Modifying I2C code not working

I am trying to communicate an arduino master using I2C to read ADC values of a slave.
When the master sends some data ( i.e 0x20 for ADC register1) an interupt service routine (Combuf_isr) is run which is supposed to interpret this data (know that I want to read adc register), then read the adc register and then write the adc values so that the master can read them.
I would like to have it so that the adc values are periodically updated and the master can just send read maybe every 100ms.
I am new to C so I can only come up with something like the code below but problem is I do not think it updates the adc values periodically.
Any help will be greatly appreciated!

Code:

---

void main(void)
{
  while(1)
  {
     
      if(COMBUF_Flag)
      {
          COMBUF_Flag = 1;
          /* Transmit received data */
          COM_MCU_TO_DIF_B = COMBUF_Rx_Buf[0];

---

.................................................. .................................................. ...........

Code:

---

interrupt void COMBUF_Handler(void)
{
      int res;
      int cmd = COM_DIF_TO_MCU_B; /* read COMBUF data */
      cmd = -1;


      /* process COMBUF data */
      switch (cmd)
      {
/* 0x20 to represent ADC reading PADC_DATA1 reg */
          case 0x20:
              res = PADC_DATA1;
              break;
/* 0x22 to represent DAC reading DAC_reg_0 register */
          case 0x22:
              res = PADC_DATA3;
              break;


          default:
                /* the cmd is invalid, so -1 will be returned */
              break;
        }


  COMBUF_Rx_Buf[0] = res;          /* write data to COM_MCU_TO_DIF? */
  COMBUF_RX_STATUS_CLEAR();            /* Clear receive status */
  COMBUF_Flag = 1;                    /* Flag to process rx data */
}

---

Before you massively complicate matters by adding an ISR to the mix, why not try polling in a loop to see if your basic idea actually works.

Code:

---

while ( 1 ) {
    // wait here until a sample is ready
    while ( /* something */ )

    // read the sample

    // display sample, print it, send it to port, something...
}

---

Hi. The basic Idea does work.
It is the ISR that I need help with...I would like the ISR to periodically read the adc data and write it to COM_MCU_TO_DIF (line 27).