149: The Hidden Signals That Make Fat Cells Grow

Bikman’s core argument is that PPARγ is permissive—it sets the table for fat storage—but insulin is the switch that starts the meal. Laboratory data show that without insulin in the culture medium, no amount of drug or fatty acid can create new fat cells. He extends this to whole-body physiology: during fasting, prolonged exercise, or a ketogenic diet, low insulin directs fats to burn. Therefore, dietary fat per se does not cause net fat gain; a high-carb meal that spikes insulin is what locks the storage program on. Controlling carbohydrate intake is thus the most practical and rational way to keep the nuclear receptors of adipose tissue in a healthy, well-regulated state.

Bikman invokes fasting and prolonged exercise as practical examples of low-insulin states where fatty acids are burned. He emphasizes that the same fatty acid that would be stored in a high-insulin postprandial state is oxidized when insulin is low. This fits neatly with the overall message that insulin is the gatekeeper, and that lifestyle strategies creating low insulin periods are as important as—if not more than—manipulating dietary fat composition.

Bikman addresses the popular online claim that seed oils are uniquely fattening. He confirms that rodent evidence shows polyunsaturated fats activate PPARγ more strongly, potentially expanding the fat cell pool and improving insulin sensitivity in a way that paradoxically allows more fat storage. But he insists the effect is far weaker than TZDs and is gated by insulin. He notes that humans who eat more polyunsaturated fat do accumulate more in their fat cells, unlike saturated fats, which don’t build up proportionally. The take-home is that reducing seed oils may be a secondary lever behind controlling insulin.

Bikman describes phthalates as plasticizers found in flexible plastics, food packaging, cosmetics, and medical tubing. Their metabolite is measurable in urine and directly activates PPARγ. He ties this to the obesogen hypothesis: environmental chemicals can partially mimic the PPARγ-driven adipogenesis seen with diabetes drugs. He doesn’t frame it as the primary cause of obesity but as an added input that, combined with a high-insulin milieu, could push the fat cell pool to expand.

Bikman explains that the standard laboratory protocol to make pre-adipocytes become fat cells requires insulin. Activating PPARγ with drugs drives adipogenesis even further, creating an abundance of small, insulin-sensitive fat cells. This clears glucose from blood and raises the metabolically favorable hormone adiponectin. Yet clinically, patients gain pure fat mass—often several kilograms in months—and many want to discontinue the drug because of this side effect. The first TZD, troglitazone, was withdrawn for liver toxicity; pioglitazone and rosiglitazone remain widely prescribed.

Bikman repeatedly emphasizes that PPARγ is the ‘master regulator’ of fat cell development, but its expression and the entire adipogenic program are orchestrated by insulin. He uses a conductor–baton analogy: insulin is the conductor, PPARγ the baton. Laboratory protocols require insulin in the culture medium to differentiate fibroblasts into adipocytes; no insulin, no new fat cells. In low-insulin states—fasting, prolonged exercise, ketogenic diet—fatty acids are burned for oxidation regardless of PPARγ binding. Therefore, the carbohydrate load that raises insulin is the real lever.

Bikman details that adipose tissue doesn't just respond to circulating cortisol; it can top up intracellular cortisol via an enzyme that converts cortisone to active cortisol. Visceral fat has especially high activity of this enzyme. Since the glucocorticoid receptor is inside the cell (a nuclear receptor), the locally generated cortisol directly activates it, amplifying the storage signal. This means that even when systemic cortisol is normal, visceral fat cells may be experiencing a high-cortisol environment, driving differentiation and fat accumulation. The effect further magnifies when combined with high insulin.

Bikman notes that the major phthalate metabolite is a direct PPARγ activator, binding the same pocket as TZDs. This can push pre-adipocytes (fibroblasts) toward becoming mature fat cells. BPA has a similar but weaker effect, possibly acting on multiple nuclear receptors. While the magnitude of activation is far lower than pharmaceutical TZDs, chronic low-level exposure provides a persistent adipogenic nudge. Even so, Bikman cautions that insulin still acts as the master gate; however, in an insulinogenic diet the combined signal may be problematic.