When working with clients who have dieted in the past, one of the most common experiences I hear is their frustration in regaining the weight they worked so hard to lose. And for most who diet, this is not an uncommon experience.

In fact, in an article published in the International Journal of Obesity, researchers analyzed 33 studies looking at people who had undergone combinations of dieting and exercise for weight loss. They found that participants regained, on average, approximately 50% of their baseline weight within one year (1).

So why does this happen? What makes keeping weight off so challenging, and is there any hope?

Understanding why we weigh what we weigh, how our body weight is regulated, and how our bodies respond to dieting will help answer these questions. This knowledge will then inform the weight loss and weight-loss maintenance strategies discussed in Part 2 and Part 3.

Understanding (changes in) body weight

Explaining why we weigh what we weigh may seem obvious – it’s the result of how much we eat and move, right? Unfortunately, it’s not that simple.

What we weigh is the result of our biology (energy balance system) interacting with environmental and behavioural factors – all of which are influenced by our genes (2,3).

Biology (energy balance system)

Energy balance is the balance of energy (calories) we take in versus the energy (calories) we expend. It’s regulated by a complex feedback loop between the brain and the periphery – stomach, liver, pancreas, intestines, fat and muscle tissue, and nutrients and hormones in the blood. The periphery conveys information to the brain about short-term food intake and long-term energy stores. In turn, energy intake and expenditure are adjusted according to the body’s energy needs (2,4–7).

Despite large fluctuations in daily energy intake and expenditure, our body weight remains relatively stable when averaged over many months (8). However, when there’s a consistent imbalance between energy intake and expenditure (over a long time scale), body weight will either increase or decrease (9,10).

But we don’t gain or lose weight indefinitely. Our energy balance system activates compensatory mechanisms to reestablish a new steady-state weight (2,11). However, it seems that our body defends more strongly against weight loss than weight gain (12) – which makes sense from an evolutionary perspective (13).

Environmental factors in the context of biology

Our modern environment is one in which there’s easy access to ultra-processed foods (14) and little need for physical activity. This combination “promotes behaviours that increase energy intake in a way that does not elicit strong biological opposition” (12,15).

For example, most ultra-processed food is less expensive than whole foods, and you can buy food in large portions. Food is also heavily advertised, and it’s become acceptable to have food everywhere (16).

So, given the modern food environment, it’s easy to see why we’re prone to eating much more than we were decades ago (16).

What role do genes play?

Because not everyone becomes overweight or obese in our modern environment, the role of genes has been an area of significant research (3,17–20).

Our genes play an important role in determining our weight – which is evident in the increased risks of people developing obesity if it runs in their family (21,22).

Twin and adoption studies also demonstrate a strong genetic basis for individual body weight.

A study investigating the weights of twins raised apart shortly after birth found the twins had similar weights despite being raised in different environments (23).

Another twin study involved overfeeding 12 sets of identical twins the same number of calories. Weight gain was very similar within twin pairs but varied widely among pairs of twins (24).

Finally, a study of 540 adoptees found their weight correlated highly with their biological parents’ weight but found no relation to their adoptive parents’ weight, despite being raised in the same environment (25).

Research shows many biological processes, including energy storage – whether in the form of fat, muscle, or glycogen (26,27) – are all, at a basic level, determined by the genes coding for these pathways.

Similarly, even some aspects that may seem behavioural, such as appetite and taste preference, have been linked to particular genes (28). These genes may explain why some people seem to have a stronger appetite and preference for certain foods.

As we can see, there’s an evident association between a person’s genes and their body weight.

How our bodies respond to dieting

A common weight loss strategy is to reduce calorie intake. While this is a useful short-term approach, it’s often not sustainable (1,29). As weight is lost, energy expenditure decreases and appetite increases in an attempt to regain the weight (30–32).

Our bodies adapting to weight loss highlights an important but often overlooked weight management concept - the need to approach weight loss and weight-loss maintenance as distinct phases. Click To Tweet

The weight loss phase is a “temporary period of negative energy balance (15).” In contrast, the weight-loss maintenance phase requires maintaining “energy balance at a new level” (15).

Weight loss phase

There are several ways to achieve a negative energy balance. You can restrict food intake, increase physical activity, or both. For the sake of this example, I’m going to explain what happens when using food restriction alone.

During the weight loss phase, energy intake will be less than energy expenditure, causing an initial weight loss. However, over time, energy expenditure decreases (11,33), and the gap between intake and expenditure narrows, causing weight loss to slow as you reach your goal weight.

Weight-loss maintenance phase

Once you’ve reached your goal weight, you’re in the weight-loss maintenance phase. Here, you can slightly increase your energy intake to maintain weight loss and prevent weight regain*.

For example, a study by Hall et al. (34) showed that for a 100 kg man to lose 20 kg in six months, he would need to reduce his energy intake from 3,000 to 1,800 kcal/day. After reaching this goal, his energy intake should increase to 2,600 kcal/day to maintain his weight at 80 kg.

*Using the Body Weight Planner, you can see how much your energy intake needs to increase to maintain weight loss. Failure to do so keeps you in a negative energy balance, which is not the goal of this phase.

What next?

In Part 2, I give an overview of the best strategy to lose and maintain weight loss. I also share the common characteristics of individuals successful at long-term weight-loss maintenance.