Understanding Weight Gain. Part One – Physiology

Understanding Weight Gain. Part One - Physiology

Understanding Weight Gain. Part One – Physiology

Author: Matt Whitaker, Digital Health Lead

In October I was invited to speak at the Diabetes Professional Care Conference on ‘Understanding weight gain and the risks associated with it’. During my research for this talk I gained such insight into how much of a massive topic this is. How we gain weight is incredibly diverse, yet also specific to any one individual gaining weight. Largely, it can be covered within three main factors: physiological, psychological and environmental. This blog will act as the first in a three part series and will cover physiological weight gain.

Before going into the main body, I will, as I did in my presentation address two basic and relevant myths:

* Weight gain is not a simple as eating a high carb or high fat diet. Any macronutrient has the capability to be stored indirectly as fat if the individual is over consuming food.

* It is not as simple as ‘energy in vs energy out’. Despite the first law of thermodynamics holding true, energy can be excreted. Intake and output can be sub-consciously influenced and requirements fluctuate daily

* Physiology is a branch of biology that covers functions within a living organism. Below are the major factors that would be considered physiological in how they impact on weight gain.

Change in basal metabolic rate

Basal metabolic rate (BMR) is the amount of energy required to sustain life at rest i.e. essential bodily functions. It is impacted largely by: muscle mass, age, body size, lack of sleep, gender and genes.

Diet can also influence, particularly when following a standard calorie controlled, low fat diet, in an attempt to reduce body weight. When one ‘diets’ in this manner, their metabolic rate can reduce to match the new energy intake, this can happen relatively quickly but can take months to increase back to the original value. Something I have covered in a previous blog.

To minimise weight gain as we age, it would be wise to: avoid crash dieting, incorporate resistance training and get enough good quality sleep.


Hormones influence our appetite and ability to prioritise stored fat as an energy source. Specifically:

Insulin acts as an anabolic hormone and puts the body into a state of storing energy. When insulin levels are high, stored energy such as glycogen and fat cannot be readily accessed. When levels are low they can.

Glucagon has the opposing effects to insulin. When levels are high the body prioritises breaking down glycogen and fat for energy and when levels are low it preserves both.

Leptin is one of the primary satiety hormones, when levels are high appetite is suppressed and one feels full. When levels are low, hunger may be prevalent.

Ghrelin is a hunger hormone, when levels are high one feels hungry and when they are low one is satiated.

There are a wealth of other hormones involved in these processes too but these are arguably the more researched ones. If levels of insulin and ghrelin are chronically high and leptin and glucagon frequently low, the body is in a position where weight gain is far more likely.

These hormones can be manipulated to favour against this by eating a diet based on unprocessed foods, eating carbohydrate to tolerance, regularly partaking in physical activity and appropriately managing stress and sleeping well.

Circadian rhythm

This refers to a sub-conscious, internal clock that dictates eating and sleeping times through releasing and supressing hormones and neurotransmitters. Habits are formed from an early age due to these physiological responses e.g. consumption of breakfast but research suggests these can be reconditioned.

It would be wise to largely listen to the bodies urges, particularly when it comes to sleep. When we eat food can also influence other health factors e.g. eating too late at night may contribute to poor sleep quality, overeating and weight gain. Reducing eating frequency may help address these, amongst other factors such as: inflammation, stress and gut microbiota health.

Sleep quality and stress management

Not sleeping enough or having poor quality sleep can contribute to weight gain by several means. It disturbs hormonal homeostasis by increasing ghrelin and supressing leptin. This combined with a heightened reward response to food, impair decision making and self-control may lead to overeating. Additionally the tiredness can lead to convenience eating which tend to be focussed on hyper-palatable, ultra-processed foods, which in-turn are easier to overconsume. If this pattern continues overtime weight gain can become apparent. There may also be a direct link with BMR reductions of up to 20% following a poor night’s sleep.

Chronic stress can elevate the hormone cortisol which in-turn can increase insulin and ghrelin. This means that physiologically speaking stress, can increase fat storage and hunger. Additionally it may lead to similar factors as the above such as convenience eating and seeking processed ‘comfort’ foods.

Aiming for seven to nine hours good quality sleep a night can help reduce risk of the above. It will aid weight management, as will taking steps to address stress, if appropriate.


It has been suggested that it takes millions of years for genes to significantly change. Considering how obesity prevalence has accelerated over the past few decades it is unlikely that genes have played a large role in this.

With this been said, some fairly recent research has proposed as there are so many different types of genes, some are indeed involved in weight management and if we can find a way of manipulating these we may be able to use this to help combat obesity. Overall more research is needed with genes and weight gain.

Thermic effect of feeding

It takes energy it digest and absorb food, the amount required varies on the macronutrient and its complexity to digest. Protein has the highest at 20 – 30%, meaning for every 100kcal protein consumed your body will require 20 – 30kcal to digest it. Carbohydrates are 5 – 10%, fats 0 – 3% and alcohol is 10 – 30%.

Generally, the more natural the food is, the more energy it will take to digest and absorb, likely due to higher protein and/or fibre. The more processed a food, the less energy required, meaning more calories are absorbed.

Non-exercise activity thermogenesis (NEAT)

This relates to any activity performed that is not predetermined e.g. walking to work, doing the gardening, house work, shopping etc. The higher ones NEAT, the more energy they expend on a daily basis, which can reduce the likelihood of weight gain.

Modern and technological living is promoting an environment with minimal NEAT and those who are overweight have lower NEAT due to sedentary behaviour.

Performing simple changes such as getting off the bus a stop earlier, avoiding taking the lift, walking where possible will increase NEAT and thus potentially reduce risk of weight gain overtime.

Fitness and strength adaptations

This is the main limitation with the ‘moving more’ message for weight loss- the body adapts to a continued stimulus. If exercise is a component in a weight loss programme then some form of progression should be put into place, otherwise its usefulness will drastically reduce overtime.

Any activity is better than no activity. To maximise benefits it is important to continue to challenge yourself and not become overly competent with a routine.

Gut microbiota

The gut microbiota are partly responsible for many essential functions, including inflammation regulation, the immune system and food absorption.

It has been suggested that overweight/obese individuals are more efficient at absorbing calories from digested food, due to a ratio of bad to good bacteria, favouring the bad. This is a fairly new concept and more research is needed but if true, taking steps to improve gut microbiota health could aid in weight management.

Prenatal and postnatal influences

Smoking/drinking whilst pregnant may impair foetus metabolism, which could lead to weight gain following birth and throughout life. Weight gain during infancy also raises risk of obesity and does not been breastfed.

Research suggests that living a healthy lifestyle can counteract these effects and reduce the likelihood of weight gain.

These factors sum up some of the physiology of why and how weight can be gained. My next two blogs will cover the psychological and environmental factors behind weight gain.

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