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Nicole Golden

The 30s are the decade of life that has its pros. Likely, you are beginning to feel established in a career, are looking at purchasing a home, and enjoy a more comfortable life. However, you have likely noticed that you may feel a little stiffer in the morning, take longer to recover from workouts, perceive higher levels of stress, and not sleep as well as you once did. As you enter your 30s, your body has begun the earliest phases of aging, and the first thing most of us notice is how quickly the scale can creep up on us. This is not in your imagination. Certain factors lead to changes in body composition as we age.

Yet, we can control these changes and get our ideal body composition by making and maintaining some healthy lifestyle choices.

If you want to put these concepts into practice, become a Weight Loss Specialist with NASM

Why We Gain Weight After Our 20s: What We Know

Common wisdom would tell us that our metabolisms simply slow down as we pass 30 because of the aging process, but this process is far from completely understood. It is downright controversial. Let us start with what we know.

The decline in skeletal muscle mass

Skeletal muscle is an important mediator of metabolic health. Skeletal muscle is metabolically expensive tissue (uses a lot of calories just to exist) and interacts with other organ systems via the secretion of peptides and cytokines. Skeletal muscle can also promote healthy glucose metabolism as it uptakes a great deal of circulating glucose, providing a place for that glucose to be stored rather than be converted to fat tissue.

Several well-known equations for predicting resting metabolic rate (i.e., the Katch-McCardle formula) is dependent on the amount of lean body mass a person has rather than their body weight (Kim & Kim, 2020).

Overall, the higher amount of skeletal muscle mass a person has, the higher their resting metabolic rate will be. This is one of the reasons why resistance training helps with weight loss.

Skeletal muscle is very responsive to stimulation in young adults, but with age, muscle cells become less responsive to stimulus. Likewise, it is well-established that humans tend to lose 3 to 8 per cent every 10 years or so after the age of 30. This may be in part due to changes in lifestyle and exercise habits. Middle age often comes with more responsibility, oftentimes, sedentary jobs, and less free time for personal pursuits (i.e., workouts).

However, the diminishing response of muscle cells is also partially responsible. The bottom line is, that it is harder (but not impossible) for us to maintain skeletal muscle as we near middle adulthood. A decrease in skeletal muscle mass can often lead to a decrease in resting metabolic rate. This makes weight gain, even if we are eating the same foods and amounts we always did, much easier.

The decline in reproductive hormone levels

The primary reproductive hormones- estrogen, progesterone, and testosterone begin a small, but steady decline in his decade of life. Although males will maintain “normal” testosterone levels into older adulthood, testosterone levels will decline an average of 1-3 per cent per year in this decade of life. Females will also experience a decline in serum testosterone levels even though the normal ranges are much lower (less than 10 per cent) than what an adult male would have at the same age.

The primary female reproductive hormones (estrogen and progesterone) will become less reliable and may be produced in irregular amounts as females approach 40 years old (Horstman et al., 2012). Human growth hormone, which is necessary for maintaining skeletal muscle and bone mass, will also decline during middle age, contributing to an increase in fat mass and decreased resting metabolic rate (Bartke, 2019).

Changes in physical activity

Individuals who remain more physically active into middle age fare better in health measures including maintaining a healthy weight. However, it is not uncommon for busy schedules filled with work and family obligations to reduce the time available for physical activity. It also becomes physically more difficult to exercise regularly as markers of physical fitness (i.e., VO2 max, muscle endurance, flexibility, and muscle strength) tend to decline after the 20s (Moreno-Agostino et al., 2020).

Westerterp (2018) describes a predictive model of physical activity over the lifecycle. The model determined that physical activity tends to be moderate until adolescence and very early adulthood at which point it peaks and then enters a steady decline in the early 30s to older adulthood. The decrease in physical activity does directly correlate to weight gain in middle age.

The mechanism of weight gain is likely due to a decrease in resting metabolic rate as a direct result of failure to attain peak fat-free mass in addition to middle-aged adults generally expending fewer calories per day via physical activity.

Why We Gain Weight After Our 20s: What We Do Not Know

Is an enzyme to blame?

Recent research has brought into question the role of genetics and normal physiologic processes being responsible for weight gain in middle age. The enzyme DNA protein kinase (DNA-PK) seems to affect the rate at which nutrients are stored as fat while simultaneously decreasing the number of functioning mitochondria (the place where cellular metabolism takes place).

Interestingly, this enzyme seems to increase in amount and activity as humans age. This mechanism would also explain why weight is more easily gained and exercise becomes more difficult as people approach middle age. Similarly, the activity of DNA-PK seems to decrease with caloric restriction and physical activity (Chung, 2018).

However, since much of the research on this topic has been conducted in laboratory mice, it is difficult to definitively say that DNA-PK is the sole cause of weight gain in middle-aged adults. Currently, researchers are looking at developing drugs that can inhibit the activity of this enzyme. Again, these drugs have proved useful in laboratory mice, however, more research needs to be conducted before it is ready for human trials.

Does our resting metabolic rate change?

This idea that our metabolism slows down at all in middle age has become very controversial with the publication of a landmark study calling into question the idea that human energy needs decrease after early adulthood. Pontzer et al. (2021) conducted a longitudinal study examining the metabolic rates of more than 6,400 individuals from infancy to 95 years old to determine if the metabolic rate does in fact decrease after early adulthood.

The researchers accounted for differences in body size and composition. The scientists reported their findings, and they were quite surprising. According to the study, infants before the age of 1 seem to have a metabolic rate 50 per cent faster than adults when adjusted for body size. Energy expenditure seems to decrease until the age of 20 when it stabilizes until 60 when another steady decline is observed into older adulthood.

These findings call into question the idea that we gain weight in middle adulthood due to a crashing metabolism. Rather, the study authors speculate that weight gain in middle age is purely due to behavioural factors such as diet and exercise habits.

The Role of Diet and Exercise in Maintaining a Healthy Weight

It is well-established that healthy behaviours such as consuming a healthy balanced diet, getting adequate physical activity, and ensuring good sleep will help to mitigate middle-aged weight gain no matter what the underlying mechanism.

Setting Sustainable Weight Goals

In my practice as a nutrition coach, setting a sustainable weight goal is the first order of business for any new client.

Many social media superstars or diet programs may advertise ultra-lean physiques- claiming this is completely attainable for an average 40-year-old is absurd and often sets up a health-seeker for failure. The truth is, there is a wide range of normal healthy weights for individuals of the same size. BMI may not even be the best measure of this.

Although a BMI of 18.0 to 25.0 kg/m2 is recommended for adults, this is an outdated metric. In a 2013 study published in the Journal of the American Medical Association, it was concluded that although having a BMI in the obese range (30 kg/m2 and above) was associated with higher mortality, a BMI of 25.1 to 30.0 kg/m2 was associated with lower mortality rates than a BMI of less than 18.5 kg/m2 (Flegal et al., 2013).

Finding a sustainable weight for an individual, often where they feel their best and can be sustained with realistic healthy habits is more important than looking “shredded” or getting back down to their “high school weight.”

Dietary Recommendations

Following a balanced diet with adequate nutrients for good health is most important when examining dietary recommendations. A healthy diet should include all macronutrients (not cutting out entire food groups), adequate fibre, limited saturated fats, and ample protein to build and sustain lean body mass. If weight loss is desired, a moderate caloric deficit (approximately 300-700 kcal per day depending on body size and activity level) is recommended to help reduce the loss of skeletal muscle mass.

In the case of most middle-aged adults with a moderately active lifestyle, 1.2 to 1.6 g/kg of body weight per day of protein is ideal for the maintenance of muscle mass, especially during periods of weight loss (Hj et al., 2015). Carbohydrate intake can comprise 40 to 50 per cent of total calorie intake. Most carbohydrates in the diet should be derived from whole grains, fruits, and vegetables. Fat intake should not be more than 30 per cent of total calories with saturated intake not comprising more than 10 per cent of total calories (Astrup et al., 2021).

Although there are equations that can guess daily energy needs, daily energy needs are better estimated via keeping an accurate food diary. I typically recommend that my clients practice using a food logging app and measuring their usual food intake for several weeks to determine the average amount of calories they are consuming. This provides a clearer picture of actual energy needs allowing the client to decide on an appropriate calorie deficit for weight loss (if desired). This strategy can also familiarize a client with how to plan and portion foods to best support a healthy weight.

Setting a Sustainable Fitness Routine

It is recommended that middle-aged adults The creation of a sustainable fitness routine to ensure adequate physical activity is also critical to maintaining a healthy weight. There are two main reasons for this. First, it is very difficult to maintain an energy balance or an energy deficit with a sedentary lifestyle because caloric needs are just too low to sustain on a reasonable diet. For instance, a typical 5’4’’woman who weighs 180 lbs. (unless she is a powerlifter) may only have 100 or so pounds of lean body mass.

Her resting metabolic rate may only be around 1,300 to 1,400 kcal/day leading to a total daily energy expenditure of around 1,600 to 1,700 kcal per day. An office party or lunch date with a friend could easily put her over her daily energy needs. Consumption of even a seemingly normal diet of 2,000 kcal/day will lead to weight gain (Summerfield, 2016).

Second, without physical activity (especially resistance training), lean body mass levels will decline leading to a decreased resting metabolic rate (recall that this is an issue as we approach middle age). This is especially true if a person is attempting weight loss via a caloric deficit. (Summerfield, 2016).

aim for 150 to 300 minutes of moderate or 75-150 minutes of vigorous physical activity over the week. This should include resistance training of all major muscle groups at least twice per week (Yang, 2019).

In practice, I find that many clients in this age group will do very well by aiming for 10,000 to 12,000 steps per day and participating in a resistance training program 2 to 3 times per week. It is important to note that steps are just a rough marker of daily physical activity. They can be obtained by walking, cycling, swimming, dancing, or completing house chores or physical labour.


Much controversy exists about the precise mechanism of what causes weight gain after we exit our 20s, however, our go-to tools for combating it will always remain the same. Setting sustainable weight goals, following a healthy diet, getting adequate sleep, and sufficient physical activity consistently will go a long way toward helping us to maintain our bodies into middle age and beyond.