious side effect is gallstone formation. Gallstones, which often develop in people who are obese, especially women, are even more common during rapid weight loss. Additionally, fatigue, constipation, nausea, or diarrhea are common complaints with severe Kcal restriction.


While considerable short term success is reported with low- and very low-energy diets, the inescapable conclusion is that the programs that are effective in producing weight loss are not necessarily successful in producing long-term weight-loss maintenance. Additionally VLCDs, may be no more effective than less severe dietary restrictions in the long run, and do pose a higher incidence of adverse events and increased need for medical monitoring.

Additionally, for most people who are obese, obesity is a long-term condition that requires a lifetime of attention even after formal weight-loss treatment ends. The fitness professional is in an excellent position to reinforce a commitment to permanent changes of healthier eating, regular physical activity, and an improved outlook about food, and may want to deemphasize the extreme Kcaloric restriction in favor of a more moderate lifestyle change program in support of successful weight loss and the prevention of regain.

Read also about High Protein/Low Carb Diets

Key Points

Low energy (versus low-fat or low-carb) dieting is gaining in popularity, as practitioners emphasize that a negative energy balance is necessary to produce weight loss.

Several proposed risks to low Kcalorie dieting, include nutrient deficiencies, metabolic depression, deprivation based behavior swings, and physiological health risks.

While some success is reported with energy restricted diets, fitness professionals are in an excellent position to widen their clients’ focus to include long-term behavior changes to support weight loss and weight loss maintenance.

For more on helping clients lose weight, check out NASM's Weight Loss Specialist. And be sure to check out our course on navigating diets for more help on the subject.


1. CDC website

2. Flegal KM et al. Prevalence and trends in obesity in US Adults, 1999 – 2008. JAMA. 2010;303(3):235-241.

3. Bray G and Champagne C. Beyond Energy Balance: There Is More to Obesity than Kilocalories. J Am Diet Assoc 2005 May;105(5 Suppl 1):S17-23.

4. Andreyeva T, et al. Trying to lose weight: Diet strategies among Americans with overweight and obesity in 1996 and 2003. J Am Diet Assoc. 2010 Apr;110(4):535-42.

5. Hill JO, Wyatt HR, Reed GW, Peters JC. Obesity and the environment (Where do we go from here?). Science. 2003;299:853–855.

6. Pace PW, Bolton MP, and Reeves RS. Ethics of obesity treatment: implications for dietitians. J Am Diet Assoc. 1991;91:1258–1260.

7. Wadden TA. Treatment of obesity by moderate and severe caloric restriction. Results of clinical research trials. Ann Intern Med. 1993 Oct 1;119(7 Pt 2):688-93.

The Author

National Academy of Sports Medicine

National Academy of Sports Medicine

Since 1987 the National Academy of Sports Medicine (NASM) has been the global leader in delivering evidence-based certifications and advanced specializations to health and fitness professionals. Our products and services are scientifically and clinically proven. They are revered and utilized by leading brands and programs around the world and have launched thousands of successful careers.


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