Your basal metabolic rate (BMR) tells you how much energy your body burns to just “keep the lights on” – it’s the energy used to power the basic functions of your vital organs, to accomplish sufficient protein and cell turnover to keep your tissues functioning properly, and more. If you didn’t leave your bed all day and didn’t move a muscle, your basal metabolic rate is the amount of energy you’d still burn in a day.
We’re approaching the end of our BMR series. We’ve discussed the (current) best formulas to estimate BMR, we’ve covered the determinants of BMR, and we’ve addressed how sex, age, and athletic status, and weight loss impact BMR. Now, we’ve reached the final factor that influences BMR: weight gain.
Type and amount of tissue gained
For starters, gaining body tissue will increase BMR, proportional to the metabolic rates of the tissues that are gained. In other words, muscle has a BMR of about 13 Calories per kilogram, adipose tissue has a BMR of about 4.5 Calories per kilogram, and the liver has a BMR of about 200 Calories per kilogram. So, if you gained 5kg, including 3kg of muscle, 1.9kg of fat, and 100g of liver tissue, you’d expect your BMR to increase by about 67-68 Calories per day due to the composition of the tissue you gained. Gaining the same 5kg, while only gaining 5kg of fat, would only be expected to increase BMR by 22-23 Calories.
| Illustration of how tissue composition affects BMR changes with weight gain | ||||
| Scenario | Tissue | Gain | Tissue-specific BMR | BMR Increase |
| A) Greater gain of fat-free mass | Muscle | 3kg | 13 Calories/kg | 39 Calories |
| Adipose Tissue | 1.9kg | 4.5 Calories/kg | 8.55 Calories | |
| Liver | 0.1kg | 200 Calories/kg | 20 Calories | |
| Total | 5kg | Varies based on tissue composition | 67.55 Calories | |
| B) Exclusively gaining fat | Adipose tissue | 5kg | 4.5 Calories/kg | 22.5 Calories |
So, the impact of weight gain on BMR will depend, in part, on the type and amount of tissue you gain. If you don’t gain much weight, you shouldn’t expect your BMR to change by very much, whereas greater weight gain generally brings larger increases in BMR. Furthermore, if you gain a considerable amount of weight, but you primarily gain fat tissue, you should expect to see a smaller increase in BMR as a result, since fat tissue has such a low tissue-specific BMR. However, if you experience a larger increase in fat-free mass (especially in the form of high-metabolic rate tissues), you should expect to see a considerably larger increase in BMR.
Thus, to maximize increases in BMR as you gain weight, you should aim to gain weight gradually, since faster rates of weight gain and larger energy surpluses lead to larger gains of fat mass (per unit of weight gain) than slower rates of weight gain and smaller energy surpluses. Furthermore, exercise – and resistance training in particular – will also help you gain more fat-free mass and less fat mass per unit of weight gain, further increasing your BMR. Finally, maintaining a relatively high protein intake will help you gain more fat-free mass as you gain weight, especially when paired with exercise and a relatively slow rate of weight gain. All three of these things will help your BMR increase to a larger extent as you gain weight.
Does weight gain also cause “metabolic adaptation”?
In our prior article about the impact of weight loss on BMR, we discussed “metabolic adaptation” extensively. As a brief refresher: in a weight loss context, metabolic adaptation refers to decreases in BMR in excess of what you’d expect, based on the amount and composition of tissue lost. In other words, if the mix of fat mass and fat-free mass you lost would be expected to decrease your BMR by 60 Calories, but your BMR actually decreases by 100 Calories, the 40 Calorie difference is attributable to metabolic adaptation.
So, does a similar thing happen when you gain weight?
Yes and no.
Plenty of studies find that BMR increases rapidly when people enter an energy surplus, in excess of what would be predicted by the amount of weight gained. This sounds a lot like metabolic adaptation during weight loss.
However, during weight loss, metabolic adaptations tend to plateau or increase over time. If your BMR is 5% lower than would be predicted after you lose 4% of your initial body weight, it will generally be at least 5% lower than would be predicted after you’ve lost 8% of your initial body weight.
With weight gain, on the other hand, these early “metabolic adaptations” decrease in magnitude over time. In other words, your BMR may jump up by 10% when your weight has only increased by 1%, so it would appear that you’ve experienced “metabolic adaptation” of about 9%. But, by the time your body weight has increased by 5%, your BMR is still only about 10% higher than it was before you started gaining weight, meaning “metabolic adaptation” has dropped to 5%. Once your body weight has increased by 10%, your BMR is still only about 10% higher than it was before you started gaining weight, meaning “metabolic adaptation” has dropped to zero. Past that point, your BMR tends to increase in proportion with the tissue you gain.
A 2012 study by Harris and colleagues illustrated this point well. It involved eight weeks of overfeeding, with the researchers assessing BMR on a weekly basis. Over the first two weeks, BMR increased by around 100 Calories per day, while weight only increased by about 2kg. Past that point, subjects kept gaining weight, but their BMRs essentially plateaued. From week 4 to week 8, the average BMR was between 1772 and 1801 Calories. In week 1, subjects burned about 30.9 Calories per kilogram of fat-free mass. In week 4, they were burning 31.5 Calories per kilogram of fat-free mass. By week 8, their relative BMR was basically identical to baseline: 31.0 Calories per kilogram of fat-free mass. Furthermore, their BMR per unit of weight was actually slightly lower at the end of the study (25.1 Calories per kilogram of body mass) than the start of the study (25.8 Calories per kilogram of body mass).
The researchers also reviewed and analyzed 21 other studies that had assessed changes in BMR following weight gain. Across the board, regardless of study duration, overfeeding appeared to result in an average increase in BMR of around 5-10%, on average. But, in shorter studies, this 10% increase was disproportionate to gains in weight and/or fat-free mass. In longer studies, it wasn’t.
So, you don’t really experience a “metabolic adaptation” to weight gain. Rather, it appears that shifting into an energy surplus gives you a short-term metabolic boost. After you’ve been in an energy surplus for a couple months, your BMR more-or-less returns to “normal” again. Per unit of fat-free mass, your BMR when gaining weight is about the same as when maintaining weight. Your total BMR will increase as you gain weight, but the increases will generally be proportional to the tissue you gain.
To be clear, if you shift directly from weight loss to weight gain, this effect can feel dramatic. If your BMR was 5-10% lower than would be expected as a result of (actual) metabolic adaptations to weight loss, shifting into an energy surplus can wipe out that 5-10% decrease, and may (temporarily) boost your BMR by an additional 5-10%. I suspect that’s one of the main reasons why there are so many positive anecdotes for “reverse dieting.” But, half of the effect simply comes from getting out of an energy deficit (i.e., it could be achieved by simply returning to weight maintenance), and the other half is a short-term adaptation that goes away as you continue gaining weight.
There is one more consideration that could impact the perceived relationship between weight gain and metabolic rate. As discussed in a previous article in this series, athletes have higher BMRs than non-athletes, even when controlling or accounting for differences in total fat-free mass. Furthermore, when people start exercising, we observe increases in BMR that exceed what we’d predict based on changes in body composition. So, if you’re a naturally skinny person who starts trying to gain weight at the same time that you begin exercising consistently for the first time, it’s very possible that you will experience a large increase in your BMR. However, the “excess” increase you experience is more attributable to the introduction of exercise than to the weight gain itself.
Wrapping it up
This article is considerably shorter than the article on BMR adaptations to weight loss because there’s much less that needs to be said. Weight gain does increase your BMR, because it takes more energy to maintain a larger body. However, the increases in BMR are predictable and roughly proportional to the amount of weight you gain and the composition of the weight you gain. For unknown reasons, people do tend to experience a relatively short-term BMR “boost” when they start gaining weight, but this effect is short-lived and distinct from the metabolic adaptations that occur and persist as people lose weight.
And with that, we’re nearing the end of our BMR series. The final article in the main part of this series will tie together everything we’ve covered, and discuss how we can use all of this information to improve upon the (current) best BMR formulas that we covered all the way back in the first article, in order to help you estimate your BMR as accurately as possible. You can also try our BMR calculator, which incorporates all of the information covered in this series, in order to estimate your BMR as accurately as possible.
