I am super excited to have Amanda Gaukroger writing a guest post for me today about Weight Loss and BMR! Amanda has been writing for me regularly over on The FODMAP Challenge blog – head on over to find out more about her!

Over to you Amanda…!

Weight Loss and BMR

When it comes to weight loss, there seems to be a new diet trend hitting the headlines every week. As a result, it is becoming increasingly difficult to dissect the facts from the myths about what actually works, and more importantly, what is safe.

When we think of weight loss, eating less commonly comes to mind. Theoretically, if you consume less energy than you expend you should lose weight. This is known as negative energy balance. But how much do we need to reduce our intake by? And are low calorie diets actually beneficial?

Weight loss can occur through increasing your energy expenditure or decreasing your energy intake. So, generally either burning more calories via exercise, or eating fewer calories. Approximately 60-70% of your energy expenditure is attributed to your Basal Metabolic Rate (BMR). BMR is the minimum amount of energy expended to sustain basic life processes, such as breathing and blood circulation. Your BMR decreases with age and loss of lean muscle mass. This is why metabolism starts to slow down with age. BMR also declines when in negative energy balance, which means eating below your BMR can slow metabolism.

Why would eating below our BMR slow down metabolism?

There are two main reasons why our BMR declines with negative energy balance:

  1. The body’s adaptive mechanism to conserve energy during starvation. Negative energy balance can cause a reduction of up to 20% in the metabolic rate per kg of lean body mass. So basically, a negative energy balance causes the body to think it’s starving, and will hold onto more energy.
  2. Lean body mass itself decreases with long-term negative energy balance.

So does eating below your BMR lead to weight loss?

Despite reductions in BMR, a negative energy balance over an extended period of time is required to lose weight (body fat). An important factor when wanting to sustain weight loss, however, is the amount of energy the diet is restricted by. Evidence suggests that a moderately reduced energy intake of 300-500 calories (1200-1500kJ) per day results in greater fat loss, compared with more severe energy restriction. Whilst those who restrict their energy intake more drastically may lose weight more rapidly, they are also more likely to regain weight compared to those with a moderately decreased intake.

It is important to remember that both fat and muscle will be lost when losing weight. The proportion of each is dependent upon the mechanism of weight loss used. Rapid energy restrictions will usually result in a higher proportion of lean muscle mass lost, and can therefore slow BMR.

What amount of energy reduction is required to lose weight?

The BMR and energy requirements of each individual differs and is influenced by factors such as age, height, weight, proportion of lean muscle mass to fat mass, activity level, occupation, gender, genetics and so on. Therefore, the amount of energy reduction required for weight loss is largely dependent on the individual and their personal goals. On average, a daily energy deficit of 500 calories (2000kj) and 1000 calories (4000kj) should produce a weekly weight loss of 0.5kg and 1kg, respectively.

How much weight loss is safe?

Weight loss of 0.5kg-1kg per week is considered a healthy amount for most individuals. An energy deficit of more than 1000 calories a day could result in poor health outcomes such as increased susceptibility to weight gain (which is more likely to be fat mass), slowed metabolism, loss of lean muscle mass, nutritional deficiencies, negative influence on mental health status, and reduced quality of life. It is important to remember though, that some individuals may lose weight more quickly, some more slowly. This depends on other factors, such as sleep, hormones and dieting history. It is common to see a bigger drop initially, especially if an individual is considerably overweight, before the weight loss starts to slow down.

Other potential benefits of low-calorie diets?

Evidence suggests low-calorie diets may be helpful in improving inflammation and blood sugar level control, which could be beneficial in managing type 2 diabetes and obesity. This effect is largely due to the weight-loss itself caused by low-calorie diets.

It is important to consider the nutritional components of the low-calorie diet rather than the numerical energy value. Negative energy balance indicates one or many nutrients are reduced. It is not recommended to undertake a very low-calorie diet at the expense of a healthy nutritional status. It is essential that individuals undergoing very low-calorie diets do so under the supervision of an accredited health professional.

Low-calorie diets can be effective in achieving weight loss and improving health status in the short term. The evidence behind long-term effectiveness remains unclear. It appears the likelihood of maintenance and long-term success of weight loss is largely dependent on the degree of energy restriction and rate of weight-loss.

 

To ensure a low-calorie diet is nutritionally adequate and safe, it is recommended to seek advice from an Accredited Practicing Dietitian.

About Chloe McLeod