As a society we have become more aware of the implications of our food choices with regards to our general health. We are taught by media and health professionals to think before we eat and to make “healthy food choices”. In many ways, watching what we eat has become an important step in achieving and maintaining a healthy lifestyle. For many individuals, however, the choice of what and how much to eat is an almost constant stress. Extreme dieting behavior and the prevalence of severe eating disorders such as anorexia nervosa are increasing.
It is well-established that malnutrition and clinical eating disorders such as anorexia nervosa are negative for bone health. Adolescents and adults with anorexia have very low bone mass and high risk for fracture (1). Weight loss through dieting or illness causes rapid bone loss (2,3). In anorexia nervosa, multiple factors cause bone loss and/or lack of bone gain including inadequate nutrition, decreased mechanical load on bone (due to low muscle mass and low body weight), lower reproductive hormone levels, and stress-related increases in cortisol levels.
In the general population, many generally healthy individuals, consciously try to limit their food intake in order to achieve or maintain a desired body weight or size. This is referred to as dietary restraint. In our research at the University of British Columbia, we have evidence from three different groups of girls or women that dietary restraint, as well as dieting behavior, influence how much bone is gained during adolescence and increase the risk for fracture. In this article we will define eating attitudes and dieting behavior, describe how stressors act to influence bone mass, and finally summarize key studies showing negative effects of dietary restraint and dieting on bone mass.
Eating attitudes and dieting behavior
Eating attitudes are assessed in both clinical and research settings with well established questionnaires. Two of those questionnaires, which we have used in our research, include the Three Factor Eating Questionnaire (TFEQ) (4) and the children’s Eating Attitudes Test (chEAT) (5). Both include questions used to define behaviors including dieting and bulimia (meaning self-induced vomiting). They also have questions designed to determine how much an individual thinks about or attempts to restrict food intake – termed “Dietary Restraint” on the TFEQ and “Oral Control” on the chEAT. Although these two terms are not identical, they are highly related and the terms will be used interchangeably for the purposes of this article.
Dietary restraint is defined as “a conscious attempt to limit food intake to regulate body weight”. Individuals scoring high on the restraint scale are extremely aware of the amount and type of food they consume (6). An example question used to assess restraint is: “how likely are you to consciously eat less than you want?”. Answers are scored from 1-5 ranging from not at all (1) to extremely (5). It is important to note that, although someone may score high on the restraint scale, they do not necessarily exhibit different patterns of eating behavior. In other words, many individuals who are extremely conscious and concerned about what they eat, do not actually eat less than those who happily eat what they want. In contrast, dieting behavior is an actual limiting of caloric intake resulting in weight loss. Weight loss schemes are often ineffective over the long-term and a rapid re-gain of weight is common. This loss and re-gain of weight is referred to as weight-cycling. We will discuss the effects of both eating attitudes/dietary restraint, and weight-cycling (or dieting) behavior. In our research, we have also studied individuals who score within what is considered to be the “normal” range for eating attitudes.
How could eating attitudes or dieting behavior influence bone mass?
Threats / Stresses:
- Physical — cold heat etc.
- Emotional — relationships
- Nutritional — undernutrition, fasting
- Overtraining — excess exercise
Figure 1. Process through which physical, emotional, or nutritional challenges cause increased release of CRH from the hypothalamus. These factors suppress the reproductive system and stimulate the adrenal axis. Abbreviations: ACTH = cortiotrophin; LH = lutenizing hormone. Adapted from Prior JC. Exercise associated menstrual disturbances. In: Adashi EY, Rock JA, Rosenwaks Z (eds). Reproductive Endocrinology, Surgery, and Technology. Raven, New York, 1996, pp. 1077-1091
Eating restraint and dieting behavior can be chronic stressors for many individuals. We hypothesized that bone loss occurs due to subtle decreases in reproductive hormones and increases in cortisol (a stress hormone) – both of these changes negatively effect bone mass.
In order to understand these relationships, it is important to discuss how the endocrine system responds and adapts to perceived stressors. In women, physiological adaptation to a number of stressors often results in changes in menstrual cycle function. These alterations range from subtle changes including shortened luteal phase length and anovulation in the presence of normal cycle length, to long cycles or amenorreha (lack of menstruation(. Menstrual cycle and ovulatory disturbances are often protective and necessary adaptations to increased stress (Figure 1).
The hypothalamus, which controls reproductive function and stress hormone release, receives information from several areas of the brain and functions to maintain homeostasis in response to internal and external demands. In situations of high stress, the body may adapt to prevent pregnancy for women or decrease libido and reproductive hormones in men, as a protective mechanism to conserve energy. Psychological or physiological stresses such as life changes, loss of a loved one, college stresses, inadequate energy intake, eating restraint, or rapid increases in exercise training, have all been associated with menstrual cycle disturbances.
These may be mediated by increased release of corticotrophin-releasing hormone (CRH) from the hypothalamus, which is hypothesized to suppress lutenizing hormone (LH) pulses through GnRH, possibly via the B-endorphin system, leading to suppressed reproductive capacity (7). Increases in cortisol also directly affect bone mass by increasing bone resorption. Thus, bone mass is negatively affected by both the increase in cortisol and the decrease in reproductive hormones (progesterone and estrogen) (Figure 1).
Similarly, dieting or other life stresses stimulate an increase in cortisol and a decrease in reproductive function (8-10).
What is the evidence that eating attitudes influence bone mass and fracture risk?
This section will summarize the results of a series of studies conducted at the University of British Columbia looking at the effects on bone mass of eating attitudes in university-aged women and adolescent girls, and of dieting behavior on fracture risk in adult men and women. Together these data provide important information about the role of eating attitudes and dieting behavior on bone mass and fracture risk.
Eating restraint and adult bone mass
Dr’s Susan Barr, Judy McLean and Jerilynn Prior have made a number of important observations about eating restraint in a series of studies in university-aged women (20-35y). The first of these studies showed that women with high restraint scores were more likely to have subtle menstrual cycle disturbances including short luteal phase lengths and anovulatory cycles (despite normal cycle lengths) (11). Women scoring high on the restraint scale also had higher cortisol levels in a 24 hour urine sample (12). Although the cortisol levels were within what is considered to be the normal range, these women also had a 1-3% lower bone mass at the spine and total body compared with women with low restraint (13). There were no other differences between the groups to explain the differences in bone mass. Interestingly, high eating restraint appeared to over-ride the positive benefits of exercise on bone mass (13). Taken together, these important data support the hypothesis that eating restraint, or a related aspect of restraint, is perceived as a stressor by the hypothalamus leading to subtle changes in the menstrual cycle, increased cortisol and decreases in bone mass.
Weight cycling and fracture risk Eating attitudes and bone mineral accrual
Childhood and adolescence are critical for the attainment of an optimal peak bone mass at adulthood (14). Peak bone mass is considered critical to preventing osteoporosis in later life. Thus, lifestyle factors should be optimized during these years. Therefore, we examined the role of eating attitudes in adolescent girls followed over 2 years (15). Girls were aged 9-12 years at the beginning of the study and were generally healthy, with adequate caloric and calcium intakes and were moderately active. Oral control scores, assessed using the children’s version of the Eating Attitudes Test, did not change during the two year study suggesting eating attitudes are established very early in life. Although we did not assess the parents’ eating attitudes, a previous study showed that 5-year old girls whose mothers dieted had high restraint scores (16). In our study, girls exhibiting high oral control had 3-5% lower bone mass at baseline and after 2 years (15). As in the studies of university-aged women, we could find no other factors to explain the differences between the groups.
Weight cycling and fracture risk
Figure 2. Proposed mechanism for bone loss with dietary restraint (from Dr. Judy McLean). Body weight is a well-established predictor of bone mineral density. People with higher body weights have higher bone mass (17) and weight gain protects against hip fracture (18). In contrast, with weight loss bone loss is inevitable (3). Individuals who diet to lose weight commonly regain what they have lost, and often more. This phenomenon, known as “weight cycling” is associated with lower spine and radius bone mineral density in premenopausal women (19) and with increased fracture risk (20).
We have recently looked at the role of weight cycling in 337 men and 728 women ages 25-96 years from British Columbia enrolled in the Candian Multicentre Osteoporosis Study (CaMOS).
In this study, men and women with 4 or more episodes of weight cycling (defined as weight loss and re-gain of 10 lbs or more) in their lifetimes had 1-3% lower bone mineral density at the hip and spine (common fracture sites) than individuals reporting no weight cycling episodes (21).
Men with more weight cycling also experienced more low-trauma fractures in their life time. This is despite the fact that the people with more weight cycling also had higher body weight – which is generally protective of bone.
There are some complicated issues in this study that we are currently examining. However, these data support the earlier work from the large Norwegian study suggesting that repeat dieting has a negative effect on bone health.
In summary, our studies and those of others suggest that:
- Dietary restraint and weight-cycling behavior can decrease bone mass and increase risk of fracture.
- Eating attitudes are established early in life and partially influenced by parental dieting behavior.
- Concern about food may prevent optimal gain of bone mass during adolescence.
- The negative influence of eating restraint on bone is at least partially mediated through a common stress mechanism due to the constant stress and worry about food intake.
- The effects of chronic stress on bone may over-ride the beneficial effects of exercise or other lifestyle factors on bone mass.
Although we have made great strides over the past decade in developing pharmecutical interventions for the prevention and treatment of osteoporosis and fractures, much less research has looked at lifestyle factors. New research suggests that stress and worry about food intake, attitudes towards eating, and “yo-yo” dieting behavior negatively affect bone mass and increase fracture risk. Eating attitudes are formed very early in life and associated to some extent with parental dieting behavior. It is important to encourage healthy eating attitudes and behaviors from a very early age. In addition to eating attitude and dieting behavior, reducing other life-stressors is also important for optimizing bone mineral accrual early in life, and slowing bone loss in later life.
Moira Petit is a post-doctoral research fellow at BC Women’s and Children’s Hospital and with the UBC Bone research group. Her research focuses on the role of physical activity, lifestyle, and endocrine factors in the prevention of osteoporosis. Jerilynn Prior is a Professor of Endocrinology at the University of British Columbia and an internationally known expert on women’s health.
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