Weight gain in one’s 40s is most often attributed to age, lack of exercise, or excess calories. The usual solution follows the same logic: eat less and exercise more.

This response does not take into account the changes occurring in the body during this period: it stores more fat, stores it in different places, and burns fewer calories. Four changes explain this: a drop in estrogen, which alters where fat is stored; insulin that isn’t working as well; a decrease in muscle mass; and a thyroid whose function is slowed by hormonal imbalance.

A few kilos—and especially my waistline

A major American study that followed nearly 1,250 women for fifteen years—measuring their body composition rather than just their weight—revealed something very insightful. During menopause, the rate at which body fat increases nearly doubles, while muscle mass begins to decline. Over the three and a half years that this phase lasts, body fat increases by an average of one and a half kilograms, and the percentage of body fat rises by nearly 4%.

The most striking thing is something else: weight itself doesn’t fluctuate, because the fat that’s added and the muscle that’s lost almost cancel each other out on the scale. Weight and body mass index therefore reveal nothing about this composition. A woman can maintain slim arms and legs and a stable weight while still seeing her belly round out. Along with waist circumference, body composition is the most telling indicator of health.

This belly fat does not serve the same purpose as the fat on the hips. Some of it accumulates around the internal organs; this is called visceral fat, and it is often associated with metabolic syndrome: excess abdominal fat combined with high blood pressure, high blood sugar, high cholesterol, or high triglycerides.

Finally, resting energy expenditure is lower after menopause, assuming comparable height and body composition, which makes it easier to gain weight even with the same diet.

Why the stomach?

Estrogens stored fat in the lower body

In women, estrogen controls the proliferation of fat cells, their filling, and their distribution. It directs them toward the buttocks, hips, and thighs—what I call our “baby reserves”—where the fat cells have many estrogen receptors, the binding sites that pick up the hormone’s signal. This is the so-called gynoid, or pear-shaped, body type.

When estrogen levels drop, these fat stores lose their signal to remain in place. Fat is released into the bloodstream and then redeposited elsewhere—mostly above the waist. This process occurs gradually and increases the risk of being overweight, inflammation, and impaired glucose tolerance.

What the Final Stages of Pregnancy Teach Us

The image stems from a simple observation: toward the end of pregnancy, estrogen levels drop sharply, and fat from the hips and thighs is mobilized to provide energy for breastfeeding. During menopause, the same process occurs, though less abruptly and over a longer period. The difference is that there is no baby to feed: since the released fat isn’t being used, it is stored elsewhere.

Insulin becomes less effective

Insulin is the hormone that transports sugar into cells, and it is also a storage hormone. Estrogens affect both its secretion and the cells’ sensitivity to its signal. One point deserves clarification, as it is often misunderstood: it is not an excess of estrogen that impairs this response, but rather its instability. When estrogen levels are stable, they actually support this response. When they fluctuate wildly, the same meal can trigger a greater insulin response, and this excess promotes fat storage, particularly in visceral fat.

The amount of insulin released is proportional to the meal's sugar content. A slight rise in blood sugar helps prevent weight gain, while a sharp rise promotes it. Without physical activity after a meal, sugar remains in the bloodstream longer, the blood sugar spike is higher, and the excess is stored as fat.

When these spikes occur repeatedly, followed by drops two to five hours later (cravings, weakness, brain fog), the cells become less and less responsive to insulin. This loss of sensitivity paves the way for fat to accumulate in the liver and around the organs, leading to metabolic syndrome.

Muscle Wasting

Muscle consumes a lot of sugar, and it does so without needing insulin. Less muscle, therefore, means less sugar is used and blood sugar levels are harder to control. With age-related muscle loss, fat infiltrates the muscle itself, and insulin sensitivity worsens: as a result, a woman can be both overweight and lacking in muscle.

Several factors accelerate this loss: a sedentary lifestyle, low-grade inflammation (subtle but persistent), and declining levels of growth hormone and androgens—so-called “male” hormones that are also present in women and help maintain muscle mass. Dieting also plays a role: it causes a loss of fat, but it also leads to a loss of lean body mass.

What about the thyroid?

The thyroid gland regulates the body's metabolic rate. It determines basal metabolic rate, and its active form, T3, is used to metabolize proteins, fats, and sugars. The gland primarily produces a storage form, T4, which the liver and intestines convert into T3.

The connection to what was discussed earlier is real, but it deserves to be stated accurately, as there are many misconceptions circulating on this subject. What is firmly established is that estrogens increase the liver’s production of the protein that transports thyroid hormones into the bloodstream. However, a transported hormone is a bound hormone, and therefore temporarily unavailable to the cells. The evidence for this is clinical and well-documented: a woman being treated for hypothyroidism who begins oral estrogen therapy will need to have her dose increased by about a quarter.

On the other hand, the often-repeated notion that progesterone promotes the conversion of T4 to T3 is based solely on animal studies. It is therefore more accurate to say that the hormonal imbalance during this period reduces the availability of thyroid hormones.

This explains why hypothyroidism—that is, a thyroid gland that no longer produces enough active hormones—frequently develops during this period. It often goes unnoticed because its symptoms (fatigue, sensitivity to cold, constipation, dry skin, hair loss, depression, and weight gain) are easily confused with those of hormonal changes. Women are several times more likely to be affected than men, and about one in ten women between the ages of 50 and 60 is affected.

There are other factors that can interfere. Stress inhibits thyroid function, and cortisol, the stress hormone, reduces the conversion of T4 to T3. Fasting and very low-calorie diets also lower thyroid hormone levels and are therefore not always recommended during this period; this should be evaluated on a case-by-case basis.

Even a thyroid that's within the normal range can affect your weight

An American study followed more than 2,400 adults for three and a half years, measuring TSH, the hormone through which the brain regulates the thyroid: when TSH levels rise, it means the gland needs to be stimulated further. The baseline TSH level did not predict future weight. However, women whose TSH levels had risen the most gained an average of 2.3 kg, compared with 0.5 kg among those whose levels had changed the least—and this finding held true even when all values remained within the normal range.

In practice, unexplained weight gain therefore warrants monitoring TSH levels over time, rather than simply checking them at a single point in time.

We also need to look beyond TSH. Sometimes TSH levels remain within the normal range even though clinical signs are clearly present. This warrants further investigation through a comprehensive evaluation—not just TSH alone: a thyroid assessment that includes both hormonal and micronutritional factors provides a more holistic perspective.

What should we focus on?

Each of these mechanisms points to a key factor: sugar intake, protein, meal timing, thyroid-supporting nutrients, and physical activity.

Focus on sugar intake rather than fat

A large American observational study compared four dietary patterns among postmenopausal women aged 49 to 81, focusing on those who consumed at least 10% of their body weight in fat. The low-fat diet was associated with a more than 40% increased risk, and this risk was even higher among women who were already overweight. The low-carbohydrate diet, by contrast, was associated with a reduced risk of about 30%. The Mediterranean diet showed no significant effect.

• No sugar on an empty stomach. A protein-rich breakfast, with high-quality protein and healthy fats, helps you avoid starting the day with an insulin spike.
• Order on your plate. Start with raw vegetables and other vegetables, then protein, and finish with starchy foods. Save sweets for dessert—never eat them on an empty stomach.
• Cooking methods and food choices that slow down the absorption of sugar. Pasta and rice cooked al dente, whole fruits rather than juice or smoothies, sourdough bread. Starchy foods that are cooked and then cooled—in a salad, for example—reduce the sugar spike by about one-fifth.

Protein at Every Meal

Protein helps maintain muscle mass and keeps you feeling full. The recommended daily intake during this period is between one gram and one gram and four per kilogram of body weight, spread out over meals, and more if you're physically active.

• Twenty to twenty-five grams per meal, or about a 100-gram serving of meat or fish. One egg provides 6 grams, and one slice of ham provides about 7.
• More plant-based protein (lentils, chickpeas, tofu) and a little less animal protein.
• A fairly light dinner, preferably with plant-based protein (such as rice and lentils). This helps you sleep better and limits the evening blood sugar spike, which is more pronounced than the morning spike for the same meal.

Eat earlier in the day

In a trial, overweight women with metabolic syndrome were given the same 1,400-calorie diet, distributed in two ways: a large breakfast and a small dinner, or the reverse. The group that ate a large breakfast lost more weight and waist circumference, with a more pronounced decrease in blood sugar and insulin levels, and a 33.6% drop in triglycerides, whereas triglycerides increased by 14.6% in the other group.

This principle is consistent with the body’s internal clock: the same amount of food raises blood sugar levels less in the morning than in the evening. A hearty breakfast (eggs, nuts, seeded bread, or sourdough bread), a moderate lunch, and a light dinner make for a healthy balance. Fasting in the evening—for example, twice a week—can be a good addition, provided you don’t overdo the restrictions, which can slow down the thyroid.

Supporting the Thyroid

• Iodine, which evaporates easily when heated: non-fluoridated iodized salt added at the end of cooking, eggs, yogurt, and seaweed in reasonable amounts.
• Selenium: One to four Brazil nuts per day, depending on their origin, as their selenium content varies widely. Choose nuts from Brazil.
• Omega-3s: small fatty fish (sardines, mackerel), flaxseed oil, and camelina oil—store in the refrigerator and consume no more than one teaspoon per day.
• Cabbage and soybeans are known to suppress thyroid function. Cooking and fermentation reduce this effect, and the priority remains ensuring an adequate intake of iodine. When consumed in reasonable amounts and with an adequate intake of iodine, a healthy balance is maintained.

Get moving—especially after a meal

A ten-minute walk after a meal is enough to significantly lower the subsequent spike in blood sugar, and is just as effective as a longer walk taken later. This is the most effective tip in the entire article, because the muscles take up the sugar the very moment it arrives. Resistance training—that is, movements performed against a load (resistance bands are sufficient)—maintains muscle mass when combined with adequate protein intake.

Other Possible Causes

Other factors contribute to this, each discussed in a separate article. Sleep that no longer restores energy: “Morning Fatigue.” Prolonged stress and cortisol: “Hypersensitivity to Stress and Noise.” Overwhelming cravings for sweets: “Sweet and Salty Cravings.” Iron deficiency or impaired cellular energy production: “Energy Slumps.” These causes are not mutually exclusive, and it is common for them to occur together.

When to Seek Medical Attention

Seek immediate medical attention if you are already being treated for a thyroid condition and your weight changes for no apparent reason. Never stop or change your treatment on your own; talk to your doctor.

Even from a distance, but without waiting too long, if several symptoms occur together (sensitivity to cold, constipation, dry skin, hair loss, loss of the outer part of the eyebrows, menstrual irregularities, low mood): the more symptoms there are, the more likely it is that you have an underactive thyroid. Similarly, if you start gaining weight without changing your habits, if your waistline increases, or if you experience hunger pangs two to five hours after meals. This is something to discuss with your doctor, who can monitor changes in your TSH, blood sugar, insulin, vitamin D, and iron levels, and may recommend a bone density test (a test that measures bone strength) during menopause.

Key takeaways

During this time, fat redistributes itself. The drop in estrogen releases fat stores from the hips and thighs, which are then redistributed to the abdomen. Insulin becomes less effective, muscle mass decreases, and thyroid hormones are less available. The scale, however, may show almost none of this.

These mechanisms explain why weight responds less effectively than before to the same diet. They do not mean we can avoid taking action. Each person’s role involves specific steps: fewer fast-acting sugars and never on an empty stomach, protein at every meal, a light dinner, adequate intake of iodine and selenium, a ten-minute walk after meals, and regular strength training.

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Frequently Asked Questions

Would a strict diet help me?

Severe calorie restriction causes you to lose muscle along with fat, and fasting lowers thyroid hormone levels. Spreading your calorie intake earlier in the day, while ensuring adequate protein intake, is more effective than simple calorie restriction. And cutting out fats is counterproductive: this approach has been linked to the highest risk of weight gain.

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Photo credit (c) Unsplash - Jacki Drexler