Perhaps the brain of an INTP consumes more glycogen, which increases the quantity of carbohydrates required to go beyond the thresh-hold of glycogen storage....Therefore, there is less likelihood of storing fat. Yet INTP's tend to be less likely to delve into physical activity, which implies less muscle-->less glycogen needed for muscle repair--> higher propensity for fat storage (relative to physically active case....Se user). Maybe, internalized thoughts causes a lack of awareness of time flow, which would cause an INTP to exhibit a sporadic eating schedule (gap between meals is not consistent)...This, in combination of Ne and lower Si would yield a larger gap between meals (-->distraction)....which in turn leads to less overall caloric intake per unit time. Either that or metabolism is correlated with the degree of NE usage, yielding more consumed energy per thought. A lower metabolism would surely slow the mind, leading to less thoughts per unit time and also may indirectly lead to negative mindset/stress. Stress/pain can trigger a higher consumption of calories to hinder the pain, but also the opposite since stress is correlated with depression....which then limits the appetite....Since INTP have inferior Fe, stress will yield a strong emotional response, which can cause impulsive acts including overindulgence. Thus, it may not hold for an unhealthy INTP in this regard. I'd say in general, a high metabolism leads to a rapid firing of thoughts (for INTP) given Ne is well developed.
Now, I will note that there exists a contradiction in regards to INFJ being "Endomorph" based on the previous assertion that INFJ's are typically "very thin" (assume true for most cases and consider that specific subset). Endomorphy is proportional to metabolism, which is related to diet...If we suppose that INFJ's are more likely to consume fast food, then it is likely they will go through puberty later....Hence, growth plates will fuse later for both Males and Females leading to longer limbs relative to torso. Further, this effect combined with poor nutrient load will yield catabolism. Thus, "mesomorphy" is unlikely.... Now, given that INFJ's are very introspective and lack Se, it would be less likely that they would enjoy various foods (exploration)....so, food is likely consumed less regularly....This would in turn lead to ectomorphy ( < calorie load), however, fast food hinders metabolism more rapidly over time than a healthy diet....Hence, an INFJ may exhibit a more pronounced transition in regards to body type relative to other types.
There is definitely a correlation though, especially higher Se user tending towards mesomorphy/ectomorphy. Food is obtained via external means and initiates a stronger response to Si/Se types for differing reasons. A strong introvert is likely to be consumed in thought, which will negate physical awareness to an extent (esp Ti users). Aesthetics will appeal to sensor's (esp Se) relative to intuitive's....hence you can make the conjecture that intuitive are less likely to be mesomorphs (this would likely apply to Ne relative to Ni.....Ni will foresee that being well dresses and physically fit will accelerate their drive to success).