Metabolism and weight – advice for midlife and beyond
Wondering how ageing impacts metabolism and weight through midlife and beyond? Dr Harriet Holme, a registered nutritionist, busts metabolism myths and shares her top tips for maintaining a healthy weight for the long haul.
What is metabolism?
Metabolism describes the chemical processes in our body that keep us alive and functioning normally, including converting food and drink into energy.
This process begins when our food is broken down into the tiny constituent building-blocks that are absorbed via the small intestine. There, the food is further broken down into a chemical called pyruvate, which will later be transformed into adenosine triphosphate (ATP) – our body’s energy currency. All bodily processes that require energy rely on ATP. Without our metabolism, we wouldn’t be able to extract this from our food.
What’s the difference between metabolism and metabolic rate?
As we have seen, metabolism describes the process of extracting energy from our food. The word metabolism is often used interchangeably with basal metabolic rate (BMR), but the two are not the same. Our BMR describes the number of calories we burn for our vital functions.
For example, our brain, liver, heart and kidneys account for almost half our BMR. Most people associate metabolism with weight gain and weight loss, when really they mean metabolic rate.
We can calculate BMR from our weight, age and gender. It accounts for most of the energy we expend throughout the day. Physical activity burns through about 10-30% of our energy stores, while thermogenesis (keeping our body at 37°C) uses 10%.
How does metabolism affect weight?
Many people assume that boosting metabolism will prevent weight gain, and that people who are overweight have a slower metabolism. In fact, people with a higher weight have a higher BMR and use more energy to perform everyday activities, as it’s harder to move about. Lean body mass contributes most to our BMR, so the higher our muscle mass, the higher our BMR. If we wanted to increase our metabolism, we’d instead need to get more energy from food, which would likely push the energy balance in our body to excess, leading to weight gain. For this reason, weight gain is less to do with metabolism and more to do with our energy balance and microbiome (more on this later).
How can I manage my weight?
Our BMR only causes weight gain in rare cases, such as in those with hypothyroid disease (an underactive thyroid). Fundamentally, what controls our weight is the balance of how much fuel we eat, compared with how much we expend throughout the day. If we eat more than we need, we will store it as fat and gain weight. If we eat less than we need, we will lose weight. When we eat fewer calories than we need, our body perceives this as starvation and drops our BMR to protect its fat stores. This means we lose less weight , despite eating less food. The evidence suggests that a slow, gradual reduction in food intake has less of a negative impact on our BMR. This is why a slow and steady approach to weight-loss often shows more sustainable results than crash diets.
As most of us know, it’s much easier to gain weight than lose it. This is probably an evolutionary adaptation to protect us in times of famine. This is a theory known as the ‘thrifty gene hypothesis’. However, using this hypothesis we might predict that everyone with freely available food would become obese. This isn’t the case. We need to understand the role of ageing, estrogen and our microbiome to get a clearer picture of influences on our weight.
What happens to our metabolic rate as we age?
Our muscle mass naturally decreases with age. This is known as sarcopenia, and it’s estimated that we lose around 8% of this mass every decade after 40. This means our muscles are using less energy (our BMR declines). We risk being in energy excess if we eat the same quantity of food and do the same amount of exercise. With age, the amount of physical activity people do often also declines, further shifting the balance of energy to excess and risking weight gain. There is hope, however. It’s possible to increase muscle mass through resistance training, which will increase our BMR and also strengthen bones.
What happens during the menopause?
As we approach the menopause, our estrogen levels decline. Estrogen promotes energy homeostasis, improves body fat distribution, enhances insulin sensitivity, improves pancreatic cell function and reduces inflammation. This means that, as estrogen levels decline, we’re more at risk of developing metabolic dysfunction. This can lead to obesity and type-2 diabetes, as well as cancer and other degenerative diseases of the skeletal, central nervous and cardiovascular systems.
We also know that estrogen affects energy balance in the brain. It acts on the hypothalamic area to both suppress food intake and also stimulate physical activity, energy expenditure and regulate body fat distribution. Experiments on rodents show that surgical removal of the ovaries leads to weight gain. This is reversed by estrogen supplements.
If we combine all these effects, it’s no wonder that many women struggle with weight during this time. Should you wish to consider body-identical HRT to supplement your estrogen levels, speak with your GP.
“Many women wrongly believe HRT causes weight gain. Studies show that more women actually lose weight when taking HRT compared to those taking a placebo (dummy pill). In its 2016 review, the International Menopause Society is clear that ‘contrary to widespread belief, menopausal hormone therapy is not associated with weight gain and may ameliorate perimenopausal accumulation of abdominal fat’.
“This reduction in fat around the middle is due to HRT helping to disperse the ‘middle-age spread’. One of the functions of estrogen is to control where our fat ends up being deposited. When our estrogen levels are low, fat accumulates around the tummy – the notorious middle-aged ‘muffin top’. This is one reason why HRT also helps cut coronary heart disease. We know that women whose fat is mostly accumulated around the middle (the so-called apple shape) are more at risk of heart attack and stroke.
“We can also help to keep the weight off by doing more exercise, although we may need HRT to give us the energy and enthusiasm to do so if we’re battling the achey joints, low mood and fatigue that so often comes with low estrogen levels.”
Metabolism and the microbiome
Our gut contains trillions of single-celled microorganisms that play an important role in our health. Research suggests that these gut microorganisms have a role in shaping our metabolism. Recent research shows that taking gut-friendly prebiotic supplements is associated with improvement of some of the parameters of the metabolic syndrome (increased blood pressure, high blood sugar, a large waistline, and abnormal cholesterol or triglyceride levels).
In studies of special germ-free mice (who have no organisms living on or inside them), they found that faecal transplant from obese humans was associated with a greater weight gain than mice that received microbes from healthy-weight humans. This suggests that maintenance of a steady healthy weight is more than the simple balance of energy in and out. Our gut microbiota are also playing a role.
Most studies of people that are obese show that they have a lower diversity of organisms making up their gut microbiota. Long-term weight gain (over 10 years) has also been associated with a low microbiota diversity. We can increase our microbial diversity by eating plenty of fibre and gut-friendly fermented foods.